Finding my second 1991 Husqvarna WMX 610 for sale in California was exiting and unexpected. The rusty and abused incomplete and mismatched pile of parts I got for $500 was in sorry condition and it took me a long time to get around to doing much with it. Finding my third 1991 Husqvarna WMX 610 for sale in California was perhaps even more unexpected, and it turned out to be practically in new condition.
The Czech Cherry
Year Long Build Plan
Cam Timing Adjustment
Rebuilding the Derelict
Riding the 1990 WMX 610
It was early Summer 2015, shortly after I got my old 1991 Husqvarna WMX 610 up and running again with a rebuilt 1997 TE610 six speed motor. I saw an ad for a 1991 Husqvarna WMX 610 that was taken apart. The seller did not seem to know much about the bike, and did not know how much she wanted. All she knew was that she wanted to sell the bike. I offered to trade an old Toyota car that was in nice condition other than needing a motor and transmission, but she wanted cash and still could not come up with any number. I offered $500 sight unseen, and she said that the bike was mine if I wanted it.
When I got out to look at the bike it was indeed mostly all there as the seller had said, but it had been sitting outside for a while and was rusty inside and out. Some parts were obviously missing, most notably the kick start lever. The motor had not been taken apart, but all the plastic was off the frame and laying in a pile. The front fender looked like it had been dragged around and chewed up by a dog, big tooth marks all over one corner. I was thrilled to get it for $500 even in that sorry state, obviously the vast majority of the bike was there and in reasonably good condition.
When I got the bike home and looked it over carefully I found that several fasteners were broken off in the frame, and there were quite a few strangely missing pieces. The lower steering head bearing had been partially removed. The rear wheel was not a 1991, but rather a 1994 rear wheel. This was particularly interesting to me as I had just replaced the trashed old 1991 rear wheel on my other 1991 WMX 610 with a 1994 rear wheel. Apparently someone else had the same idea, but they had not gotten around to making the custom wheel spacer required to actually use the 1994 rear wheel on the 1991 bike. It was just flopping around on the axle. It was a start though.
The carburetor was off of the motor, but still dangling by the throttle cable. Looking in the intake ports I could see that the motor had been sitting out in the rain with the carburetor off. This told me that the motor was probably in poor condition. It also felt seized up. I figured I would pull the top end off to get a look inside. I broke the motor free turning the flywheel nut, and then the cylinder came off fairly easily. The motor was in rather good condition, but a chunk of the plating had been stuck to the compression ring and pulled off of the cylinder wall. The cylinder would need re-plating. When I got the piston out I also found that the rings were quite stuck, and I broke the oil control ring in removing it. The piston skirts were worn down a small amount so that the skirt clearance had opened up to close to the 0.0071" wear limit listed in the 1991 Husqvarna Owner's Service and Tuning manual. The compression ring end gap was also way out at a huge 0.030". It seems that this motor did get run quite a bit at some point. The transmission and bottom end seemed to be in good condition. It went through all the gears nicely, and the connecting rod big end radial play was what the 610 motors usually are at about 0.0013".
I sent the cylinder off to U.S. chrome and bought a new Woessner piston kit. The Woessner gaskets were however not available. I thought I was going to put the bike right back together, but somehow the project lost priority. When the cylinder came back from Wisconsin I laid it out on my desk with the piston and the cylinder head and it became my conversation piece and inspiration. Mostly I was just fascinated with the huge bore diameter. I have taken quite a few dirt bikes apart, and that bore is just really extremely large. The cylinder and head look small on the outside, but the actual bore diameter is something that a clenched fist goes through. Really really big for a dirt bike.
Where the project really got held up was in trying to decide what to do about compression ratios and camshafts. I laid out the 1991 camshaft next to the huge 1992 camshaft on my desk to contemplate which one to use. This was at the time that I was getting a lot of really extremely slow flame front travel speed gasoline that ran great at 3,000 to 5,000RPM but would not rev out well past 7,000RPM with 16 to 21 degrees BTDC on the static timing setting and then got extremely unusably harsh at all engine speeds bellow 3,700RPM with 24 degrees BTDC on the static timing setting that also was able to rev out and make big power to 8,000RPM. I had been going back and forth on the spark timing on that slow flame front travel speed gasoline and it just could not be made to work without a more substantial advance curve or a bigger camshaft. I knew the giant 260 degree at 1mm valve lift 1992 camshaft would cure the slow flame front travel speed problems, but I also knew that was way too big of a camshaft for the Husqvarna 610 motor.
Then the slow flame front travel speed gasoline became less common and I was getting all sorts of gasoline everywhere from some pretty weak low energy density watered down stuff that seemed like it had about 20% ethanol in it to extremely high pressure race gas that was requiring tons of spark advance in the 11:1 motor and was just supper harsh at all engine speeds while not making quite as much power as lower pressure gasoline. It was hard to figure out what should be done about camshafts and compression ratios. I had all sorts of ideas, but each idea only worked with one type of gasoline and just about every week or two the gasoline supply was dramatically changing at all the gas stations I tried. Part of the problem was also that I did not know if the stock 1991 SEM ignition system was going to work or not, and the best compromise engine setup was considerably different depending on whether I was building it for the stock SEM CDI or for a points ignition system. I didn't really need another dirt bike, as I already had my trust old 1991 WMX 610 up and running registered and insured as well as my 1991 WXE 350 with the 386 stroker motor up and running registered and insured. Two was enough, and I also had the 1992 TE 350 chassis with my worn out old 1991 WMX 610 motor up and running as well as the 1986 400 WR two stroke. That was plenty of dirt bikes. I was however always riding the 1991 WMX 610, that was by far my favorite bike. Both because of the broad big power and because of the very fast and fairly comfortable suspension. All the other bikes were weird slow toys by comparison. Then I bought another 1991 WMX 610.
This one was mostly all together, but also rusty. It was advertised for $500 and I just couldn't pass it up. Where were all these 1991 WMX 610 bikes coming from. I had never seen another one other than mine in the first 18 years that I had owned it. I had seen a few 1993 through 1996 TE610 Husqvarnas around here and there. Even they seemed to be quite rare.
In any case $500 for another nearly complete 1991 WMX 610 was too good to pass up, and this one was not even disassembled. At least not much. The ignition cover was off and it had some other ignition system on it. At first I thought it was one of the late 1990's Cagiva ignition systems that people were calling the "Ducati" ignition. When I looked more carefully though it did not say Cagiva on the flywheel, instead it said "Czech Republic". Very interesting. The guy that had the bike said that he had never had it running, he wasn't really even much into dirt bikes. I guess he got stuck with the bike thinking it would be an easy fix. Well it was an easy fix, it just took some creative repairs.
The motor was free, but the cranking compression seemed low. When I poured a half ounce of lube oil in the spark plug hole the cranking compression came up a whole lot, so it had to do with the ring and cylinder not the valves. Probably just a stuck ring from sitting outside for so many years. The 40mm DellOrto carburetor was however gunked up solid. Every jet and passage was stuffed full of that same strange smelling green residue that was in the 1997 TE610 40mm DellOrto that I had put in my original 1991 WMX 610 chassis. It was the dirtiest and hardest to clean carburetor I have ever seen, it took me hours of poking through holes and cleaning to get it ready to go. New ones are also available from the English company that now owns DellOrto carburetors, they are just a bit expensive and come all the way across the Atlantic. Instead I persevered with cleaning the gunk out. All the parts were in reasonably good condition except that someone had drilled the main jet out to about a 195 size. The first step in fixing a drilled out jet is to drill it out much farther to give some room to work with. Then the jet can be soldered up with plumbers solder and a new hole drilled. It is not perfectly precise, but it gets the jet down to a reasonable size. The jets are available also, I just seem to be extremely resistant to ordering English made DellOrto parts across the Atlantic.
I drilled the jet size to something close to 175 or 178 and I set the needle clip position to the stock recommended third groove. I was actually thinking that I would start out with the needle clip on the stock K32 needle at the 2nd groove position, but somehow I put it together with the clip on the stock 3rd groove position. It was only later that I realized the mistake I had made, so at first I was running the extremely excessively rich stock 3rd groove needle clip position.
The brakes were mush both front and rear. When I bled them through a bunch of thick black and dark brown gunk came oozing out. Once I had flushed the brake systems through they both seemed to work perfectly. The cables were also dry and sticking. I worked some chain lube through them and they freed up pretty well. Then the end of the de-compressor cable broke off. I repaired that by putting a bicycle shift cable in with a new end brazed on. The air filter cage and hold down were missing, but I had those parts off of my original 1991 WMX 610 from having put the 1997 airbox on it. The clutch actuator shaft was also broken off at the clutch arm, but I had an extra one from my spare junk Husqvarna 350 motor. The intake boot on this 1991 WMX 610 was also in very poor condition, but it had not yet broken all the way through to be useless. Overall the bike seemed to be in really good condition.
I pulled the clutch cover off to inspect the oil reed valve and that was installed correctly and in perfect condition. While I had the clutch cover off I noticed that the primary reduction gears were not yet broken in. This bike had hardly been ridden at all. It was like brand new 25 years old. It wouldn't start though. The ignition was providing what looked like a pretty strong spark, so that was encouraging. At first it was that I had missed the idle passage when I cleaned the carburetor, so I took the carburetor off again to run a little piece of fine wire trough the passage to push the gunk out. The boots survived another carburetor removal and installation, but this thing is going to need an intake boot also.
Eventually I got the bike to fire up but it would not idle. I turned the idle stop all the way in and it would sort of idle once warmed up. There was no power though. I rode it around a bit but it was totally flat and would not rev up. It seemed like the spark timing was very late and it just would not pop off. I found top dead center through the spark plug hole and I made marks on the flywheel and the cases. When I put a timing light on it I found that it was low idling at a spark timing of 10 degrees ATDC and then it advanced 15 degrees to run at a spark timing of 5 degrees BTDC above about 3,000RPM. Obviously the flywheel had either not been set correctly or it had slipped. I thought the key was broken and it had slipped. I did not have a puller for the flywheel, but it turned out to be a standard size that I was able to easily get.
While I was waiting for the puller to arrive I rode the bike around a bit and changed the oil several times. After the first little test ride of a quarter mile the oil came out looking like thick brown Chocolate milk. I changed the oil again and it came out less contaminated looking. The magnet was clear though, no metal shavings. After just this small amount of test riding I noticed that the cranking compression had come up substantially. The low cranking compression had just been a dirty and slightly rusty compression ring from sitting all those years out in the elements. Once the ring seated in on the short test ride the engine seemed like new. I checked the valve lash again also, and it was still right where I had set it.
Once I had the puller I popped the flywheel off and found that it was indeed an entirely custom ignition system installation. The flywheel was for some other bike with a smaller crankshaft end tapper, but someone had turned it out to fit the Husqvarna. Well it sort of fit. The tapper was not a perfect match and there was no key. There was no keyway cut into the turned out flywheel tapper at all. I guess it had been easy to pop the flywheel on a lathe to turn it out to a bigger diameter, but cutting a keyway would have taken some doing.
I set the flywheel in position to where it looked like it would be low idling at 10 degrees BTDC and running at 25 degrees BTDC above 3,000RPM. That seemed like a good starting point for the stock 10.2:1 compression ratio.
The bike started much more easily, and I had to turn the idle stop way out to get it to idle down. When I rode off the engine pulled for like two seconds and then went flat. The flywheel had slipped again. When I put the timing light on it the running spark timing above 3,000RPM was back down below 20 degrees BTDC. I reset the flywheel at a spark timing of about 29 degrees BTDC and torqued the nut down tighter. This time the bike ran stronger for a while, but when I really jumped down on it the flywheel slipped again. When I first rode the bike with the 29 degree BTDC spark timing it was rather crisp, but it was also very harsh at 3,000 to 4,000RPM. When the flywheel slipped a bit the torque actually increased as the engine got much smoother and quieter. This time it did not slip as much and the bike was still running and making torque, for a while. But the flywheel continued to slip, and within a few miles it just had tons of lag and hesitation and would not rev up much. When I put the timing light on the engine the running spark timing above 3,000RPM was at about 23 degrees BTDC.
I took the flywheel off again, and this time I detailed the chewed up surfaces with 400 grit silicon carbide paper. I set the flywheel at 26 degrees BTDC and tightened the nut down a lot more. I also put high temperature thread locker on the tapper this last time, so this was going to be the setting.
Sure enough that did the trick, the flywheel did not slip any more. All of a sudden I had another running 1991 Husqvarna WMX 610. With locked down spark timing. By this point that was starting to sound appealing. I had been having to make large adjustments to the static timing setting on my points ignition practically every time I went to the gas station, and that was getting old.
Indeed the stock motor with "locked down" spark timing was seeming to get a more consistent gasoline supply. It was not exactly the same every time, but a lot of days the motor actually would run and make torque with that 26 degree BTDC spark timing setting. It was often revving out to 7,000 and even 7,500RPM, but power up at the top was not spectacular. Near peak power was seeming to come down around 6,000 to 6,500RPM. It was pulling plenty good for a dirt bike, but the difference between the stock motor and my hot rod 610 motor is really very dramatic. I spent several days swapping gasoline back and forth, and each time I put the gasoline out of the stock motor into the hot rod motor I was just blown away by how much more power I was getting everywhere from 5,000 to 8,000RPM. It was like 40% more power from the hot rod 610 motor. Especially up at 7,500 to 8,500RPM the hot rod motor was just giving a huge pull compared to the stock motor on the same gasoline. On the same gasoline with the same main jet and the same exhaust system the 12.2:1 hot rod motor was running a static timing setting of 22 degrees BTDC and the stock, presumably 10.2:1, motor was running a spark timing of 26 degrees BTDC. I did not know it at the time, but the stock 10.2:1 motor was also running the stock 3rd needle clip position, where the 12.5:1 hot rod 610 motor was running the 2nd needle clip position. No doubt that was part of why I was able to run such similar spark timing values with such a large 10.2:1 to 12.2:1 compression ratio difference. I had also been calling the rebuilt 1997 motor with the 30mm compression height Woessner piston and a 0.020" base gasket a 12.5:1 motor. I had made that estimate when I did not have a Mahle piston to look at, and I had assumed that the valve reliefs were of similar size. It turns out though that the stock Mahle piston has extremely small valve reliefs, practically non-existent, so the hot rod 610 motor actually is more like 12.2:1 with the huge valve reliefs on the Woessner piston. In any case it is still a rather large compression ratio difference between the stock 10.2:1 motor and the 12.2:1 hot rod 610 motor.
All these compression ratios are also relative to the assumption that the stock 1991 Husqvarna WMX 610 motor actually does have a compression raio of 10.2:1. I have never precisely measured the combustion chamber volumes using a piece of clear acrilic sheet and a syringe like I used to do on automotive engines. I have several times estimated the combustion chamber volumes by measuring with a caliper and calculating the volumes of the shapes, and I have always come up with a combustion chamber volume that is fairly close to what it would be for the stock 10.2:1 compression ratio. When I did a slightly more precise estimate using the gram scale and pouring water into the combustion chamber I was getting about a 37cc water volume, which with the stock Mahle piston and the stock head and base gaskets works out to a somewhat lower 9.8:1 compression ratio. The 1991 Husqvarna Owner's Service and Tuning Manual lists the compression ratio of the WMX 610 motor at 10.2:1, but the 1992 Husqvarna Owner's Service and Tuning Manual lists the comprssion ratio of the TC 610 at 9.9:1. Later Husqvarna 610 motors were listed at a 10.0:1 compression ratio. Slight over estimates of compression ratios have been common. I have taken bone stock 1980's automotive engines appart and found the actual compression ratio to be as much as 6% lower than advertised, which is a big deal on engines that had excessively low advertised compresion ratios generally well bellow 9:1.
When I rode the bone stock 1991 WMX 610 I could really feel that heavy 406g stock Mahle piston also. The stock motor vibrates more right from 2,000RPM, although it is a mild sort of vibration down at lower engine speeds. Then up at above about 5,500RPM there is just this huge crashing around and rattling with the heavy stock piston. Interestingly though this new stock 1991 WMX 610 motor did not seem to be vibrating quite as bad as I remember my other stock 1991 WMX 610 motor had back in 1998 through 2006. Certainly this new stock 1991 WMX 610 motor does vibrate a whole lot more than the hot rod 610 motor with the cut down 332g Woessner piston, and it also vibrates a whole lot more than my old stock 10.2:1 motor with the Mahle piston cut down to 368g. It was just really seeming like the vibration was noticeably less on this new stock WMX 610 motor than I remembered. It could have something to do with the rather long 10 year interim between running the two bone stock 1991 WMX 610 motors, or there could also be some slight differences in the balance jobs on the two motors. The Mahle pistons in the 1991 WMX 610 motors are really very crude looking things with sloppy machine work on them, so there could even be some significant weight variations from piston to piston. If I had to guess how much the piston in this new stock 1991 WMX 610 motor weighs just comparing it to how I remember the 406g piston running and how the 368g cut down piston runs I would say it is about 390 or 395g. Right in the middle between the two, but perhaps a bit closer to the violent crashing around of the 406g stock piston.
Another interesting thing I noticed was that the lighter 2.3 pound Check Republic flywheel felt noticeably better than the big heavy four pound stock SEM flywheel. I could really feel the difference when slipping the clutch around 2,500 to 3,500RPM. The 2.3 pound flywheel has noticeably more inertia than the points ignition system with no flywheel, but that 2.3 pounds was much less of a problem than the extremely heavy four pound SEM flywheel.
The suspension on this 1991 WMX 610 was exactly like the suspension on my original 1991 WMX 610. It seemed like someone had even put the correct 5 weight oil in the forks on this bike, as they felt very much the same as the forks on my other 1991 WMX 610. I set the shock pre-load on both bikes to the same setting and they rode exactly the same. It was interesting that with the pre-load ring set to the same position on both shocks the back of my street legal 1991 WMX 610 was sitting noticeably a small bit lower than on the new stripped down 1991 WMX 610. The difference in weight was partly the license plate, tail/brake light and turn signals, but those come to only a small amount of weight. The difference was also in my fender bag with tools, CO2 inflator ext. that I had on my original 1991 WMX 610. The ignition coil mounted to the front of the rear fender liner also added some extra weight towards the rear of the bike. This all added up to the back of the street legal 1991 WMX 610 sitting like a quarter inch lower with the same setting on the shock pre-load. When riding the bikes though the weight was not really noticeable, they both felt pretty much exactly the same. Really all I could notice in terms of a difference was just that the bike with the hot rod 610 motor was accelerating a whole lot faster and getting up to much higher speeds even on a small twisty dirt road.
Later I changed the fork oil on my original 1991 WMX 610, and the fresh Bell Ray brand 5 weight oil felt much better. It was like the oil had thickened over time. The oil change made such a dramatic difference on that 1991 WMX 610 that I changed the oil on the new 1991 WMX 610 also. Interestingly this seemed to make less of a difference, even though I rode the bike in the morning, changed the oil and then rode the bike again on the same day. In any case the two sets of 1991 WMX 610 suspension have been seeming nearly identical, which is not at all surprising considering that they are in fact exactly the same.
I started out with the stock 12/52 gearing using some old sprockets and the old chain that came with the bike. This was expectedly way too low of gearing. With the engine only revving out to about 7,000RPM sometimes top speeds were barely breaking 70mph. When I ordered a new chain and 13 tooth front sprocket I was planning to run the 13/52 gearing I had started out with on the stock 10.2:1 610 motor in the 1992 chassis back in early 2015. That gearing had been seeming low enough for all purposes on the five speed transmission. Somehow I mixed up the chain lengths though and ordered a 114 link 520 O-ring chain. When the new chain arrived I realized it was four links too short. Instead of sending the chain back I put an old 48 tooth rear sprocket on for some very tall 13/48 gearing. This actually did work, although first gear felt very tall and the rear axle was pretty much all the way forward. At least fifth gear was not seeming so low that the bike got topped out on small trails. I have always liked the feel of the 1991 WMX 610 with the axle forward, it turns quicker and feels more nimble. I have however mostly run the rear axle back rather far to keep the front end down with the huge 577cc displacement. The stock 610 motor with the heavy stock Mahle piston is always somewhat weak on power though, so the forward axle position has worked out just fine.
At first I was running the stock 1991 WMX 610 with just the stock motocross muffler extension tip that came with my original 1991 WMX 610. It was early spring and there was no danger of lighting anything on fire without a spark arrestor. I did try one of my Cobra Sparky spark arrestors though, and I was disappointed to find that it caused worse hesitation. The spark arrestor also made the bike somewhat quieter even once it was up and running pulling hard. The quite dramatic increase in hesitation and reduction in power output was however totally unacceptable. I went looking for another Supertrapp muffler. The local motorcycle salvage yard did indeed have some old Supertrapp mufflers, but they were also in much worse condition than the two nice ones I already had on my other 1991 bikes. With parts from two old junk Supertrapp mufflers I was able to get a nice stack of 13 disks together on a muffler that was only moderately dented and rusty. Another custom mount made out of 0.1" thick mild steel and I had my favorite exhaust system up and running on my new 1991 WMX 610. It even looks pretty good, mostly just because I am getting better at custom fabrication of Husqvarna parts.
I was careful to start the project by running the bike around both with and without the spark arrestor and then I got the muffler installed for a test ride without having let the gasoline out of my sight. The only way to make a fair comparison these days it seems. What really surprised me was that the bike ran better with less hesitation and more power with the Supertrapp than it had with the Cobra Sparky spark arrestor. That screen type spark arrestor was a huge flow restriction! The Supertrapp did add some small noticeable additional hesitation compared to just the straight motocross muffler extension, but it was only over a range of medium engine speeds around 5,000RPM and it was very slight. Up on the top the bike was pulling really very nearly just as good as it had with the straight tail pipe extension. The other thing that surprised me was that the bike was a lot louder than my hot rod 610 motor with the same exhaust system. Part of this is probably that I used a stamped steel end cap on this Supertrapp for the stock motor, where my other Supertrapp mufflers have the later style two piece cast aluminum end cap. I think some of the noise is just going straight out the back right through the thin stamped steel end cap. In any case the Supertrapp muffler system does quiet the bike down dramatically with hardly any performance penalty.
With the USFS legal Supertrapp spark arrestor I headed out for some more substantial test rides. Often I was getting slow flame front travel speed gasoline that would not rev out, but the bike was running pretty good down bellow 5,000RPM. One day I got some dramatically cold burning gasoline and the engine was pulling pretty strong down to amazingly low engine speeds around 2,500RPM. On that same gasoline it was also revving out to 7,0000RPM but it was getting extremely harsh around 4,500 to 5,500RPM. Obviously it was hitting the earlier and easier to hit 5 degree ATDC time of late compression ignition at way too low of an engine speed around 4,500RPM which was causing very loud and harsh operation. The power output was pretty weak with the cold burning gasoline, but having torque down to 2,500RPM was useful for tight trails.
Something that sort of surprised me about the cold burning gasoline was that the engine became touchy and jumpy right at 3,000RPM. With the "Czech Republic" CDI ignition system advancing all the way up to 3,000RPM the engine would hesitate around 2,500 to 3,000RPM. Then once it hit 3,000RPM it would light off and jump forward. This is a problem I never had with either the stock 1991 SEM CDI ignition system or the points ignition system. On more normal hotter burning gasoline the engine seemed fine, it just got harsh and mostly unusable under a heavy load bellow 3,000RPM so the spark timing down there was not critical. Even when I did open the throttle for some very harsh torque bellow 3,000RPM I was not having any trouble with jumpy power delivery on the more normal hotter burning gasoline in the three inch stroke engine. When there is a dramatic shoulder to the advance curve, as is often the case on CDI ignition systems, that shoulder has to come down at an engine speed bellow where late compression ignition works well. If the shoulder comes up above where the engine is starting to make good torque in late compression ignition mode then it gets jumpy and hard to control right there at the shoulder in the advance curve. On normal gasoline 3,000RPM seems to be low enough on the three inch stroke engine, it is only on unusually cold burning gasoline that the three inch stroke engine gets jumpy at 3,000RPM. I see now why the SEM ignition systems have the shoulder of the advance curve so low. I had actually guessed as much, but running the "Czech Republic" CDI confirmed that an advance curve shoulder up at too high of an engine speed for the fuel and stroke length being used can cause severe lunging. Really the 2,000RPM shoulder of the SEM ignition system is irrationally low though. Even on the most unusual dramatically cold burning gasoline there appears to be no reason to place the shoulder bellow 2,500RPM, and on more normal gasoline 3,000RPM usually seems to work just fine. On the hottest burning race gas a shoulder up at 3,500RPM on the three inch stroke motor would probably work just fine.
Most automotive distributors from the 1950's through the 1990's had the shoulder at around 3,000 to 3,300RPM for the three and a half inch stroke length automotive engines. For a three and a half inch stroke length a 3,300RPM shoulder on the advance curve is probably too high for anything but the hottest burning race gas, but that is what they always used. Of course the location of the shoulder was not all that critical on poorly running automotive engines with 7:1 and 8.5:1 compression ratios because they did not enter late compression ignition much on normal gasoline. And running around 30 to 35 degrees BTDC those automotive engines just got supper harsh at all lower engine speeds anyway because they could not attain the latest possible time of late compression ignition with all that spark advance. It is sort of a case of the advance curves having been wrong on automotive engines, but that was the least of the problems when they were running way too much spark advance and way too low of compression ratios. On the 10.2:1 Husqvarna 610 motor running 26 degrees BTDC the latest possible time of late compression ignition is attainable so the location of the shoulder of the advance curve is important. That 26 degree BTDC spark timing is pretty early, but on somewhat slow flame front travel speed gasoline it does work. It is only on the race gas that was coming out of the pumps all the time for many years that 26 degrees BTDC is way too much spark advance. On any gasoline 20 or 23 degrees BTDC is better, but slightly slow flame front travel speed gasoline and/or slightly cold burning gasoline allows 25 and even 27 degrees BTDC to seem like it works acceptably well. Going up to 30 and 33 degrees BTDC at 3,000 to 5,000RPM though just never works on any sort of gasoline, the latest possible time of late compression ignition just can't be hit at all and the engine is very harsh and severely lacks torque at those lower engine speeds.
The fuel consumption was also pretty high on the stock motor with the Czech Republic ignition system. The motor seemed to run well in full flame front travel mode down at 2,300 to 3,000RPM under light cruising loads, but it was just an illusion of running well. When I rode the bike a lot at low engine speeds in full flame front travel mode the fuel consumption remained up at around 0.70GPH. With the points ignition on the hot rod 12.5:1 610 motor I often see fuel consumption rates as low as 0.63GPH even when riding faster on long mixed rides with some substantial higher speed 45 to 50mph cruising in sixth gear. Full flame front travel mode combustion at 2,300 to 3,000RPM sounds good on the three inch stroke length engine, but efficiency is actually much better up at 3,000 to 4,000RPM in late compression ignition mode going faster and using more power.
With a smooth linear advance curve advancing 15 degrees from 1,500 to 3,000RPM the Czech Republic CDI ignition system tends to provide very smooth operation down bellow 3,000RPM in full flame front travel mode. The points ignition system on the other hand gets harsher and harsher bellow 3,000RPM in full flame front travel mode. Even when running just 21 degrees BTDC on the static timing setting with fast flame front travel speed gasoline full flame front travel mode operation bellow 3,000RPM tends to sound very harsh and loud. Part of this is that the points ignition system actually often provides more spark advance at 2,300RPM. With the Czech Republic CDI at 26 degrees at 3,000RPM and 11 degrees at low idle the spark timing is perhaps about 20 degrees BTDC at 2,300RPM. If the points ignition system is set at a static timing setting of 23 degrees BTDC it is providing more spark timing everywhere bellow about 2,500RPM. The reality though is somewhat different. I never run the points ignition bikes much bellow 2,500RPM, and usually the engine speed stays above 3,000RPM pretty much all the time. The points ignition 12.2:1 hot rod motor is in practice always running considerably less spark advance than the 10.2:1 stock motor with the Czech Republic CDI.
The lower fuel consumption of the 12.2:1 hot rod motor comes from three different things. One is that I always tend to set the spark timing at the ideal value for each ride. If the bike will run stronger and use less fuel with the spark timing advanced another two or three degrees I stop and advance the spark timing. On the hot rod 12.2:1 610 motor I have usually been running between 20 and 24 degrees BTDC with good results, but there have been days when it required 26 or even 27 degrees BTDC on slow flame front travel speed gasoline. I have also been all the way down to 14 degrees BTDC on the static timing setting with the 12.2:1 hot rod 610 motor, which on weak cold burning and slow flame front travel speed gasoline is an exceedingly late fixed spark timing value for a four inch bore engine. The 0.63GPH fuel consumption seems to come most easily when the engine will run without undue hesitation at static timing settings of 20 to 23 degrees BTDC. Up at 26 and 28 degrees BTDC I have also gotten this low 0.63GPH fuel consumption, but it is trickier requiring both more precise spark timing settings and more precise riding, generally keeping the engine speed up slightly higher over a narrower range of engine speeds with more shifting. The easiest fixed spark timing value to run is about 20 to 24 degrees BTDC. More spark advance than this is very tricky to get to work at all well.
One thing I have been noticing though is that 50 to 60mph cruising tends to use less fuel at 26 degrees BTDC than at 20 degrees BTDC. The 577cc engine is so big that it usually just won't cruise at a steady speed in late compression ignition mode. When I can get the big 577cc engine to cruise in late compression ignition mode it tends to be up at more than 55 or 60mph so more spark advance tends to be better for normal 50 to 60mph steady cruising in full flame front travel mode. At least on somewhat cold burning and somewhat slow flame front travel speed gasoline more spark advance is better up at higher cruising speeds in full flame front travel mode. On the hotter burning and rather fast flame front travel speed gasoline that was always coming out of the pumps for many years the Husqvarna 610 tended to be louder, harsher and less efficient with more spark advance in full flame front travel mode all the way up to 3,500 and even 4,000RPM.
On the CDI ignition system the spark timing just is what it is, and this usually means more hesitation and higher fuel consumption. Then there is the fact that the gasoline had nearly always been for much higher compression ratio engines throughout 2014 and 2015. The 10.2:1 motor would run stronger with 30 degree BTDC spark timing on the higher pressure and/or slower flame front travel speed gasoline, but I know it gets way to harsh everywhere from 3,000 to 5,000RPM with that much spark advance so I just won't set the CDI ignition up there. The 26 degrees BTDC at 3,000 to 4,000RPM is already marginally too much spark advance for a fixed advance curve ignition system, and only just barely works. The third factor in fuel consumption is the compression ratio itself. On the same gasoline a 12.2:1 engine simply is able to attain higher thermodynamic efficiency than a 10.2:1 engine can. The result is that the points ignition 12.2:1 hot rod motor goes noticeably much faster on somewhat less fuel.
By summer time I was having a lot of trouble with highly variable energy density gasoline. I started smelling a methanol like smell from the exhaust on all of the bikes sometimes. I had actually first noticed this acrid methanol like exhaust smell on the 386 stroker motor sometimes back in late spring of 2015, but it was always from old gasoline that had been sitting around unattended for days or weeks. For an entire year I never noticed this acrid exhaust smell from gasoline straight from the gas station. Then all of a sudden in August of 2016 the gasoline right from the gas stations was producing this acrid alcohol exhaust smell that I for some reason associate with methanol. Ethanol does not seem cause quite the same exhaust smell. Back in the first decade of the 20th century I had often added 10% ethanol to gasoline just to see what would happen, and there was not much change. I even tried 20% ethanol a few times, but this did produce some slight noticeable change. I determined that the stock jetting on the 40mm DellOrto on the 1991 WMX 610 was sufficiently rich that 10% ethanol produced essentially no noticeable change, even when running the leaner 2nd needle clip possition instead of the stock 3rd needle clip possition. I also determined that 20% ethanol was too much, as even on the rather rich jetted carburetor I did notice that the engine then felt a bit weak. It was not a huge difference even with 20% ethanol, but there was a difference. It just seemed a bit weaker, and I never noticed any unusual exhaust smells. I ran 10% ethanol probably a few dozen times, but I only tried 20% ethanol twice I think back around 2002 and 2003.
When the extremely strong acrid exhaust smell started showing up right from the gas station in the summer of 2016 I did some more ethanol testing just to make sure I knew what was going on. I found that I could get the 12.2:1 hot rod 610 motor to start and idle on 100% ethanol with the choke on. The choke on the 40mm DellOrto carburetors is not actually a choke at all, but an additional starting circuit that allows more air and a richer mixture to bypass the slide when the lever is pulled. What I did to run 100% ethanol was to drain gasoline from the tank, and put a quart of Clean Strip brand ethanol in the tank. I then started the bike on the gasoline in the carburetor bowel with the petcock closed. Only once the engine had warmed up after a half mile did I open the petcock to let the ethanol in. With jetting for gasoline the engine would not make power at all on the 100% ethanol, but it would idle and run at very small throttle openings with the choke on. With the choke off it just stalled right away. The engine did start on 100% ethanol once warmed up, but not always with the kick starter. Often I had to roll start it even once hot.
I then changed the needle clip position to the richest, 4th groove, position. This got the engine to actually sort of run on the 100% ethanol. Not only would it idle with the choke on, but I could actually open the throttle up about 1/3 and get some power. It would run at all smaller throttle openings around 1/5 to 1/2 even with the choke off, but as soon as I closed the throttle with the choke off it just stalled right away. I did not bother to change the main jet, and this meant that the engine would not run at all with the throttle opened up past about halfway. The engine ran very similarly on 100% ethanol, but there were of course some significant differences. One of course was that power output was low. It felt very lean, and it probably was still far too lean for 100% ethanol. Another thing I noticed was that the temperature of combustion potential of the ethanol was very low. It was seeming to require earlier times of late compression ignition all the way down to around 3,500RPM, and advancing the spark timing more delivered more power up at 4,000 to 5,000RPM. Running 27 degrees BTDC in the 12.2:1 98mm bore engine seemed to work fairly well, although the mixture was really far too lean.
Running 100% ethanol there was a noticeable alcohol smell to the exhaust, but it was not the same as the strong acrid methanol smell that had been caused by the gasoline coming out of the pumps. The Clean Strip brand ethanol is called "denatured alcohol", so it does have some unspecified small amount of methanol in it. I don't think the exhaust smell from the 100% ethanol was caused by that small bit of methanol though. The smell from burning ethanol is different. It is not as acrid or strong smelling, it is a milder sort of smell that is hardly noticeable.
Not long after I did the 100% ethanol test the gasoline from gas stations stopped having the acrid methanol exhaust smell. It was only a few months that the heavily methanol laced gasoline was coming out of the pumps, but the weak watered down low energy density nature of the gasoline remained. I started noticing some of that milder ethanol smell from the exhaust, and it was seeming like the gasoline was about 30% ethanol with 70% gasoline.
The energy density of the gasoline was swinging around so much that I never knew what sort of crap was going to be in the tank when I took off to go for a ride. And even buying gasoline at the beginning of a ride did not help much. I decided to go back to the leanest needle clip position that had been working well in 2015. This did help, as the gasoline supply then became somewhat more consistent. First I changed the needle clip position back to the first groove on the 12.2:1 hot rod 610 motor, and performance was about the same most of the time. Then I took the top off of the 40mm DellOrto on the stock 10.2:1 WMX 610 motor, and I found that the needle clip was in the stock 3rd groove position. I went all the way to the first groove, and the bike ran much better. What was really amazing was that the Czech Republic CDI ignition system seemed to work better with the much leaner needle clip position. The surging around 3,000RPM was less noticeable, and power delivery was smoother and stronger across all normal engine speeds from 2,500 to 6,000RPM. The old problem of needing to twist the throttle up to 3/4 sometimes to get the engine to run at 5,000 and 6,000RPM was back, but most of the time the engine was running fairly well at small throttle openings on the lean mixture at 2,500 to 4,000RPM. It seems that the 40mm DellOrto with the stock K32 needle is not actually all that lean even with the needle clip in the leanest position.
When I first got the stock 10.2:1 motor running with the Czech Republic CDI ignition it was usually not quite crisp enough with the 26 degree BTDC spark timing. It needed those few more degrees up at 28 and 29 degrees BTDC to pop off reliably with a minimum of lag. I just refused to tolerate the excess harshness at 3,000 to 4,000RPM though, so I "locked down" the spark timing at what I felt was the earliest spark timing that had any chance of working on the big 610 motor. At first there was often a lot of lag with the spark timing at 26 degrees BTDC, but the engine was running and making torque from 3,000 to 7,000RPM. At least down at 1,000 feet of elevation. Then lower pressure gasoline started showing up in the tank of the Czech Republic CDI equipped WMX 610. Much lower pressure gasoline. Such low pressure gasoline that there were just huge amounts of excess crispness from the 10.2:1 motor with 26 degree BTDC spark timing. With lower pressure gasoline the stock engine was however reliably able to rev out to 8,000 and 8,500RPM. Power was not spectacular up there with the heavy stock piston and rather weak gasoline, but it was able to rev out. After I switched from the stock 3rd needle clip position to 1st needle clip position the engine ran much better, being able to rev out to 8,500 and even 9,000RPM without so much excess crispness around 4,000 to 6,000RPM.
The like new totally stock 1991 Husqvarna WMX 610 has been fun to work on and ride. It is interesting to see that the 1991 WMX 610 is really a very capable dirt bike in stock form. The Czech Republic CDI ignition upgrade is great in that the bike starts easily and does not stall, that's great. The advance shoulder being up at 3,000RPM has however been a bit of a problem sometimes on the unusually cold burning gasoline. The surging around 3,000RPM was worse with the needle clip in the stock 3rd groove position, but even with the mixture leaned out there is still a noticeable problem with the shoulder of the advance curve being up at too high of an engine speed for the cold burning gasoline that has been common lately. This ignition system really needs fairly hot burning gasoline to work well, and it does not work well with slow flame front travel speed gasoline either. Well, it does actually work fairly well with slow flame front travel speed gasoline, but the engine then won't rev out much. The slow flame front travel speed gasoline works fine for many types of trail riding, but totally loosing the 5,500 to 7,500RPM range of engine speeds is disappointing. What it comes down to is that the Czech Republic CDI ignition system on the bone stock 1991 WMX 610 requires high quality premium gasoline. Not necessarily the hottest and fastest race gas, but certainly some pretty powerful gasoline. And even with rather powerful gasoline the stock motor with the big heavy 395g or 406g Mahle piston gets pretty weak up above about 6,500RPM.
I knew my other "barn find" 1991 WMX 610 could be built to do better. I had been debating back and forth many ideas of how to build this old Junker of a 1991 WMX 610 motor. Since weak rather cold burning gasoline has been available sometimes lately I have been trying to come up with some way to make use of it. Obviously what the colder burning gasoline really needs is a shorter stroke length. With a shorter stroke length the colder burning gasoline would still be able to rev out well and make good strong power to 8,000RPM. On all of my old Husqvarnas I am pretty well stuck with the three inch stroke length though. The motors can be de-stroked, but that is a crappy sort of way to build a motor. Using the same connecting rod for a smaller displacement motor just makes it worse. The whole motor, transmission and clutch is sized for the 577cc displacement so going smaller on the displacement reduces efficiency and makes the bike seem excessively heavy. The 577cc displacement is too big for a dirt bike, but using less displacement on the same platform just makes everything worse.
Instead of using the shorter stroke length that colder burning gasoline would tend to require the three inch stroke length Husqvarna 610 motor can just be optimized for best performance across the lower to midrange engine speeds from 3,000 to 5,000RPM to do as well as it possibly can on the colder burning gasoline. What this means is sticking with the stock 242 degree at 1mm valve lift 1991 camshaft.
For the low engine speed oriented build the stock SEM ignition system would be great. With the shoulder of the advance curve down at 2,000RPM there is never any kind of problem with excess surging or jumpiness at any engine speeds from 2,500 to 3,000RPM, even on the weakest coldest burning gasoline. The 2,000RPM engine speed is always way down below where late compression ignition will work well on any kind of gasoline. Just keeping the shoulder of the advance curve down to 2,500RPM would be good enough for even the coldest burning weak gasoline in the three inch stroke length engine, but the SEM ignition with the shoulder down at 2,000RPM would also work. The weakest coldest burning gasoline also favors a rather large amount of spark advance up at around 25 to 27 degrees BTDC in the three inch stroke length engine, where hotter burning gasoline does much better with spark timing around 18 to 22 degrees BTDC. The reason that the weak cold burning gasoline does well with a large amount of spark advance in the three inch stroke length engine is that the earlier and easier to hit 5 degree ATDC time of late compression is required to make power up at only slightly higher engine speeds. Keeping the spark timing up at 25 to 27 degrees BTDC means that the engine very easily falls over to the earlier and easier to hit 5 degree ATDC time of late compression ignition with a bit of a twist of the throttle.
The problem of course is that earlier times of late compression ignition really don't work well at low engine speeds. The three inch stroke length results in such high mean piston speeds at elevated engine speeds that the cold burning gasoline needs to be running at the earlier 5 degree ATDC time of late compression ignition to make power. On the most dramatically cold burning gasoline the earlier and easier to hit 5 degree ATDC time of late compression ignition can seem to be required down to even lower engine speeds around 4,000RPM or even 3,500RPM, but this is far too low of an engine speed for that earlier 5 degree ATDC time of late compression ignition. It just does not work well. Peak cylinder pressures are so high that engine wear is accelerated and both efficiency and torque generation suffer.
A better strategy is to use the narrow range of engine speeds where good torque can be generated at the latest possible time of late compression ignition. For moderately cold burning gasoline in the three inch stroke length engine this is 3,000 to 4,000RPM. For the extremely unusually cold burning gasoline this is about 2,700 to 3,500RPM in the three inch stroke engine. To get as much torque as possible down at those low engine speeds earlier intake valve closing times are required. The 242 degree at 1mm valve lift 1991 camshaft installed straight up with split overlap at top dead center is pretty good at delivering good cylinder filling down to 3,000RPM. These lowest engine speeds can however attain even higher peak cylinder filling with even earlier intake valve closing times. The 242 degree at 1mm valve lift camshaft could also be installed advanced a few degrees. Advancing the camshaft by four degrees of crankshaft rotation would tend to lower the torque curve by about 300RPM. For the colder burning gasoline this would result in considerably more torque generation down at 3,000 to 3,500RPM.
Then there is the compression ratio question. With the Woessner piston the compression ratio is 11.0:1 with the stock gaskets, but other compression ratios can also be used. There always seems to be room to fit about a 0.020" thicker base gasket to lower the compression ratio back down to close to the stock 10.2:1 compression ratio. Increasing the base gasket thickness by 0.020" would actually result in a 10.4:1 compression ratio, a very slight increase over the stock 10.2:1 compression ratio. Going the other way a 0.030" thinner base gasket can be used to increase the compression ratio with the Woessner piston all the way up to 12.2:1. And any compression ratio in between is also possible with different base gasket thicknesses. Using the stock Mahle piston the compression ratio could be made even lower than the stock 10.2:1 compression ratio. Increasing the base gasket thickness by 0.020" with the stock Mahle piston would result in a 9.7:1 compression ratio. Quite a lot of material could also be removed from the top of the crown on the stock Mahle piston to get an even lower compression ratio. Removing 0.030" from the top of the crown on the stock Mahle piston is no kind of a problem at all, there is way plenty of material thickness on that quarter inch thick crown to take 0.030" off without the slightest bit of structural or thermal problems. Taking 0.030" off the crown of the stock Mahle piston along with using a 0.020" thicker base gasket would result in a 9.0:1 compression ratio. Obviously this motor can easily go down to very low compression ratios.
The weakest spectacularly cold burning gasoline I got a few times in early 2016 also worked in very low compression ratio engines, so it would be possible to use a very low compression ratio down at 10:1 or even 9:1. Most of the gasoline had however for some time remained quite high pressure stuff usually requiring 20 to 24 degrees BTDC or even 26 degrees BTDC in the 12.2:1 hot rod 610 motor. With the 11.0:1 610 motor back in the summer and fall of 2015 I often had to run static timing settings up at 28 and even 30 degrees BTDC. The stock 10.2:1 610 motor had been all over the place in 2015, sometimes I was able to run as little as 21 degrees BTDC, but during the summer of 2015 it was up at around 29 to 31 degrees BTDC most of the time. Then recently both the stock 10.2:1 610 motor with the points ignition in the 1992 chassis and my new 1991 WMX 610 with the Czech Republic CDI ignition have been running very crisply with 26 degree BTDC spark timing. The stock 10.2:1 610 motor with the points ignition does however rev out much farther and make much more power with the 368g lightened Mahle piston and the extra spark advance above 6,500RPM from the crankshaft wiggle advancing effect. That old stock 10.2:1 motor with the points ignition system works pretty well, the main problem was that the low 10.2:1 compression ratio had often been requiring so much spark advance that the engine got hugely harsh at less than about 5,000RPM.
The points ignition motors have also been kicking back again with static timing settings around 27 degrees BTDC. There for quite a while in 2014 and through most of 2015 the gasoline was never kicking back at any static timing settings from 15 degrees BTDC to 35 degrees BTDC. Then all of a sudden all the bike started kicking back again. With the hot rod 610 motor with the centrifical de-compressor on the 1994 camshaft kicking back is hardly even a problem on any gasoline at any static timing setting. That hot rod motor has kicked back violently a few times with 27 degrees BTDC on the static timing setting, but what I figured out was that it only kicks back if I give it a huge kick. With moderate kicks the centrifical de-compressor prevents the motor from kicking back at all static timing settings, and this is much appreciated. The 386 stroker motor never kicked back at any static timing setting from 15 degrees BTDC to 33 degrees BTDC from February 2015 to about October of 2015. Then all of a sudden it started kicking back all the time with 27 degrees BTDC on the static timing setting. The smaller displacement motor does not kick back as violently, but it is still not nice at all to have the motor try to rip the soles off of your boots. The old stock 10.2:1 motor in the 1992 chassis still has never once kicked back on me, but I have been roll starting it each time ever since the other two points ignition bikes once again started kicking back on me.
All of this was relevant to the build plan for the 1991 WMX 610 I had spread out in pieces mostly ready to go back together. The mostly much higher pressure gasoline tends to need much higher compression ratios. I had almost never felt that the 12.2:1 motor had too high of a compression ratio, and many days when I have had to run 27 degrees BTDC on the static timing setting it has seemed like it actually needs an even higher compression ratio. With the gasoline kicking back again at static timing settings earlier than about 24 degrees BTDC it also seemed to make sense to use a rather high compression ratio so that those large amounts of spark advance would not be not required. A 1994 or later Husqvarna 610 camshaft with the centrifical de-compressor would be a good addition to the build project in terms of preventing kicking back, but those later camshafts also have a longer 250 degree at 1mm valve lift duration that is not as good for advancing the cam timing to go after more torque out of cold burning gasoline down at around 3,000 to 3,500RPM.
Even though the colder burning gasoline tends to favor 25 to 27 degree BTDC spark timing for making power up at 4,500 to 5,500RPM it still makes sense to try to keep the spark timing down to around 20 to 23 degrees BTDC. Torque production down at 3,000 to 3,500RPM is both more reliable and somewhat smoother and stronger with later than 23 degree BTDC spark timing, even on rather weak cold burning gasoline in the three inch stroke length engine.
What all this adds up to is that a 610 motor build for low end torque and the ability to run as efficiently as possible on weak cold burning gasoline can still use a fairly high compression ratio with a points ignition system. If spectacularly weak cold burning low pressure gasoline is encountered then the spark timing can be backed off. As long as the flame front travel speed of the cold burning gasoline remains reasonably high, as it usually does, then static timing settings as low as about 15 degrees BTDC can be used on the 610 motor. With the stock SEM ignition system it is not really possible to use spark timing values less than 25 degrees BTDC, and even at that running spark timing of 25 degrees BTDC results in low idling spark timing of 5 degrees BTDC which is really too late for best starting and idling performance. The four inch bore engine usually does much better with low idling spark timing up in the 10 to 15 degree BTDC range.
Raising the compression ratio up to 11.0:1 with the Woessner piston is easy, just drop the new piston into the motor in place of the old 10.2:1 Mahle piston. Going higher than 11.0:1 though requires lowering the cylinder head, and this changes the camshaft timing. On the hot rod 610 motor going up to 12.2:1 on the compression ratio with a 0.030" thinner base gasket changed the camshaft timing on the 250 degree at 1mm valve lift 1994 camshaft from advanced 3.5 degrees of crankshaft rotation to being installed straight up with split overlap at top dead center. This makes the 250 degree camshaft seem very large, perhaps too big for the 610 motor. The bigger camshaft does indeed result in more power production up at 6,000 to 8,000RPM, but it also cuts torque down at 3,000 to 3,500RPM. All the way up to about 4,500RPM the bigger camshaft results in noticeably lower cylinder filling. The stock 242 degree at 1mm valve lift 1991 camshaft or the 250 degree at 1mm valve lift 1994 camshaft installed advanced 3.5 degrees of crankshaft rotation yield broader and smoother power. The 250 degree at 1mm valve lift camshaft installed straight up results in a more peaky power around 6,000 to 8,000RPM often with a more abrupt fall off of power from 8,200 to 8,500RPM when running slower flame front travel speed gasoline. The big camshaft does quite well on somewhat slower flame front travel speed gasoline as it tends to prevent excessive harshness down at 3,000 to 3,700RPM. When running slower flame front travel speed gasoline, even if it is running well and making good power, the power tends to end rather abruptly at about 8,000 or 8,200RPM.
The camshaft timing can however be set to whatever value is desired. It is just a matter of elongating the cam sprocket mounting holes and installing a new locating pin. The sprocket centers on the camshaft, so the bolts are only required to clamp the sprocket down and don't need to do any locating themselves.
Particularly on the 1991 camshafts it is very easy to change the cam timing. On the 1994 camshaft I had a bit more trouble with changing the cam timing. And I did put the 12.2:1 hot rod 610 motor back to stock cam timing not long after raising the compression ratio to 12.2:1.
The idea I had come up with was to use a 0.020" thinner base gasket for an 11.7:1 compression ratio, but to re-position the camshaft sprocket so that the 242 degree at 1mm valve lift camshaft would be installed advanced about one to two degrees of crankshaft rotation. I could go even farther to advance the 242 degree camshaft a few degrees more for even higher cylinder filling down at 3,000RPM, but that would likely only be better on the most dramatically colder burning gasoline that has hardly ever been available. The 242 degree at 1mm valve lift camshaft installed straight up with split overlap right at top dead center does pretty well all the way down to 3,000RPM even if the cylinder filling is dropping off somewhat everywhere bellow about 4,000RPM.
The last big question has to do with the piston weight. It seems sort of ridiculous to consider increasing the weight of the piston when it is not required for strength or heat dissipation, but the weight of the piston is a critical part of the points ignition system. Especially on either slow flame front travel speed gasoline or weak cold burning gasoline getting the engine speed where wide open throttle crankshaft wiggle advance comes on down to less than 6,500RPM is highly beneficial. With the wide open throttle crankshaft wiggle advance coming up at 7,000RPM slow flame front travel speed gasoline sometimes causes cutting out, stumbling and severe hesitation from 6,500 to 7,000RPM.
The stock 369g weight of the Woessner piston is just right for running slow flame front travel speed gasoline or weak cold burning gasoline with the points ignition system. And really even on more normal gasoline getting the wide open throttle crankshaft wiggle advance to come down at 6,500RPM instead of 7,000RPM probably is better. At least the lower 6,300 or 6,500RPM wide open throttle crankshaft wiggle advance is good for reliably getting the engine to easily rev out past 7,000RPM when the engine is not running quite crisply enough.
For the slowest flame front travel speed gasoline or the weakest coldest burning gasoline the 5,500 or 5,800RPM wide open throttle crankshaft wiggle advance provided by the stock 406g Mahle piston would actually be best. For most gasoline though that is way too low of an engine speed for the wide open throttle crankshaft wiggle advance and it just causes extreme harshness and even a modest reduction in torque generation.
The main determinant of the engine speed where the wide open throttle crankshaft wiggle advance comes on is the weight of the piston and connecting rod, but the compression ratio also plays a role. A higher compression ratio increases the downward force on the piston and causes the wide open throttle crankshaft wiggle advance to come at a higher engine speed. From the perspective of the points ignition system a lower compression ratio would be better because it allows a lighter piston weight with a lower engine speed where the wide open throttle crankshaft wiggle advance occurs. The compression ratio is actually much less significant than the weight of the piston though. Increasing the compression ratio from 10.2:1 to 12.2:1 causes a small increase in the engine speed where the full throttle crankshaft wiggle advance comes on, perhaps just 200 or 300RPM. Lightening the piston weight from the stock 406g down to 332g on the other hand increases the engine speed where the full throttle crankshaft wiggle advance comes by a dramatic 1,200 or 1,300RPM.
The irony and difficulty in choosing compression ratios and piston weights is that both higher compression ratios and lower piston weights dramatically increase power output on the same gasoline. How much of an increase in power and efficiency is seen with higher compression ratios of course depends on what type of gasoline is being used. On high pressure gasoline that will hardly pop off on late compression ignition in a 10:1 engine even with large amounts of spark advance going up to a 12:1 compression ratio makes a huge difference in torque production, power output and efficiency. If on the other hand the gasoline is working in the 10:1 engine with reasonable amounts of spark advance then going up to a higher compression ratio will result in only modest increases in power and efficiency. Lighter pistons always result in dramatic increases in power and efficiency regardless of compression ratio, amount of spark advance or what type of gasoline is used.
What I have come up with on the torquer 1991 WMX 610 build is that a heavier piston is just not worth considering. It is too ridiculous to consider increasing the weight of the piston just to get the crankshaft wiggle advance to come at a lower engine speed. Even for an engine setup that is oriented towards best possible torque production at low engine speeds around 3,000 to 5,000RPM a lighter reciprocating assembly is still a large advantage. The lighter reciprocating assembly dramatically increases light load efficiency at moderate engine speeds around 4,500 to 5,500RPM so even if the engine is not normally revved out much a lighter piston is still going make the engine feel more responsive and use less gasoline.
The other reason that a lighter piston seems like a good idea even for the torquer engine setup is that a four valve per cylinder Husqvarna thumper is bound to rev out quite a bit even when it is setup to do as well as it can down at 3,000 to 5,000RPM. On anything but the slowest flame front travel speed gasoline the Husqvarna 610 motor with the stock 242 degree at 1mm valve lift 1991 camshaft installed just one or two degrees advanced is going to very easily twist 6,000 and 7,000RPM. Up at 7,000RPM the lighter piston is a significant advantage and it frees up quite a bit of power.
What the Husqvarna 610 motor really needs to work as well as it could at lower engine speeds would be a longer intake stack length to give some intake stack boost down to about 6,000 or 6,500RPM. It would be a pretty good setup to get the intake stack boost first at 6,000RPM, and then get the crankshaft wiggle advance up at 7,000RPM. That is however not possible, the longer intake stack just does not fit in the chassis. That big frame tube is right in the way. As it is the short seven inch total intake stack length results in the intake stack boost coming up at 7,500RPM and above, which is marginally too high for a three inch stroke engine. On hot burning gasoline the three inch stroke engine has no trouble revving past 8,000RPM to make good use of the intake stack boost at 7,500RPM and above. On colder burning gasoline though the 7,500RPM engine speed on the three inch stroke engine is just a whole lot of mean piston speed. The Husqvarna 610 motor can be made to rev up to 7,800 or even 8,000RPM on the coldest burning weakest gasoline known to man, but there is not much power up there above 7,000RPM and the large amount of spark advance required to get the cold burning gasoline to hit 8,000RPM causes a dramatic reduction in torque at 5,000 to 6,500RPM. Intake stack boost down at 6,000RPM would be much better for most types of gasoline.
The way to look at the torquer Husqvarna 610 engine build is that it is oriented towards best possible torque production down at 3,000 to 4,000RPM, but the crankshaft wiggle advance and intake stack boost will still be up there at the top of the engine speed range. If the engine happens to make it up to 7,000RPM then it gets the crankshaft wiggle advance and the intake stack boost at about 7,600RPM to carry on and make more power up to 8,000 or 8,500RPM. On race gas the torquer Husqvarna 610 motor will pull strong everywhere from just over 3,000RPM up to 8,500RPM and will have overrev to about 10,000RPM.
On race gas the hot rod 610 motor with the big 250 degree at 1mm valve lift camshaft installed straight up with split overlap at top dead center will make more power at 6,500 to 8,500RPM than the torquer 610 motor. The hot rod 610 motor will actually make more peak power than the torque 610 motor on any type of gasoline as long as both motors are running the same gasoline. What the torquer motor will deliver on race gas is a broader and smoother power curve. The torquer motor will deliver more torque down to 3,700 and even 3,300RPM and the power will build more gradually up to peak power output at 7,500 to 8,500RPM than the big cam 610 motor when running the same race gas.
The irony is that the torquer motor runs better with higher peak efficiency on really any type of gasoline, but the big cam hot rod motor does make more power. Especially on slow flame front travel speed gasoline the big cam hot rod 610 motor makes a whole lot more power than the torquer 610 motor and also runs over a much broader range of engine speeds.
What it comes down to is that with the gasoline supply changing dramatically and frequently the only solution is to have both the big cam hot rod 610 motor and the torquer 610 motor. The torquer 610 motor would usually be better, but when it won't make power and won't run over a wide enough range of engine speeds on slow flame front travel speed gasoline then the big cam hot rod 610 motor is required to take up the slack. The big came hot rod 610 motor is also useful just for the shear thrill of laying down more power on the same gasoline. Whatever type of gasoline it happens to be the hot rod 610 motor will make more power up to higher engine speeds.
In a way it would seem better to put the new torquer 610 motor in my street legal 1991 WMX 610 with the six speed transmission and put the big cam hot rod motor in the other 1991 WMX 610 chassis with the five speed transmission for racing. That sort of makes sense, but I am actually going to do it the other way. The big cam hot rod 610 motor was developed exclusively on pump gas and has always been maintained primarily for use on public roads and highways. The big cam hot rod 610 motor was not something that I designed and then built, it is just something that happened. The big 250 degree camshaft was the result of my wanting to use the centrifical de-compressor to make starting easier. The higher 11.0:1 compression ratio was the result of lots of very high pressure gasoline coming out of the pumps all the time which just would not work in the stock 10.2:1 motor. The later higher 12.2:1 compression ratio and installation of the big 250 degree camshaft straight up with split overlap at top dead center was the result of slow flame front travel speed gasoline coming out of the pumps all the time. The later intake valve closing time was required to get a broad range of engine speeds out of the slow flame front travel pump gas that was universally available for a period of time in June and July of 2015 and that has been sporadically available sometimes since then. The big cam hot rod 610 motor is an extreme engine setup that I would have stayed away from were it not for the pump gas changing to require that extreme setup. I knew better than to go above about a 10.5:1 compression ratio, but the high pressure gasoline that was always available demanded a higher compression ratio. I also knew better than to make the intake valve closing time later, but the slow flame front travel speed gasoline that suddenly became quite common demanded a later intake valve closing time to work with a fixed advance curve. The big cam hot rod 610 motor is a street motor simply because it was developed on pump gas to be run on the street.
The torquer 610 motor will work better for delivering a smooth broad torque curve well suited to trail riding and most types of racing. The 577cc 610 motor is not lacking in power, so the 242 degree at 1mm valve lift camshaft installed a degree or two advanced will make enough power. Especially on race gas the torquer 610 motor will make plenty of power. Aside from having a bike to ride the main utility of the torquer 610 motor will be in demonstrating high efficiency on unusually weak cold burning gasoline. With the earlier intake valve closing time and a high compression ratio good torque and efficiency can be obtained down at around 3,000 to 3,500RPM even on spectacularly cold burning gasoline. It is not that the torquer 610 motor will able to do all that well on spectacularly cold burning gasoline. No, the spectacularly cold burning gasoline needs a much shorter stroke length to attain high efficiency and good performance. What the torquer 610 motor will do is demonstrate how well a three inch stroke engine is able to do on the spectacularly cold burning gasoline.
That all sounded great, but I did chose to set the cam timing on the hot rod 610 motor back to stock. I just got tired of the abrupt two-stroke like hit of the power at 5,000RPM all the time.
I also adjusted the cam timing on the 386 stroker motor to get it to attain higher cylinder filling down at 3,500 to 4,500RPM. As it was the 245 degree at 1mm valve lift camshaft installed three degrees retarded was causing the engine to only attain high cylinder filling up at 5,000RPM and above. That would be fine for getting power out of the slightly shorter stroke length small engine, but that engine has turned out to not be about big power production. With the very low 9.7:1 compression ratio all that the 386 stroker motor was doing was to demonstrate that the gasoline was mostly for much higher compression ratio engines. When I occasionally got low pressure gasoline for the 386 stroker motor it had often also been extremely cold burning gasoline that just did not want to make power above about 6,000RPM even on the shorter 2.68 inch stroke length of the 386 stroker motor. On that dramatically colder burning gasoline even with the shorter 2.68 inch stroke length the good torque and high efficiency would be down in the 3,000 to 4,000RPM range of engine speeds, and the later intake valve closing time just was not delivering high enough cylinder filling to do really well down there at those low engine speeds. I expected 386 stroker motor to be all around worse with the camshaft advanced two or three degrees of crankshaft rotation, but I chose to advance it anyway to deliver good torque and best possible efficiency on the spectacularly weak cold burning gasoline that had been becoming more and more common. The 386 stroker motor in the same cases and on the same transmission as the 610 motor is not so much about big power output, it is about running better and using less gasoline. What the 386 stroker motor really needs to deliver high efficiency and low fuel consumption is a higher compression ratio. With the only practical means of increasing the compression ratio being to increase the stroke length though it is sort of stuck with the 9.7:1 compression ratio.
I keep thinking that 50 years of 8.5:1 and 9.5:1 automotive engines and 80 years of 9.5:1 and 10.5:1 motorcycle engines probably indicates that normal gasoline is supposed to be for roughly these compression ratios. What I was seeing though was mostly gasoline for about 12:1 to 14:1 engines. There is a large discrepancy here, but I figured that for the time being I wold ride a bike with a 12.2:1 compression ratio and keep the 9.7:1 386 stroker motor in the garage in case lower pressure gasoline should become widely available.
The very low 9.7:1 compression ratio on the 386 stroker motor is good for one thing, and that is running similar amounts of spark advance to the stock 10.2:1 Husqvarna 610 motor. When both the Husqvarna 350 and the Husqvarna 610 motors have the stock 10.2:1 compression ratio the smaller bore 350 motor needs considerably less spark advance. With the compression ratio of the small bore motor down to 9.7:1 the motors are better able to run the same amount of spark advance. Running the same amount of spark advance is not really necessary, as the small bore motor runs very well with less spark advance. There does however seem to be a tendency for people to want to run the same amount of spark advance on different bore diameters. The stock Husqvarna specifications called for exactly the same spark timing setting for both the 350 motor and the 610 motor, which is sort of insane considering that they also had the exact same 10.2:1 compression ratio. Of course the whole idea of 33 degree BTDC full load spark timing is rather insane itself.
Lowering the compression ratio of the small bore motor to 9.7:1 actually does not provide enough of a compression ratio difference to be able to run the exact same spark timing as the stock 10.2:1 Husqvarna 610 motor. That half a point compression ratio difference does substantially reduce the difference in spark timing required, but to actually run the exact same spark timing the small bore motor would have to go down to an even lower compression ratio or the 610 motor would have to go up to an even higher compression ratio. How much of a compression ratio difference would be required?. Not usually as much as the 610 motor running 11.0:1 and the small bore motor running 9.7:1, that is quite a bit of a compression ratio difference. Perhaps about 10.5:1 or 10.7:1 on the 610 motor would allow the 9.7:1 386 stroker motor to run the exact same spark timing. It does of course depend on the type of gasoline being used. Slower flame front travel speed gasoline is more sensitive to compression ratio differences, where faster flame front travel speed gasoline is more sensitive to spark timing differences.
An engine with such a dramatically smaller bore, only 86% the diameter of the 610 motor, would be expected to require a dramatically lower compression ratio to run the exact same spark timing value. The reason that only a moderately lower compression ratio appears to allow the same spark timing value also has to do with the fact that the smaller motor with the shorter stroke tends to rev higher where more spark advance is an advantage. The 2.68 inch stroke length of the 386 stroker motor is only eleven percent shorter than the 3.01 inch stroke length of the 610 motor. That eleven percent shorter stroke length would tend to allow an eleven percent higher engine speed. The eleven percent higher engine speed would not however require so much more spark advance as as to fully compensate for the 14% smaller bore diameter. Eleven percent is less than 14% percent. And then there is the fact that shorter stroke motors typically don't run at quite as high of mean piston speeds as longer stroke motors. This all means that the smaller bore motor can run somewhat closer to the same spark timing as the big bore motor than might at first be expected.
Even when running the exact same spark timing values the small bore motor is not going to run exactly the same as the big bore 610 motor though. With the same spark timing value the small bore motor is going to burn more of the gasoline in full flame front travel mode before late compression ignition occurs. This means that the small bore motor will be louder, harsher and less efficient than the 610 motor running a higher compression ratio. It is usually much better for both motors to run the same compression ratio and for the small bore motor to run less spark advance.
With a large four inch bore very small amounts of spark advance and slow flame front travel speed gasoline can result in tricky tuning and an on-off sort of power delivery. For this reason many people have wanted to stick with low compression ratios for running slow flame front travel speed gasoline in large bore engines. This sort of make sense, but an excessively low compression ratio on slow flame front travel speed gasoline can also very easily result in a situation where huge amounts of spark advance are required to get the fuel to pop off on late compression ignition. On any engine with any fuel using spark timing earlier than about 26 or 28 degrees BTDC means that the latest possible time of late compression ignition around 15 or 20 degrees ATDC is absolutely unattainable and the engine will only run at the earlier and easier to hit 5 degree ATDC time of late compression ignition. On a three inch stroke engine this means that operation at engine speeds around 3,00 to 5,000RPM gets very harsh and inefficient. If the engine is to run at all in this 3,000 to 5,000RPM range of lower engine speeds the compression ratio has to be high enough that spark timing values earlier than about 27 degrees BTDC are not required. As bad as it is to get stuck with too little spark advance on slow flame front travel speed gasoline it is even worse to get stuck not being able to attain the latest possible time of late compression ignition if those lower 3,000 to 5,000RPM engine speeds are to be used. Even though the gasoline supply has been extremely variable in the last few years the vast majority of the gasoline has been pretty high pressure stuff for amazingly high compression ratio engines. The torquer Husqvarna 610 motor is going to get a high compression ratio around 12.1:1, anything lower just does not make sense on the gasoline that has actually been available.
It had been seeming like the temperature of combustion potential of the gasoline was always somewhat low. It had been seeming that way for many months, ever since the fall of 2015. There for a while I was getting extremely low temperature of combustion potential gasoline straight from the gas stations, but that did not last long. Then from March of 2016 to May of 2016 what I was always getting form the gas stations was rather slow flame front travel speed gasoline that had only a slight to moderately low temperature of combustion potential. Compared to the extremely dramatically cold burning gasoline that had been available sometimes over the winter this slow flame front travel speed gasoline was seeming like some pretty hot burning fuel. I was however still having lots of trouble with the 12.2:1 hot rod 610 motor having an annoying flat spot around 4,000 to 4,500RPM nearly all the time. What was most problematic was that it ended up being an abrupt hit right at 5,000RPM that made trail riding somewhat more difficult. It was obvious that the real problem was that the three inch stroke length was just too long for the colder burning gasoline. Since all I have are three inch stroke length motors though I was sort of stuck.
As a first step I went back to stock cam timing on both my 12.2:1 hot rod 610 motor and my 9.7:1 386 stroker motor. I did the 610 first, and it took me two days as I was somewhat undecided about how to actually go about rotating the timing sprocket on the camshaft. I had not adjusted the valve lash in 80 hours of operation, so I was interested to see what the lash setting was when I started taking the bike apart. I was somewhat amazed but not really surprised to find that the intake valve lash was nearly exactly where I had left it 80 hours before. They had opened up only about 0.001" in that time. The exhaust valve lash had actually closed up in 80 hours, by nearly 0.004" on one exhaust valve and by 0.002" on the other exhaust valve. Once the parts have all worn in to match each other the rate of wear of the valve stems and adjusters drops off considerably. It is probably still a good idea to check the valve lash at the recommended 30 hour intervals, but the Husqvarna 610 motor certainly is well capable of going for 100 hours or more between valve lash adjustments.
I had been thinking about drilling the camshaft flange and the sprocket for a new locating pin. What I ended up doing though was just filing out the mounting holes in the sprocket and bolting it on rotated back for a more advanced camshaft timing setting. The mounting holes on the 1994 and later Husqvarna camshafts are on a 54mm bolt circle so I filed out the holes 0.035" aiming for about three and a half degrees of crankshaft rotation of change in the cam timing. This would bring the cam timing back to the stock setting. Reducing the base gasket thickness to go up to the 12.2:1 compression ratio from the 11.0:1 Woessner compression ratio had retarded the cam timing about 3.5 degrees of crankshaft rotation, so this adjustment would bring the cam timing back to the stock setting which is 3.5 degrees of crankshaft rotation advanced for the big 250 degree at 1mm valve lift 1994 camshaft.
The 1994 Husqvarna camshaft with the centrifical de-compressor mechanism didn't seem to center very well in the timing sprocket. There are large cutouts, and the fit just seemed somewhat sloppy. I was concerned about the alignment of the sprocket, but when I test fit the sprocket and measured the alignment it was looking like the sprocket was centered within 0.005". Not perfect but pretty good.
I bolted the sprocket on with liberal application of high strength thread locking compound and I torqued the 8mm bolts down as tight as I thought they could reasonably handle. I also removed the washers from the bolts so that they would come all the way through the threaded holes in the camshaft flange. With the 1/16" washers removed the threads fully engaged, where with the washers in place the bolts did not quite come all the way through to the back of the threaded holes.
With the camshaft back in the engine and the chain tensioner installed it was looking like I had gotten the cam timing to advance about two and a half or three degrees of crankshaft rotation. Not quite back to stock, but a substantial change in that direction. With the cam timing within one degree of crankshaft rotation of the stock setting the difference from stock is extremely small and would not be expected to be easily noticeable.
When I took off on a test ride I immediately noticed that the flat spot around 4,000 to 4,500RPM was less pronounced. Torque seemed to build smoothly from 3,500RPM up to 6,000RPM as it had with the stock cam timing. Since it was two days that I had the bike apart I was not sure that it was the same gasoline still in the tank that I had started with, and in fact the gasoline had seemed to change quite dramatically over night. Before the cam timing change I had been running a static timing setting of 23 degrees BTDC and the engine had been reluctant to light off requiring a big twist of the throttle at all engine speeds down to 3,000RPM. After I advanced the cam timing about three degrees I set the static timing setting dramatically lower at 18 degrees BTDC and the engine ran much more crisply with instant torque across a wide range of lower engine speeds. It really seemed like much lower pressure gasoline. Gasoline changing overnight has not been uncommon of late, so I was not all that surprised even if it is always rather alarming when the gasoline dramatically changes overnight.
Wanting some better information about cam timing changes I decided to advance the cam timing on the 386 stroker motor all within one day. I started the morning by going for a substantial little test ride, and then I checked the static timing setting when I got back. I made notes about exactly how the engine was running at various engine speeds under different conditions.
When I first began to take the bike apart though I ran into an unexpected problem. The cheap $8 Italian made Mazda Miata valves I had put in the 1992 cylinder head had smashed down considerably at the lash adjuster. They are soft valves. I scratched them with a pocket knife and they deformed easily. I then went around with the same pocket knife checking other intake valves, and I could not find anything nearly that soft. The Husqvarna valves that came out of that head were much harder, so hard that I could not get them to scratch at all with the pocket knife. When I checked a Suzuki automotive intake valve I had in the garage it was a bit softer than the Husqvarna intake valves, but not anywhere near as soft as the cheap Italian Mazda Miata intake valves.
The valves had not exactly failed. They were still somewhat intact after 75 hours of operation in the 386 stroker motor, but I was alarmed at the extreme softness of the valve stems compared to other intake valves. I contemplated halting the whole project to order some Mazda Miata valves from a different manufacturer, but in the end I figured that those cheap soft valves would probably easily go another 75 hours or more if I kept the valve lash a bit tighter.
Once I decided to just go ahead with the cam timing change as planed the project moved along rapidly. Advancing the cam timing on the 1991 camshaft was much easier as it centers well in the timing sprocket. What I noticed was that there was already some small amount of rotational play in the sprocket mounting holes. The bolt circle on the 1991 camshaft is only 33mm, so a smaller amount of wiggle results in more degrees of variation in cam timing. I filled the holes out 0.015" which yielded a total of about 3.5 degrees of crankshaft rotation of adjustment.
Again with large amounts of high strength thread locking compound all over the bolts and mounting flange I bolted the sprocket up and tightened the bolts aggressively. When I checked the cam timing on the assembled motor it was looking like I had gotten that full 3.5 degrees of crankshaft rotation of cam timing change. The cam timing was certainly a degree or so advanced from straight up.
Having just done the same procedure to another bike three days previously it all went back together quickly and easily. By late afternoon I had the bike up and running again on the same gasoline I had ridden on earlier in the day.
When I rode off I was amazed that there was a rather noticeable increase in torque around 3,5000 to 5,000RPM. It was some very low pressure gasoline and I was running a static timing setting of 20 degrees BTDC so torque was pretty good down at those low engine speeds. Unsurprisingly the engine ran extremely similarly to how it had in the morning with the same 20 degree BTDC static timing setting. The difference was just that the torque built more smoothly up to 5,000RPM, where with the retarded cam timing setting there had been a dead spot and then an abrupt hit right at 5,000RPM. Up on top there seemed to be actually very little change. The engine was a slight bit more reluctant to rev all the way out, but power output at 8,000RPM seemed really exactly the same. Later when I carefully checked the static timing setting again it was looking like it actually was a degree later than it had been in the morning, the points probably got bumped while I was turning the engine over to check the cam timing. In the morning it had been pulling 8,900RPM, but very reluctantly and only when well warmed up on a big pull. In the afternoon with the advanced cam timing and perhaps about a degree later spark timing the engine was only revving out to 8,000RPM, but it was not hitting a wall there. It felt like it would go higher if I pushed it harder.
It seems obvious that the stock cam timing is correct for the Husqvarna motors. Going with a three degree of crankshaft rotation later intake valve closing time does give a bit more pull up around 7,500 to 9,000RPM, but it is actually a surprisingly small difference. The lack of torque bellow 5,000RPM on the other had is rather significant with the three degree later intake valve closing time. It is not so much the actual loss of torque at 4,000RPM as it is the very noticeably more abrupt hit at 5,000RPM that is problematic with the three degree later intake valve closing time. The stock cam timing seems to deliver very nearly as much power up on the top and a rather significantly better feel to the power down bellow 5,000RPM. On more normal hotter burning gasoline the 2.68" stroke length 386 stroker motor seemed really very good with the later intake valve closing time, but with the somewhat colder burning gasoline the stock cam timing was a whole lot better even on the shorter stroke length motor. Of course the 1991 WXE 350 camshaft is not exactly the same as the 1991 WMX 610 camshaft either. They look exactly the same, and are in fact very similar, but the 1991 WXE 350 camshaft does have about three degrees of crankshaft rotation more duration at 1mm valve lift. The way it turned out though I did get the camshaft in the 386 stroker motor in about a degree of crankshaft rotation advanced from straight up, so the actual intake valve closing time is in fact extremely close to that of the 610 motor.
From this testing of slightly different intake valve closing times it may at first seem easy to understand why three and a half and four inch stroke length automotive engines have had such very early intake valve closing times. The thing that has to be kept in mind though is that a 240 or 250 degree at 1mm valve lift camshaft is a pretty big stick. With that much duration cylinder filling is dropping off dramatically bellow 4,000RPM. With that late of an intake valve closing time there are gains to be had in efficiency and peak torque generation with earlier intake valve closing times. Essentially a 240 degree at 1mm valve lift camshaft is easily big enough for at least some types of rather long stroke gasoline engines. A 200 degree at 0.05" valve lift automotive camshaft on the other hand is excessively small for any gasoline engine. Down at those much earlier intake valve closing times advancing the cam timing would tend to do nothing but cause more excessive harshness at 2,000 to 3,000RPM. Unless of course the engine has such a low compression ratio that it never enters late compression ignition mode, then a smaller 190 degree at 0.05" valve lift camshaft could be used to increase torque at 2,000RPM. If the camshaft is big enough or too big then advancing it a few degrees can result in significantly better low engine speed performance from around 3,000 to 4,500RPM. If the camshaft is too small then advancing it does not accomplish anything constructive.
So for overall best performance on pretty much any type of gasoline the Husqvarna 610 motor works best with the stock intake valve closing time of the 242 degree at 1mm valve lift 106 degree lobe center 1991 camshaft installed straight up with split overlap at top dead center or the 250 degree at 1mm valve lift 106 degree lobe center 1994 camshaft installed three and a half degrees of crankshaft rotation advanced. The shorter 2.68" stroke 386 stroker motor also generally seems much better with approximately this same intake valve closing time, although on more normal hotter burning gasoline the shorter stroke motor certainly can make use of later intake valve closing times to run only between 5,000 and 9,000RPM. Even on the hottest burning race gas that could easily support 11,000 or even 11,500RPM operation in the 2.68 inch stroke length engine very good operation at the latest possible time of late compression ignition around 15 or 20 degrees ATDC would also be possible down to 4,000RPM or even 3,500RPM. The 2.68 inch stroke engine might be able to rev high enough to make good use of later intake valve closing times, but even on the hottest burning race gas the stock intake valve closing time would also work very well for providing strong low end torque way down to below 4,000RPM. The 386 stroker motor with it's giant 43mm Honda sized rod bearing certainly can take some harsh operation down at low engine speeds, so there is really no reason not to run it down as far as it will make useful torque. On normal pump gas this means some harsh torque available down to somewhat less than 3,500RPM, perhaps as low as 3,000RPM. On the hottest burning race gas the engine might get rather harsh bellow 3,700RPM, but some harsh torque would still be available all the way down to close to 3,200RPM.
The same is sort of true for the three inch stroke length 610 motor, but there are some slightly different realities. Because of the very small 38mm rod bearing avoiding extremely harsh operation is a good idea. The engine seems to be able to hold up to quite a bit of harsh operation, but by any standards that is a very small rod bearing. What this means is that on the hottest burning race gas it is generally a good idea to avoid the harsh operation down at less than 3,500RPM even if some harsh torque is still available down to somewhat lower engine speeds around 3,200RPM at the latest possible time of late compression ignition around 15 or 20 degrees ATDC. On any sort of normal pump gas the three inch stroke engine runs pretty smooth down to about 3,300RPM at the latest possible time of late compression ignition, and on colder burning gasoline operation down to 3,000RPM can be useful.
The later intake valve closing time can be good for avoiding harsh operation down at less than 3,500RPM. With the big 250 degree at 1mm valve lift camshaft installed straight up it seemed like the engine would have worked well on hotter burning race gas. During that six months that I had the 250 degree camshaft installed straight up though the hotter burning gasoline was completely absent. I had been suffering through very hot burning and very high pressure race gas coming out of the pumps all the time when all I had were 10.2:1 and 11.0:1 engines, then once I had a 12.2:1 motor that could make use of that higher pressure race gas what was coming out of the pumps was mostly considerably colder burning gasoline. I did often get some slow flame front travel speed gasoline that had a reasonably high temperature of combustion potential, and that did make some big power up at 7,000 to 8,000RPM. The slow flame front travel speed gasoline was however much more difficult to use and the power always seemed to shut off abruptly at 8,0000 or 8,300RPM with very little overrev. It was fun to pull big power like that on the slow flame front travel speed gasoline, but it was not really very useful for a dirt bike. On the trails the engine speed usually stayed bellow 6,000RPM with an annoying abrupt hit at 5,000RPM right in the middle of the range of engine speeds that got used the most. Race gas like had been coming out of the pumps before would have increased torque at 4,000RPM, smoothed the abrupt hit at 5,000RPM and provided good usable overrev to 8,500 and 9,000RPM. I have however never wanted to run race gas. I have always wanted a dirt bike that runs on normal pump gas. Instead of buying expensive $10 or $15 per gallon race gas I advanced the cam timing back to stock to make best use of what was actually coming out of the pumps. It is mostly the higher compression ratio that is required to make use of the higher pressure race gas, the earlier intake valve closing time can be made to work on any type of gasoline other than extremely slow flame front travel speed gasoline that just won't rev out without a programmable advance curve. The stock cam timing really is better for the three inch stroke Husqvarna 610.
The question still remained as to what sort of build strategy to use for the 610 motor in frame 130. The big question really was still on the compression ratio. It had gotten to be a quite extreme situation where every time I bought gasoline at the gas station it was some pretty high pressure stuff that required more than 26 degrees BTDC in the 12.2:1 motor. But then when I let the gasoline sit around unattended for a few days it mysteriously changed to a much lower pressure gasoline that would run in the stock 10.2:1 610 motor at 26 degrees BTDC and would also run in the 9.7:1 386 stroker motor at around 20 to 23 degrees BTDC. This had been repeated over and over again, and it still happened pretty much like this after I changed the cam timing on both the 12.2:1 hot rod 610 motor and the 9.7:1 386 stroker motor. So what to do? Build the new 610 motor for the gasoline that was coming out of the pumps, or build the new 610 motor for the gasoline that was showing up overnight?
Sometimes the gasoline from the gas stations breaks down in storage, but this can be caused by two different things. When I ride to the gas station and fill up with two gallons of fresh 91 (RON+MON)/2 octane rating premium and then go for a two hour ride I have sometimes experienced the gasoline breaking down into something different towards the end of that ride. This could be caused by someone adding a reactive type gasoline additive to the tank on my dirt bike the night before. Only part of the additive is used up in reacting with the gallon or half gallon of gasoline in the tank so the reactive additive is still available to break down the two gallons of fresh pump gas when it is added at the gas station. This may happen sometimes. It may also be possible for two grades of gasoline to remain stable in separate storage tanks but then react when mixed together. The common type of electronically controlled single handle gas pumps are probably capable of mixing gasoline from the two storage tanks in any proportions on any of the three grade selection buttons. Even when pressing the 91 or 87 buttons that traditionally drew from only one tank it may sometimes instead be a mixture from the two tanks that is sold.
A clue that something strange is going on with the pumps themselves is that I am constantly being charged for around 2.53 to 2.55 gallons of gasoline to fill my 2.40 gallon gas tank. This has been pretty consistent for almost a year now. When I buy 2.54 gallons of gasoline and take it straight home in a sealed container it just fills the empty tank on the bike right to the top. When I buy 2.40 gallons and add it to the empty tank the level in the tank is down substantially from the top. In the past it always seemed like the 2.40 gallon tank held something very close to 2.4 gallons. I have five of the 1991 and 1992 Husqvarna gas tanks, and they all seem to hold very nearly the same amount of gasoline. I suspect that it is close to the 2.40 U.S. gallon rated volume. There would of course be some variation in volumes of the flimsy molded plastic gas tanks, but the consistency of the tanks I have points to a variation of less than 0.05 gallons. When it takes an extra 0.15 gallons to fill the tank every single time this points to a calibration problem with the pumps. The pumps do seem to be very consistent, within the last year it has always been about 2.53 to 2.55 gallons that is required to fill the 2.40 gallon tank. This has been true of dozens of different gas stations in several different counties.
Another clue about something strange going on with the pumps lately is that when filling cars and light trucks the pump often shuts off early. In the past the pumps were very consistent, the automatic shut off always occurred right when the liquid level reached the end of the nozzle. With the same vehicles I have been driving for 20 years the pumps are now sometimes shutting off early and inconsistently. There is certainly something very strange going on with the gasoline pumps.
What has been most alarming about the gasoline lately is just that it is so dramatically different at different times. There has been a certain amount of consistency day after day and week after week sometimes, but when a different type of gasoline then becomes available the change is often really very dramatic. One thing that I have been seeing over and over again is that the gasoline seems to be formulated specifically to require large amounts of spark advance. This is mostly in the gasoline that shows up in my tanks overnight, but has also been somewhat true of the gasoline straight from the gas stations. It has often seemed like an unusually weak low energy density fuel that just won't run without a rich mixture and lots of spark advance. It is however also a lower pressure gasoline that does actually sort of work in the 10.2:1 motors. What I ran into over and over again from late 2015 through the summer of 2016 was that this lower pressure and weaker gasoline would seem to sort of work in the 10.2:1 stock 610 motor at around 26 or 28 degrees BTDC, then when I put the same gasoline in the 12.2:1 hot rod 610 motor it would run at 21 or 23 degrees BTDC, but up on the top end at 7,000 or 8,000RPM it would backfire and cut out hard. With the crankshaft wiggle advance, a static timing setting of 21 degrees BTDC would be a spark timing value of around 24 or 25 degrees BTDC up above 7,000RPM. Hardly less than the 26 degree BTDC spark timing on the 10.2:1 motor. It was seeming like the crankshaft wiggle advance just was not doing any good. The engine would still cut out hard sometimes at 7,200 or 7,400 or 8,200RPM. Often I had noticed the engine cutting out a bit down at 6,500 to 7,00RPM, then when the crankshaft wiggle advance hit at 7,000 or 7,100RPM it pulled better for a short time and then started cutting out again at 7,400RPM, then when the intake stack boost hit at 7,500RPM it again pulled hard for a while until it cut out really bad at around 7,900 to 8,200RPM.
It is like this weak specialty gasoline just always needs more spark advance. Even down at around 4,000 to 6,000RPM the weaker colder burning gasoline seems to favor more spark advance. The problem then is that in both the stock 10.2:1 motor and the 12.2:1 hot rod 610 motor the colder burning gasoline starts to whine and surge at 5,000 to 6,000RPM with more spark advance for crisper operation. In the stock 10.2:1 motor with the Czech Republic CDI ignition it is sort of unavoidable that the engine gets overly crisp over a range of lower engine speeds if it is to be able to rev all the way out. It has often continued to pull to 7,800 and even 8,300RPM, but the engine then tends to get extremely crisp around 4,500 to 6,500RPM. The totally fixed spark timing of the CDI ignition system works better when it is tuned for good torque at 3,500 to 7,000RPM on the stock 610 motor. The 10.2:1 stock 610 motor with points ignition on the other hand has no trouble revving out to 8,000RPM without becoming overly crisp at 5,000RPM. That is the motor that can run on really very spectacularly weak gasoline. With the crankshaft wiggle advance coming down at less than 6,500RPM it always seems to give a good strong pull everywhere from 5,000 to about 8,000RPM and it is also capable of running strong down to 3,500 and even 3,200RPM as long as the gasoline will pop off easily enough in the 10.2:1 motor. The only two things that can stop the 10.2:1 stock 610 motor with points ignition are extremely slow flame front travel speed gasoline where the crankshaft wiggle advance just is not enough additional advance or extremely high pressure gasoline that is very reluctant to pop off in the low 10.2:1 compression ratio motor. On very high pressure gasoline the 10.2:1 stock motor with points ignition has a narrow power band from about 5,500 to 7,000RPM and it gets extremely loud and harsh at all engine speeds. Lots of noise, lots of heat and power output is actually lower because the engine won't rev all the way out to 8,000RPM.
The 10.2:1 stock 610 motor with the points will of course also get whinny and surge badly on extremely dramatically cold burning gasoline, but even on that garbage power tends to be better through the 5,500 to 7,000RPM range of engine speeds because the wide open throttle crankshaft wiggle advance is well placed right in the middle there to help the engine rev out to 7,000RPM without losing torque at 5,000 to 6,000RPM. The problem of course is that the heavier 368g piston does cause a noticeable reduction in power output compared to the much lighter 332g piston in the hot rod 610 motor. The heavier piston places the crankshaft wiggle advance lower so the engine runs better on weaker gasoline, but the extra piston weight is even more of a problem on the weaker gasoline and the hot rod 610 motor always makes dramatically more power even if it is harder to tune. The combustion tends to be more efficient on weak gasoline with the heavier 368g piston that delivers the crankshaft wiggle advance down at a very low 6,300RPM, but less of that efficient combustion actually makes it to the rear wheel as power output because such a large and significant portion of the power is wasted in throwing the heavy piston up and down.
The really big change though was that very low pressure gasoline had been showing up in my tanks overnight. Low enough pressure gasoline that it runs with just 19 or 20 degrees BTDC on the static timing setting in the 9.7:1 386 stroker motor. This is very different than 2014 when the stock 10.2:1 1991 WXE 350 was always requiring static timing settings up at around 30 to 33 degrees BTDC to run at all. Most of 2015 also the 9.7:1 386 stroker motor was requiring around 29 to 31 degrees BTDC to run, and sometimes it would not make power at all even when I went all the way up to 40 degrees BTDC.
Then in late summer and fall of 2016 very low pressure gasoline started coming right from the pumps at gas stations. I took a longer ride on the 1991 WXE 350 with the 9.7:1 386 stroker motor, and it was very crisp tank after tank with around 20 to 22 degrees BTDC on the static timing setting. Even up to 6,000 feet of elevation it was running and making power with just 22 degrees BTDC on the static timing setting, although there was lots of lag and hesitation up at those highest elevations. The 12.2:1 hot rod 610 motor has also been running with much less spark advance on gasoline straight from the gas stations. As little as 14 degrees BTDC on the static timing setting, although that small of an amount of spark advance tends to not work well at all in the big four inch bore engine. With 17 degrees BTDC on the static timing setting the 12.2:1 hot rod 610 motor has been running consistently and crisply, without cutting out or stumbling at any engine speeds.
What the big three inch stroke length engine has also been doing on the very weak and also very low pressure gasoline though is going flat around 4,000 to 5,000PRM. With small amounts of spark advance on weak cold burning gasoline the engine runs amazingly smoothly and powerfully all the way down to around 2,500RPM, but then torque falls flat as the engine speed is increased past 3,500RPM. To get power out of the weak cold burning gasoline earlier times of late compression ignition are required, and the late spark timing values mean that the engine is resistant to falling over to the earlier and easier to hit 5 degree ATDC time of late compression ignition. The 12.2:1 hot rod 610 motor has still been revving up and making some power up at 6,000 to 8,500RPM, but it is somewhat weak compared to more normal gasoline. It is interesting that the stock 10.2:1 motor at a 26 degree BTDC spark timing value has an easier time making torque at 4,000 to 5,000RPM on the weaker colder burning gasoline because the time of late compression ignition so easily falls over to the earlier and easier to hit 5 degree ATDC time of late compression ignition and even earlier times of late compression ignition.
The 12.2:1 hot rod 610 motor still makes more power and more torque on the same gasoline, but getting torque down to 4,500 and 4,000RPM requires that the engine be running very crisply and well heated up on some big pulls when the static timing setting is down at less than about 18 degrees BTDC on the weaker and somewhat lower temperatuer of combustion potential gasoline. The weak and somewhat low temperature of combustion potential gasoline certainly favors earlier spark timing around 23 to 26 degrees BTDC in the big three inch stroke length engines. It is interesting though that the flame front travel speed of the very weak and somewhat low temperature of combustion potential gasoline is really quite high. With a fast flame front travel speed and just 17 degree BTDC spark timing the 12.2:1 hot rod 610 motor does rev feely, and there is no cutting out of any sort up in the 6,500 to 8,000RPM range of engine speeds. The engine usually revs freely to 8,000 or 8,500RPM, but power up there is strikingly low compared to running more normal gasoline in the same engine.
I had been back and forth many times on what to do with the pile of 1991 WMX 610 parts I had in the corner of the garage. I had collected up most of the missing pieces I needed, including ordering a new kick start engagement pawl and spring. I had the re-plated cylinder and new 11.0:1 Woessner piston sitting on my desk, but the gasoline had turned very weak and very low pressure so that it was the 10:1 motors that were seeming to work best. It was interesting though that the dramatically weaker but also rather fast flame front travel speed gasoline that had become common was actually very sensitive to compression ratio changes. The difference between 10:1 and 12:1 was huge. The stock 10.2:1 1991 WMX 610 motor with the Czech Republic CDI ignition set at 26 degrees BTDC was running crisply down at 1,000 feet of elevation, but taking it up to higher elevations it often was hesitating way too much and power delivery was very poor. The 12.2:1 hot rod 610 motor was seeming like it had too high of a compression ratio down at 1,000 feet of elevation running 17 degree BTDC spark timing, but up above 4,000 feet of elevation running 21 degree BTDC spark timing was working much better.
It was seeming like I needed an 11:1 compression ratio again. If 10.2:1 was marginally too low and 12.2:1 was marginally too high then 11.0:1 should be just right, so I decided to just put the motor together bone stock with the 11.0:1 Woessner piston. I also decided to give the stock Swedish SEM CDI ignition a try.
When I pulled the flywheel off I found that the stator was set closer to the middle of the range of adjustment than I had even seen on the SEM ignitions on other bikes. On my original 1991 WMX 610 as well as the stock 1992 TE 350 and the stock 1991 WXE 350 I had found the stator turned almost as far as it would go towards earlier spark timing. On the derelict 1991 WMX 610 though the stator was in the middle, about where I had first run the stock SEM ignition on the rebuilt 1997 motor back in early summer of 2015.
At this point it begins to seem necessary to name this derelict bike. It is beginning to get to be like counting chickens out there in the garage with all of these 1991 Husqvarnas running around. I had been calling it the derelict, since it was in by far the worse condition than any of the other Husqvarna four strokes I had. As I looked over the parts I began to see that there was a difference. This was actually a 1990 bike. The front fender was a little bit different, the fender liner and rear fender did not fit as perfectly as on the 1991 bikes and the graphics were noticeably different. The colors on the remaining stickers were somewhat different than the colors on the 1991 stickers I have, and the text was even somewhat different. The 1991 Husqvarna WMX 610 has "WMX 600" across the tops of both number plates. This 1990 Husqvarna WMX 610 instead has "WMX 610" across the tops of both number plates. The welds look different also. The 1990 bike looks like a very early attempt, with somewhat haphazard welds on the frame. The 1991 and 1992 frames are all uniform, with standardized welds similar to the older Electrolux Husqvarnas from the late 1970's and 1980's. The 1990 Husqvarna WMX 610 is really identical to the 1991 Husqvarna WMX 610, all the parts are interchangeable and essentially identical. It is the same bike, there are just those few little clues that the 1990 model was actually built earlier.
The stator hold down bolts on the 1990 were in very tight, the same as I had found on my 1991 Husqvarnas. As is typical I managed to get one of the 5mm stator hold down bolts to come out, but the other two Allen heads stripped out. I had to drill those two bolts out, which was not all that difficult. Once the heads were off, the bolts came out fairly easily. They were not rusted or stuck badly, they had just been run in extremely tight. So tight that the steel washers were squished down into the slots on the stator. Talk about locked down spark timing! To run the 11.0:1 compression ratio though I was going to need later spark timing. I turned the stator all the way towards later spark timing, which I have been calling about 25 degrees BTDC.
The assembly of the motor went quickly, as it was a stock build and I had all the parts. The only hiccup I ran into was that the 1994 cylinder head I had been planning on using turned out to be garbage. When I pushed an intake valve open I found that the valves were pounded way down into the seats, and both the valves and the seats were garbage. Instead I had to refurbish the original cylinder head that had been on the 1990 WMX 610 motor. The bike had been left out in the rain with the carburetor off, so water had gotten on the intake valves. The valves and seats were rusty, but the damage was not very deep. Some lapping in with coarse valve grinding compound got a clean sealing surface without tanking much material off.
When I turned to the exhaust valves I found the reason the bike had been parked all those years ago. The exhaust valve seats were burned and had started to leak. The damage was moderately deep, but I was able to get it to clean up just by lapping the valves in. It took fairly aggressive material removal, but the valves and seats cleaned up without the seat contact width distorting all that much. Those Husqvarna valves are extremely high quality stainless steel valves, and they always seem to remain usable even after the seats burn and pit rather badly.
Since there would be no penalty for lighter piston weight with the CDI ignition I cut the new Woessner piston down to the same 332g weight as the Woessner piston I have in the 12.2:1 hot rod 1997 610 motor. The second time around it was easier to cut the piston down, and I managed to stay even farther away from problem areas that might get close to weakening the piston in a high cycle life application.
I reused the original gaskets with RTV silicon on them, so the cylinder head height is stock. With the head bolts torqued down to 41 foot pounds the distance from the cylinder head down to the cases was exactly the same as on the other two 1991 WMX 610 motors I had sitting there, within about 0.005" anyway. When I installed the stock 242 degree at 1mm valve lift camshaft it came out straight up with split overlap right at top dead center. A bone stock WMX 610 motor, just with the taller 30mm compression height Woessner piston.
Getting the rest of the bike together did take some creativity, but it all went together well. I had to replace a broken spoke on the 1994 rear wheel, and the wheel bearings were also toast. When I knocked the bearings out I got a bit of a surprise. Although the drive side uses the same pair of ball bearings as the 1991 wheels the brake side on the 1994 wheel uses a wider bearing. I did not have a 1994 bearing, but I came up with an interesting solution. Instead of using a custom wheel spacer to put the 1994 wheel on the 1991 bike I just put two 1991 bearings in the 1994 wheel, and the width came out close enough to work fairly well. A bit wider than it should be by about 0.10", but the swing arm spreads out that far with no difficulty. The alignment in the rear brake caliper is not perfect either, but it is close enough to work. I also had to make up a drive side wheel spacer, not because the stock part would not work, but simply because I did not have any extra 1990's Husqvarna drive side rear wheel spacers. I slapped together a fabricated spacer by brazing a piece of 3/4" bronze pipe onto a steel washer. I finished the part in the lathe, and it is pretty much just as good as the stock one-piece steel spacers.
The front tire just held air as it was, and I put a new tube and rim strip in the rear tire. The old style 80/100-21 Dunlop K490 front tire and 110/100-18 IRC VE-33 rear tire are like I used to run sometimes back in the 1990's. They are slower and more difficult to ride than modern tires, but they do sort of work. The tires on the 1990 WMX 610 were old and a bit cracked, but the tread was nearly all the way up so I figured I would run them to start with.
The front brake hydraulic system worked after I drained the thick brown old gunk out and flushed it through with a big can of fresh DOT 3 fluid followed by DOT 4 fluid to fill the system. The inner boot on the main front caliper pin was torn, but the caliper and pin were still in good condition. I removed the boot and installed an O-ring instead, a repair I have done on other 1991 front brake calipers with good success. The O-ring does not hold the grease in quite as well as the original boot, but it does keep the mud and water out. The caliper pin just has to be greased after about a year or 100 hours, where the original boots hold grease in fairly well for up to a decade.
The rear brake system on the derelict 1990 bike also worked when I bled it through, but there were some parts missing. The boot was missing off the master cylinder, and one of the rear caliper boots ripped when I pulled on it. Luckily I had a spare caliper boot from a worn out caliper I took off of my other 1991 WMX 610. All that had been wrong with that caliper was that the pins had not been kept greased before I got the bike, and eventually the wiggle in the pins was so much that the rear brake would not work well anymore. The rear caliper on the 1990 WMX 610 was in pretty good condition, there was actually still a bit of dried up old grease on the pins and they had not gotten rusty at all. I also had a spare master cylinder boot, so it all went together just like stock. I took the brake pedal off to grease the pivot, something that is often overlooked on these bikes. The pivot was in good condition, better than can be said for some of the 1991 rear brake pedals I have taken off.
One of the really big jobs on the 1990 bike was repairing the steering head. Someone had started the project years before by popping the rollers and cage off of the lower inner race. I knew that the bearings were the same top and bottom, so getting new bearings was easy enough. They were the same on Husqvarnas for many decades. Getting that lower race off was a bit of a challenge as I did not have a large enough gear puller to span the large cast aluminum lower triple clamp. I managed to modify an old "clam shell" puller so that I could install it offset to one side, and this got the race to come off in the 20T shop press.
The outer races stubbornly would not come out of the steering head though. I beat on them mercilessly, but they were rusted and stuck solid. The upper race was still in good condition, but the lower race was rusty and pitted. It looked pretty bad, but I was not out of tricks. The steering head bearings turn only slowly, so they do not heat up at all. A rusty and pitted race is not the same kind of problem as on a higher speed wheel bearing or transmission bearing. And the huge angled roller bearings used on these steering heads are about a hundred times oversize for the application. I just scraped the rusty lower race smooth with a half round file, and put the steering head together with new rollers and inner races on the old outer races. With the new bearings well greased and adjusted just tight enough to take all the play out the steering head moved easily and smoothly, with essentially no noticeable roughness.
The clutch cable on the 1990 WMX 610 was in pretty good condition, but it was stuck with hardened old lube and the barrel end at the lever was missing. These Husqvarna levers us a large brass barrel that fits over the smaller standard cast end on the cable. This brass barrel goes on the cable before the standard end is cast, so it is not replaceable. Someone had cut this large brass barrel off of the clutch cable for the 1990 bike. I had a few old cables around, but how to move the brass barrel from the toasted cable to the good cable.
I cut the brass barrel off with a small cutoff wheel, and then I squeezed it back into shape on the good cable. This looked like it would have worked just like that, but to finish the job I soldered the barrel back together with some high strength plumbers solder and filed it down smooth. A perfect repair.
Getting the stuck old lube freed up was just a matter of squirting Lucas chain lube into the cable and working it back and forth. It took some doing, but in about 15 minutes I had the cable back in operation. I don't know if replacement 1990/1991 clutch cables are available at all, but I was recently able to buy a new 1992 and up Husqvarna clutch cable from Motion Pro. Those cables had been discontinued, but now they are once again available. All that is required to use the 1992 and up cable on the 1990/1991 Husqvarnas is a custom extension link at the lever, a fairly easy modification. For now though the 1990 WMX 610 is together with an all original clutch cable.
The clutch adjuster bolt, plastic adjuster wheel and boot were all missing on the 1990 WMX 610, but these parts are the same as on the early 1990's Husqvarna/Cagiva two strokes, so I was able to get replacement parts fairly easily. It was the whole clutch lever and cable I got off of an old 1991 Husqvarna/Cagiva 250 two stroke. The Cagiva two-stroke lever looks different, but the adjuster bolt and wheel are interchangeable so I was able to put them on the original 1990 four stroke clutch lever.
The manual de-compressor lever was also broken on the 1990 WMX 610, but the broken off piece was still there. It does not take much cable force to open the exhaust valve with the de-compressor mechanism, so I figured that the broken off stub would probably be sufficient. The manual de-compressor cable was also in poor condition. The cable and housing were actually still good, but the hollow bolts on the ends were both broken. Amazingly I was able to get the cable to work anyway just by threading the broken pieces of bolts back into the perches on both ends. I have another extra manual de-compressor cable around somewhere that just needs a new cable installed in the housing, but I was not able to find it. That is the manual de-compressor cable I took off of my original old 1991 WMX 610 when I installed the 1997 motor with the 1994 camshaft and centrifical de-compressor mechanism.
I decided to start out with the same very tall gearing I had been running on the stock 10.2:1 motor with the Czech Republic CDI ignition. For the 1990 WMX 610 though I went with 14/52 gearing, figuring that I could go down to a 13 tooth front sprocket on the same chain if I ended up wanting lower gearing. One tooth on the front sprocket changes the axle location only 3/16", which is not a huge amount.
When I pulled the forks apart to install new seals I found evidence of some suspension modifications. The left side fork leg seemed perfectly normal, but the right side leg was different. With the spring out there was noticeably a lot less damping on the right leg. The damper rod just went up and down with a whole lot less resistance than normal. When raising the damper rod there was hardly any oil coming out of the hole where oil usually squirts out on the rebound stroke. There was also a bit of metallic grinding feel down near the bottom of the stroke. Obviously something was wrong. When I tried to take the right hand cartridge out of the lower leg though it was stuck. Even with the bolt removed the cartridge would not budge, it was stuck solid in the bottom of the lower leg.
I put oil back in the right hand fork leg and moved the damper rod up and down at different clicker settings. The clickers were not hooked up to anything. It was the same very low damping at both ends of the ranges of adjustment. In moving the damper rod up and down I began to get the impression that it was an intentional modification of that cartridge. The damping was low, but it was seeming pretty good. The damper rod would go down very easily with a sharp push, but there was some noticeable compression damping. The rebound damping was also low, but there was certainly some rebound damping still working. I decided to assume the best, and I reassembled the forks with new seals and new five weight Bell Ray fork oil. Since the right hand leg obviously had diminished damping I set the clickers on the left leg up at more substantial damping levels than I usually run. I started with five clicks in on the rebound adjuster the compression adjuster at the #3 setting.
I cleaned the 40mm DellOrto carburetor, and I was careful this time to make sure the idle port was clear. The stock 62 pilot jet and 60 starting jet looked original, with the 62 pilot jet noticeably very slightly larger than the 60 starting jet. I verified that the needle jet was the stock size, and I set the main jet at close to the 175 size. Again I drilled an old 145 main jet out much larger, soldered up the hole and then drilled it with a 0.0695" diameter number 50 drill. I drilled through with a #52 drill first, and then finished up with the #50 drill by hand. I only ran the #50 drill just through the solder until it came out the other side, so the size is as small as a #50 drill can reasonably produce. The 0.0695" diameter drill stock end of the #50 drill still does not quite go through the hole. It is probably about a 176 size main jet. This is the same technique I had used when I set the main jet size on the rebuild 1997 motor back in 2015, but I have since changed that jet a few times and it is drilled out a small bit bigger to about a 178 size now. I set the needle clip on the stock K32 needle to the leanest position and installed the carburetor on the 1990 WMX 610.
The air boot on this bike was also torn through, but only over a short section on the top of the carbu he carburetor flange. This had previously been sealed with some clear silicone, but it was not sticking all that well. I pealed all the silicone off, and glued it together with Pro Taper brand grip glue, some very tough gelled cyanoacrilate resin. This seemed to repair the air boot pretty well, but I certainly will be avoiding taking the carburetor off this bike if it can at all be helped. Even if I later decide to change main, pilot or choke jets jet I'll probably just take the bottom off the carburetor and pull the jets out with the carburetor on the bike.
At first the engine would not kick start, but when I rolled it down a hill I got it to fire up. It ran great right away, with strong torque across a wide range of amazingly low engine speeds down to slightly below 3,000RPM. Noth the slightest hint of the surging at 3,000RPM that has been a problem with the Czech Republic CD ignition.
Wow, a running stock Swedish SEM ignition system! Amazing. And it was even kick starting. The idle speed was very low, but it was not stalling. It seemed to idle best when I left the idle stop out quite a bit to let the low idle drop way down bellow 1,200RPM. It was kick starting easily once warmed up, mostly on the first kick but sometimes on the second kick. I had gapped the new C7E spark plug down somewhat less than the stock 0.023" gap at a 0.017" gap. Small, but not dramatically small.
There was none of the bobbling and stumbling at small throttle openins around 1,200 to 2,000RPM like I had experienced with the 0.009" plug gap I had been running on the 1992 SEM ignition on the 1997 motor back in early summer of 2015. no, with the 0.017" plug gap the 1990 SEM ignitino system was seeming to actually work. It had a stable, if very low, idle. It did not stall when riding, and there was no strange bobbling or stumbling at the first crack of the throttle.
After just a short test ride I shut the motor off and changed the cheap 10-30 lube oil I had in the engine as break in oil. The motor then fired right back up with the kick starter. i rode it a bit more and changed the oil again, this time putting the more expensive full synthetic in. Even after the engine sat for about an hour it fired right back up with the kick starter.
The motor ran great that first day, but it was not revving out much. Torque was reasonably good from 3,000 to 5,000RPM, but it was reluctant to rev out and felt lean at wide open throttle. I ttok some of the gasoline out and pout it in the empty tank on the stock 10.2:1 motor with teh Czech Republic CDI ignition. The lower compression ratio engine with a slighty fatter main jet revved out and made much more power up top on the same gasoline. Obviously it was the slight main jet size that was making the difference on the weak watered down gasoline.
The next day the 11:1 1991 motor with the stock SEM ignitino would not kick start. It fired a little bit when I first kicked it, but then nothing on subsequent kicks. I had to roll it down a hill to get it started. Again once it was warm it was able to kick start easily every time, and again the idle was stable with no stalling what so ever.
Again the 1990 motor would not rev out and power was low despite pretty good perfromance bellow about 5,000RPM. I had been running a Cobra Sparky spark arrestor on the 1990 motor, where I have a Supertrapp muffler behind the stock motocross muffler on the 1991 WMX 610 with the Czech Republic CDI. I remembered that the Cobra SParky spark arrestor actually seemed to deliver worse performance than the Supertrapp, so I took the Cobra Sparky spark arrestor off of the 1990 motor and replaced it with the race tip. Since it had already been raining quite a bit in October this seemed perfectly reasonable.
The motor revved out much better and made a lot more power with the race tip. Yes, those Cobra Sparky spark arrestors do hurt exhaust flow. Amazingly the 1990 motor was not even all that loud with the race tip. Not nearly as loud as when I have run just that stock motocross muffler and the race tip in the past on other 610 motors. It must be the weak wattered down gasoine that makes less power nd less noise. Of course backing off on the spark timing from the stock 33 degree BTDC value to the latest possible setting at about 25 degrees BTDC does cut the noise a lot. In the psat though the stock muffler with the race tip was very loud at any spark timing value that would run and make torque.
When I again swapped the same gasoline back and forth between the 11:1 1990 motor with the race tip and the 10.2:1 motorperformance was more similar. The 10.2:1 motor still seemed like it had a bit fatter main jet, but the difference was slight. The main jet sizes are in fact very similar. They were both drilled out with the same 0.069" diameter #50 drill, it is just that I let the drill spin until it turned freely in the jet on the 10.2:1 motor.
I then did a timing check on both of the CDI ignitino systems. The Czech Republic CDI was still right at the same 26 degrees BTDC where I had set it earlier in the year. When I put the timing light on the SEM ignition on the 1990 motor though I was surprised to see that it was earlier than 25 degrees BTDC. It was looking like it was also up at 26 degres BTDC. The advance on the 1990 SEM ignition was the same as on the 1992 SEM ignition. It advanced a total of 20 degrees of crankshaft rotation from the low idle value bellow 1,400RPM to the running value above 2,000RPM. And again most of that 20 degrees came right at 1,400RPM, with the remainder of the advance coming as the engine speed was increased from 1,500 to 2,000RPM.
When I revved the engines out far with the timing light hooked up I noticed that both of the CDI ignition systems did continue to advance another degree or two. From about 5,000 up to about 7,000RPM there was that little bit of extra advance. hardly anthing, just perhaps a degree and a half of cranshaft rotation.
At first this really surprised me, that both engines were running with the same 26 degree BTDC spark timing value arund 3,000 to 5,000RPM. How could it be that the lower compression ratio 10.2:1 motor was running on the same gasoline with the same spark timing value? This seemed impossible. There was the slight difference in main jet sizes, but this was a very small difference. And down on the needle jet at bellow 3/4 throttle the carburetors are really very nearly exactly the same. The main jet does change the mixutre ratio some all the way down to about 1/2 throttle, but it is a rather small effect that the main jet has down at 1/2 throttle. Especially with such extremely similar size main jets. And even down at 3,000 to 3,500RPM at 1/4 throttle both engines were running amazingly similarly at the same spark timing value. Really the 11.0:1 engine was someewhat crisper at 3,000RPM though, and it did make quite a bit more torque everywhere down there at 2,500 to 4,000RPM also.
What I came up with is that it is a spark energy difference. The Czech Republic CDI on the stock 10.2:1 motor blasts huge spark energy and always kick starts easily with the stock 0.023" plug gap. The stronger spark of the Czech Republic CDI allows a wider spark plug gap to be used, and this larger spark plug gap gets the flame front going a bit faster. The smaller 0.017" plug gap introduces a small delay as the flame front expands, and this tends to require slightly more spark advance. Or a slightly higher compression ratio.
For three days in a row the 1990 motor with teh stock Swedish SEM ignition would not kick start cold. Each morning it would fire a little on the first or second kick. Either a solitary pop, or a few revolutions at low engine speed before stalling. Then after that first pop or very brief run it would not pop or start at all no matter how many times I kicked it. When rolled down a hill though it always fired right up without even having taken the spark plug out. After the engine had started that first time it always kick started easily, even after sitting for up to two hours. This is considerably different starting perfrormance than I have ever gotten out of the stock SEM ignition systems. I have never had one that started so easily every time once warm, and I have never seen the SEM ignitions to start, then stall and not restart. This 1990 SEM ignition system appears to be working. The hard starting when cold is probably due to a gasoline problem.
I tried taking some of the gasoline out of the 1990 motor and putting it in the empty tank on my original 1991 WMX 610 that now has the 12.2:1 hot rod 610 motor. I leaned the bike over to drain the gasoline out of the carburetor bowel, and then I let the bowel refill with teh gasolien from the 1990 motor. The 12.2:1 hot rod motor then would not kick start, which is highly unusual. I kicked and kicked, but all I could get were some little pops adn it would not start with or without the choke. When I rolled it down a littel hill it fired up and ran. When I shut the 12.2:1 hot rod 610 motor of it then was very hard to start. I actually had to use the choke to get the engine started, even though it was fully warmed up. Twice I shut it off, and each time I had to use the choke to get it started hot. The one time I shut the motor off and restarted it immediately while it was still hot. And the second time I let it sit for a few minutes before trying to restart it. Both times it would not start until I put the choke on, and then it fired right up with the choke on. This is something that has never happened on any of my Husqvarnas in 20 years, that the choke was required to start the engine once it was already warmed up. Obviously something very strange is going on with the gasoline.
The rebuilt 1997 motor does have the slightly smaller 60 pilot jet, where all the other 1991 WMX 610 bikes have the stock 62 pilot jet. A bigger difference is that the rebuilt 1997 motor has a much smaller 45 choke jet versus the 60 choke jet in all the other 1991 WMX 610 bikes. The much smaller choke jet would not however explain why the motor would not start without the choke. The choke circuit actually seems rather rich with the smaller 45 choke jet, the bike was after all able to run on 100% ethanol with the choke on. The stock 60 choke jet is extremely rich. Normally the engine will stall if the choke is pulled on, even when the engine is cold in freezing conditions. The normal starting procedure for all of the 1991 WMX 610 bikes has always been to kick it once with the choke on, and it usually does not start. Then on the second kick with the choke off it fires right up. Very occasionally the 610 motors have started with the choke on, but they normally stall within about one second if the choke is not immediately taken off. Lately though all of the bikes have sometimes been starting and continuing to run with the choke on. This is perhaps understandable with teh rebuilt 1997 motor that has the much smaller 45 choke jet, but the same thing has sometimes been happening on the stock 10.2:1 motor with the Czech Republic CDI and all stock carburetor parts.
A few days later I tried to kick start the 12.2:1 hot rod 610 motor, ostensibily with the same gasoline still in the tank. It would not kick start. It would not even give a pop, with or without the choke. Just nothing. When I roll started it in second gear it fired up somewhat reluctantly and ran. It ran weak though, hardly andy torque and it jst got weaker the farther I revved it. When I shut it off it would not restart with the kick starter, with or without the choke. Again it would not even give a pop, just absolutely nothing. It restarted easily when rolled in second gear. I repeated this four times, and each time it just refused to start or fire at all with the kick starter.
When I drained the tank and put some gasoine out of a different bike in the tank it still would not fire up on the gasoline in the carburetor bowel. I had let the carburetor bowel run down by shutting off the petcock with the motor running, but the engien still did not kick start. It fired up easily when roll started, and when I shut it off again after a mile it fired back up very easily with the kick starter. Several times I shut it off, and each time it fired right back up with the kick starter like normal. The 12.2:1 hot rod engine was also running much stronger, well actualy rather weak but similarly to how it has been running most of the time over the past months. The static timing setting was still at 17 degrees BTDC, and the cranking compression turning the flywheel nut with a wrench was still way up there as it has been or the entire 220 hours I have run the rebuilt 1997 motor since it first went together in April of 2015.
The 14/52 gearing was working just fine on teh 11:1 1990 five speed motor, but again first gaer felt pretty tall. Interestingly the very tall gearing felt somewhat differetn with teh SEM ignition than it did with teh Czech Republic CDI ignition. With teh spark timing backing off substantialy bellow 3,000RPM on the Czech Republic CDI ignition it was generally possible to lug the engine along fairly well way down to 2,000RPM in full flame front travel mode. The power was of course weak down there, but it is after all a huge 577cc motor so the bike moves along fairly well for slow casual cruising. Especially in lower gears. WIth the surging around 3,000RPM these lower en engine speeds were not all that great, but the bike did work and it would move along at any lower engine speeds without much excess harshness. The SEM ignition was quite differetn in that there certainly was harshness at the lower engine speeds down towards 2,000RPM. This meant that the bike was not quite as good for slow casual cruising in high gears at low engine speeds. The advantage of the SEM ignition though is that there is never any surging at 3,000RPM, just increaseing harshness and progressively less torque at lower engine speeds. For faster, more aggressive riding the SEM ignition is better. No mater how low the engine is lugged the power delivery was consistent, without surging. It just got harsh and weak down low. This means that the tall gearing is less of a performance penalty on the SEM ignition, even though the tall gearing can certainly seem to result in considerably more harshness on the SEM ignition.
I had been running very tall gearing on teh 12.2:1 hot rod 610 motor. First 15/50 in 2015 and then 16/50 for a while in early 2016. The 15/50 gearing was realy only good for highway cruising, but the six speed transmission was still allowing for some reasonably good trail riding. The jump from first to second just felt immensely large. Then I switched back to lower 14/48 gearing on the hot rod 610 motor, and this was much better for trail riding. The 13/48 and 14/52 gearing on the five speed bikes actually seems better matched to the 14/48 gearing on the six speed transmission. First gear is a whole lot lower on the 14/48 geared six speed, but the jumps between the rest of the gears are more similar. Running 13/52 gearing on the five speed and 15/50 gearing on the six speed to get both first gears more similar means that the jumps between the upper gears then feel very different on teh five speed and six speed bikes. The reality is that the huge 69% jump down from second to first on the six speed transmission means that first gear needs to be down very low and rarely used.
When actually tooling along on trails the stock SEM ignition can tolerate lower engine speeds down somewhat bellow 3,000RPM on weak gasoline better than the Czech Republic CDI ignition can, but the stock SEM ignition does get harsher as the engine speed is reduced bellow 3,000RPM. For the lowest speed manouvering though the stock SEM ignition is actually smoother with less harshness. Way down at less than 1,400RPM the approximately 6 degree BTDC spark timing yields smoother and more powerful operation. Down at those very low engine speeds around 1,200RPM the 11 degree BTDC spark timing of the Czech Republic CDI ignitoin also does fairly well, but there is noticeably considerably more harshness and not quite as much torqeue. Despite the fact that 6 degree BTDC spark timing delivers better full load operation down at 1,000 to 1,400RPM I would chose more like 12 to 18 degree BTDC low idle spark timing for the Husqvarna 610 motor. The low idle spark timing obviously is not all that critical, anthing from 5 degrees BTDC to 20 degrees BTDC can work fine in the four inch bore engine. On slow flame front travel speed gasoline, or even just unusually weak gasoline, very late low idle spark timing around 5 to 10 degrees BTDC can easily cause somewaht harder starting and dirty inefficient idling in the four inch bore engine. Even on more normal gasoline low idle on the four inch bore engine is probably considerably cleaner with 15 degree BTDC spark timing than with 6 or 8 degree BTDC spark timing.
The ideal fixed advance curve CDI ignitino for the Husqvarna 610 motor would advance the spark timing about eight or ten degrees of cranksahft rotation from 1,000 to 2,500RPM. This would work well with anywhere from 18 to 27 degree BTDC running spark timing above 2,500RPM. On the most powerful types of gasoline the running spark timing could be set down around 18 to 22 degrees BTDC for best possible low end torqeu at 3,300 to 5,000RPM at the latest possible time of late comprssion ignition. On this powerful and clean burning race gas style gasoline low idle spark timing down at 10 to 14 degrees BTDC would work just fine. On very weak gasoline the running spark timing could be up around 25 or 26 degrees BTDC for best possible instant torque around 4,000 to 6,000RPM at the earlier and easier to hit 5 degree ATDC time of late compression ignition. On very weak gasoline low idle spark timing up at 17 or 18 degrees BTDC does not seem all that early in the four inch bore engine. Normal gasoline can also well tollerate backing off to a 20 degree BTDC running spark timing so long as the engine is still crisp enough that the earlier times of late compression ignition can reliably be attained when the engine is pushed hard at somewhat higher speeds.
The only way to do substantially better than this ideal fixed advance curve would be with load dependant spark timing, as has become universal on modern dirt bikes of the 21st century. Load dependant spark timing can deliver cleaner and more efficient light load operation at all engine speeds, and it could also allow very late 5 degree ATDC full load spark timing for best possible torqeu way down at 1,000 and 1,400RPM. All the way through the very low engine speeds from 1,000 to about 2,000RPM or 2,500RPM load dependant spark timing to deliver precise late spark timing under heavier loads would mean smoother and more powerfull torqeu in full flame front travel mode. This does however require either a consistent gasoline supply or a full tune up at each fill up. And it also probably requires a general awarness of the fact that gasolien engines always tend to be rather dirty and inefficient in full flame front travel mode, no mater how well they are tuned.
When I took the 1990 WMX 610 with the stock SEM CDI ignition out for a longer ride it worked pretty well, and I was amazed to have power all the way up to 6,000 feet of elevation. On the gasolien that day it was very crisp down at 1,000 feet of elevation, but it was ridable and made power. Up at 4,000 feet of elevation there was some noticeable extra lag at all engine speed above 2,800RPM, but it continued to run and make power. Up at 6,000 feet of elevation there was a lot of lag, and torqeu was not exactly instant. The power was there though, and once well warmed up on a big pull I was able to rev the motor out and get some substantial power.
What really surprised me though was that there was some bobbling and stumbling at very low engine speeds and very small throttle openings up at 6,300 feet of elevation. The engine did not stall, but it sounded like it might. It was still able to low idle, and it was still able to restart with the kick starer after I shut it off. When Riding slowly in first and second gears though there was a bunch of bobbling and stumbling when the throttle was first cracked open a small amount. Very similar to how the 1997 610 motor ran in early summer of 2015 with the SEM ignition and the plug gap set way down at 0.009".
WHen I descended back down to 5,000 feet of elevation the bobbling was totally gone. It was only up there above 6,000 feet of elevation that the small 0.017" spark plug gap had been seeming problematic. Down at lower elevations the idle speed was still very low, but it was stable and the engine revved up crisply as soon as the throttle was cracked open.
On the longer ride I had also been impressed with how the White Power suspension on the 1990 WMX 610 was woring. I thought it was perhaps even slightly better than on my other 1991 WMX 610 bikes with all stock White Power suspension. The five clicks of rebound damping on the left leg was actually seeming excessive, and I went down to just four clicks in. I also went out from the #3 compression settting to the #2 compression setting that I normally run on the other 1991 WMX 610 bikes.
The front end felt a bit divey at times under heavy braking, but it was working great all around. Really just as well as the other 1991 WMX 610 bikes. I think the modification to the right fork leg was probably done to allow the stock recomended 10 weight fork oil to be used. With the five weight fork oil that works well in the stock 1991 WMX 610 40mm White Power forks the modified damping in the right leg was perhaps only very marginally better or about the same. Even with the five weigh oil I was noticeing that the odified suspension was perhaps a bit more complient over small bumps at medium speeds.
The front brake was weak at first, but this seemed to be due just to contamination on the rotor. After some big high speed applications of the front brake the rotor and pads were burning clean and the front brake was working great.
Mostly what I was impressed with was just the reliable and powerfull torque from 3,000 to 4,000RPM over a very wide range of elevations. The amount of power output was low compared to what the 610 bikes normally do, but the torqeu was fairly good and very reliable. Each time I twisted the throttle the torque was there, and this was very good for trail riding. The lack of any surging at 3,000RPM, as has plagued the Czech Republic CDI on the stock 10.2:1 motor, was also very nice. Torque was of course progressively harsher and weaker as the engine speed dropped bellow 3,000RPM, but there was no trouble with stretching the torque down rather low to around 2,600RPM on the weak and somewhat low temperatuer of combustion potential gasoline.