Since the spring of 2015 my 1992 Showa Husqvarna mostly just sat there. I never wanted to ride it because the Showa suspension was so atrociously harsh, uncomfortable and pathetically slow. Then in October of 2017 as other projects came to a close I noticed that several 1999 Husqvarna Sachs shocks were advertised for sale used. The one that caught my eye particularly was a 1999 Husqvarna TC 610 Sachs shock for under $200 delivered. The 1999 Sachs shocks are the same 17-5/8" length as the 1992 Showa shocks, and the lower mount is also of the same type. Without even bothering to take the 1999 Sachs shock off of my 1999 Husqvarna 410 I just went ahead and ordered the used 1999 Husqvarna TC 610 Sachs shock hopping that it would fit on the 1992 linkage. I figured I could make it work somehow, even if I had to modify a 1999 linkage to go on the 1992 frame or modify the 1992 frame to take the 1999 linkage. As it turned out the 1999 Sachs shock went right on in place of the 1992 Showa shock; they are fully interchangeable in terms of sizes and mounting.
The Slow Demise of the 1992 Showa Husqvarna
1999 Sachs Shock Install
1999 TC 610 Sachs Shock Performance
An Attempt at the 1992 Showa Forks
Main Jet Shenanigans
Excessive Spark Advance Causes Surging
Chain and Sprocket Tricks
It was really only just for a few months in the spring of 2015 that I rode the 1992 Showa Husqvarna much at all. It was nice to have some sort of a chassis to put my old 1991 Husqvarna WMX 610 motor in, but the 1992 Showa suspension was very disappointing. As soon as I got my old 1991 Husqvarna WMX 610  going again with the 1994 rear wheel, the rebuilt 1997 TE 610 six speed motor, 1997 TE 610 air box and other new and used replacement parts I had little use for the 1992 Showa Husqvarna and it's brutal harsh uncomfortable ride. Pretty much the only thing I ever took the 1992 Showa Husqvarna out for was just to compare the stock 10.2:1 points ignition 610 motor to the rebuilt 11:1 hot rod 610 motor. Since the 10.2:1 motor didn't work anywhere near as well as the 11:1 motor did back in 2015 there really wasn't much reason at all to ride the 1992 Showa Husqvarna.
Then after I got the stock 10.2:1 Czech Republic CDI 1991 Husqvarna WMX 610 going early in 2016 (see A Tail of Two Huskies) I had a stock 10.2:1 motor for comparison without having to deal with the crappy 1992 suspension at all. Then the 1992 Showa Husqvarna really just sat there hardly ever getting ridden at all. Just because the 1992 Showa Husqvarna didn't get ridden much didn't mean that things didn't change on it though. Crazy things started going wrong with the 1992 Showa Husqvarna Chassis shortly after I put the 610 motor in it.
The first crazy failure happened back in late February of 2015 just one week after I got the old 610 motor going in the 1992 Showa Husqvarna Chassis. I came back from a ride and I saw that one side of the linkage mount on the 1992 swing arm was broken. What the heck? How could one side break and leave the entire suspension load hanging cantilevered off the other side without that second side breaking? That just didn't make any sense at all.
The next crazy failure was that the right hand side rebound clicker stopped working on the 1992 Showa forks. The left hand side rebound clicker was still the same, but turning the right hand side rebound clicker didn't do anything. The rebound clickers on the 1992 Showa forks seemed useless anyway, the forks didn't work no matter where the clickers were set. There seemed to be tons of rebound damping even with the rebound clickers all the way out, and there was also tons of harshness with all of the clickers turned all the way out. Especially at the rear end there was just a huge amount of rebound damping even with the rebound clicker turned all the way out on the 1992 Showa shock, and the rebound clickers on the 1992 Showa forks didn't seem to be useful either. Then that right hand side rebound clicker seemed to totally stop working. It just kept turning and turning without any stop. This hardly seemed to matter though since the 1992 Showa suspension was just so horrible in nearly every imaginable way. Going in on the compression clickers at both ends did increase hold up to resist bottoming, but then the bike also got harsher and even more uncomfortable. Even with the compression clickers in substantially both ends still bottomed easily. It just seemed like the wrong kind of compression damping. The 1992 Showa suspension didn't work.
The next crazy failure was that both of the 1992 Showa fork legs and the 1992 Show shock started leaking at the same time. None of the 1992 Showa suspension components had leaked before, then one day I noticed that both forks and the shock were all dripping oil. Despite dripping what seemed like large quantities of oil the 1992 Showa suspension continued to work pretty much exactly the same for many months. All through 2016 the oil continued to drip on the garage floor, but each time I took the 1992 Showa Husqvarna out for a little ride it was the same horrible harsh uncomfortable suspension.
Then for most of 2017 the 1992 Showa Husqvarna just sat there without ever firing up. At one point I drained the gasoline out of the tank, and the last little bit out off the bottom was thick, oily and didn't evaporate. I did run that old gasoline through another engine without much difficulty though, so it was just a bit of oily residue that had developed on the bottom of the tank as opposed to the gasoline completely failing.
Then when I finally got around to firing the old 610 motor up again it wouldn't start. It wouldn't fire at all without the choke when I tried to roll start the bike, then when I put the choke on it fired up but ran for only a few seconds and died. That was very strange. Usually all of the 1990's Husqvarna motors fire up very easily when roll started, and it doesn't usually require the choke at all even in cold conditions so that was very strange that it wouldn't fire at all until I put the choke on. Then it was even stranger that the engine ran for only a short time and died even though I had flipped the choke off right away as soon as the engine fired.
I pushed the bike back up the hill and tried again with the choke on, and this time I left the choke on. Again the engine fired right up with the choke on, but then it died after a few seconds with the choke on. I pushed the 1992 Husqvarna back up the hill and rolled it without the choke and this time it fired but ran for only a few seconds and died. I pushed the 1992 Husqvarna back up the hill several more times, and each time it was the same. The engine would fire up but it would only run for a short few seconds before cutting out bad and dying altogether after another second or so.
I thought the problem might have something to do with the oily residue I had found in the bottom of the 1992 gas tank, so I drained the carburetor bowel and let it refill with fresh gasoline from the tank. That didn't seem to change much. The engine would still only fire up for a few seconds before cutting out and dying. I found that I could get the engine to run longest by putting the choke on and then flipping it off as soon as the engine fired up. Putting the choke back on after a second didn't seem to help at all though. The engine still just died each time.
Eventually I did get the engine to fire up and continue to run, so part of the problem did seem to be the oily residue in the carburetor bowel. Once the engine continued to run it would only idle though. It was idling along without stalling, but it wouldn't rev up. I was able to ride the bike around, but there was no torque at all. It would only run at around 2,000 to 2,500RPM. This might seem like some sort of a carburetor problem, but I knew better. I knew it was another failed ignition condenser. The engine actually was able to take wide throttle openings way down at 2,000RPM, but as soon as the engine speed came up just a bit there was a huge amount of cutting out and no torque. The engine would also rev up a bit to around 4,000RPM with extremely small throttle openings, but again there wasn't any torque. All it would do was idle, and this obviously was another failed ignition condenser.
I went down to the auto parts store for another new ignition condenser, and when I got back I took the 1992 Showa Husqvarna out again with the new ignition condenser in my shirt pocket and the old failed condenser still on the engine. The engine fired up when I roll started it, but it wouldn't low idle. All it would do was high idle with the throttle cracked open a very slight bit, and then it wouldn't rev up, wouldn't take wider throttle openings and wouldn't make any torque. It was however just the same in terms of being able to take wide throttle openings for a second down around 2,000RPM, and it was also the same in terms of being able to rev up a bit with extremely small throttle openings. The failed condenser was still exactly the same, but the gasoline had also disappeared while I was gone. Yes, it literally had disappeared. The tank was essentially empty. The quart of gasoline I had put in earlier was nearly totally gone. There was however sill a few ounces sloshing around on the bottom of the tank, so it was obvious that the carburetor bowel was still remaining full. The 1990's Husqvarnas have petcocks on both sides of the tank, so if there is any gasoline in the tank at all the carburetor bowel stays full.
At this point I installed the new ignition condenser. The engine then still wouldn't low idle, but it did make torque and power when I opened the throttle. There was tons of lean stumbling at all small throttle openings, the engine was very reluctant to get going and it still wouldn't low idle but there was some torque and power at wider throttle openings once it did get going. When I added more gasoline the 10.2:1 points ignition 610 motor was again running and pulling and it was also able to low idle again.
At a 19 degree BTDC static timing setting the stock 10.2:1 points ignition 610 motor was kick starting easily and idling along well. It was able to make torque and power, but there was tons of lag around 3,000 to 5,000RPM and even some bad hesitation. Mostly it was just lots of lag. It took a long second to get going and make torque after the throttle was opened at all lower engine speeds. There was quite a bit of smooth and very quiet torque at small throttle openings around 3,000 to 4,000RPM, and then opening the throttle farther the engine gave a loud bark and delivered a lot more torque. The lag was in the delivery of that barking stronger torque. Opening the throttle nothing happened at first, and only a second later did the barking torque come on strong. Sometimes around 4,000RPM nothing at all happened when I opened the throttle. I had to start down lower and get the barking torque going before it would rev past 4,000RPM and up to about 5,000RPM where the stronger power took off. There was power up higher, but it also was somewhat reluctant to get going and then the power seemed to be ending abruptly rather early.
I then bumped the static timing setting up to 21 degrees BTDC, and this got the stock 10.2:1 points ignition 610 motor going much more substantially. There was still a lot of lag, but the hesitation was gone and torque and power were reliably available at all engine speeds. The lag was considerably diminished also, and the engine seemed to be working fairly well. Top end power was more instant also, and the power continued up higher without such an early abrupt end.
This was all with the stock 1992 Showa suspension, but it wasn't working like stock. The oil had finally all leaked out of the 1992 Showa shock, and there just wasn't any damping at all at the rear of the bike. Obviously a total lack of all rebound damping didn't work at all. The bike was out of control and scary. The ride quality and comfort was however much improved with the harsh Showa damping entirely removed. The 1992 Showa forks didn't feel exactly good, but it was apparent that a large portion of the extreme harshness and discomfort of the 1992 Showa suspension had been on the rear end.
Installing the 1999 Husqvarna TC 610 Sachs shock was fairly straight forward, but not without some frustration. The 1992 Husqvarna rear end is just tricky to take apart and put back together. Things have to be done in the correct order, and these procedures are not well described in the 1992 Husqvarna Owner's Service and Tuning Manual. The 1992 Husqvarna Owner's Service and Tuning Manual says that the exhaust system has to be removed to remove the shock. This isn't true. The shock comes out fine with the exhaust system fully installed. The 1992 Shock does not however come out the top like on the 1990/1991 Husqvarnas. Instead the 1992 shock comes out the back of the bike with the exhaust system and sub frame still fully bolted in place.
The real difficulty with the 1992 Husqvarna is installing the shock on the linkage, or rather just in re-installing the 1992 linkage on the bike. It turned out that the 1999 TC 610 Sachs shock does indeed go right on the 1992 linkage without any difficulty, but that 1992 linkage is always a bit tricky to put back together. I remembered that from when I replaced the broken swing arm. The bolts just have to be installed in the correct order. The lower shock mount has to be torqued down before the linkage is stuffed up into the swing arm because there isn't any room to torque that bolt down once the linkage is assembled. The rear drag link bolt on the other hand has to be left loose until the linkage is up in the swing arm. If that rear bolt is torqued down first then there isn't quite enough clearance for the linkage to swing up into the swing arm. This is all straight forward and not that difficult, but the 1992 Husqvarna Owner's Service and Tuning Manual makes no mention of this strictly required order of bolt installation and tightening. That's frustrating.
The 1999 Husqvarna TC 610 Sachs shock I got seemed to be in fair condition. There was a bit of rust on the body of the shock, and the pre-load adjuster rings were somewhat stuck, but overall it looked faily good. The pre-load adjuster rings had also been beaten on severely enough that the edges were mushroomed over and jamming on each other a bit which also made it difficult to turn the rings. Once I filed the protruding ends of the mushroomed over tangs down and got the threads moving freely the pre-load adjuster rings worked fine. There wasn't any sign of oil having leaked out of this 1999 Husqvarna TC 610 Sachs shock, so it looked like it would probably work.
As soon as I rode the 1992 Husqvarna with the 1999 TC 610 Sachs shock I was very impressed with the large improvement over the 1992 Showa shock. What I noticed right away was actually that the low speed compression clicker didn't work, which was very disappointing. I also noticed that the action of the 1999 TC 610 Sachs shock felt thick and harsher than the 1999 TE 410 Sachs shock on my 1999 Husqvarna WMX 410. Flat, no compression damping and also harsher and thick feeling. That was disappointing. What was good about the 1999 TC 610 Sachs shock though was that it was otherwise very similar to the 1999 TE 410 Sachs shock, and compared to the very bad 1992 Showa shock that was very good.
The really big thing that was readily apparent was that the packing up of the rear suspension on the 1992 Husqvarna over wide spaced bumps was gone. That made the bike a lot faster over uneven terrain. For the first time I could really open up the 610 motor in the 1992 Husqvarna and go fast over uneven terrain like on the 1991 Husqvarnas. That was obviously a really big improvement. Interestingly the 1999 TC 610 Sachs shock was also a lot smoother and more comfortable than the 1992 Showa shock. The 1999 TC 610 Sachs shock might have felt thick and somewhat harsh compared to the 1999 TE 410 Sachs shock, but it was still a very large improvement in comfort and smoothness compared to the 1992 Showa shock.
The thick feeling of the 1999 TC 610 Sachs shock compared to the 1999 TE 410 Sachs shock felt like it was just in the weight of the oil. The flat 1999 TC 610 Sachs shock felt like it had thicker oil in it, and that was somewhat hiding the lack of a functional low speed compression damping circuit. I thought perhaps I was noticing a small increase in hold up and impact absorption when I went in a few clicks on the low speed compression clicker on the 1999 TC 610 Sachs shock, but it was hard to tell. Mostly it seemed like there was just no response at all from the low speed compression clicker. That is very different from the TE 410 Sachs shock on my 1999 Husqvarna 410 which has a very noticeable increase in hold up and impact absorption with each single click in on the low speed compression adjuster.
The 1999 TC 610 Sachs shock was also feeling like it had too much rebound damping with the clicker in 12 clicks. Much more rebound damping than on the 1999 TE 410 Sachs shock with 15 clicks in. Overall the flat 1999 TC 610 Sachs shock was just feeling very thick and over damped, and that seemed to be due to thicker oil.
Even though the 1999 TC 610 Sachs shock was feeling very thick with too much rebound damping it was still working fairly well. It seemed like a lot of rebound damping, but it was fairly competent rebound damping that wasn't slowing the bike down much. I did however notice that the rear wheel felt slow to come down after going over bumps and this was causing the rear end to step out sometimes under acceleration. The 1992 Showa shock had done this much worse even with it's rebound clicker all the way out. On the 1999 TC 610 Sachs shock with the rebound clicker in 12 clicks there was only seeming to be a little bit of loss of drive traction due to the excessive rebound damping.
I tried going all the way out to just 2 clicks in on the rebound adjuster on the 1999 TC 610 Sachs shock, and this made a very noticeable difference. Without the low speed compression damping working going all the way out on the rebound clicker didn't work well. The rear end was moving around too much even though it still felt like there was quite a bit of rebound damping. I then went in on the rebound clicker to 6 clicks in, and that seemed like a good compromise setting. The 1999 TC 610 Sachs shock seems to have potential, it just needs an oil change and a nitrogen gas charge. It needs the oil changed because the oil that is in it is obviously far too thick, and it needs a nitrogen gas charge so that the low speed compression clicker will actually increase the low speed compression damping when it is turned in.
The 1992 Husqvarna chassis was working much better with the 1999 TC 610 Sachs shock installed, but it was still harsh at both ends and a bit awkward feeling at speed. The 1992 Show forks seemed to be the main problem; they just remained very harsh and uncomfortable at anything above about 10mph. At extremely low speeds the action of the 1992 Showa forks has always seemed pretty good, with good compliance over small bumps and a fairly smooth feel. As soon as the bike speed comes up just a little bit though the unreasonable harshness sets in very quickly and remains problematic at all higher bike speeds.
Turning the compression clickers all the way out on the 1992 Showa forks did help a small bit, but the front end remained very harsh and then the forks also bottomed extremely easily. Going in on the compression clickers seemed to increase the harshness only slightly while providing quite a bit of increased hold up and impact absorption capability. I got the idea that the 1992 Showa forks might be better with thinner oil.
I started with 4W Motorex fork oil from the local KTM dealer. I was going to try the 2.5W Bel Ray fork oil that I had seen in stock at another local store months earlier, but when I went back they only had 30W, 20W, 15W, 10W, 7W, 5W and no 2.5W Bel Ray fork oil. When I asked what had happened to the 2.5W the guy at the counter said that they had never sold any of it, so they eventually stopped offering it.
When I installed the 4W oil I also replaced the fork seals, which is pretty simple on the 1992 Showa forks. They come apart a bit differently than the 1991 White Power forks, but the fork seal replacement is still straight forward and simple. The only trick is getting the order of the bushings, seals and shims correct on the lower fork tubes before sliding it all back together. I did this wrong a few times, and had to pull it apart again. In this regard the procedure outlined in the 1992 Husqvarna Owner's Service and Tuning Manual is correct, but I was still so pissed off about the lack of information on the shock linkage that I hadn't even bothered to look at the manual until after got the order of parts wrong a few times on the fork tubes. There is however an error in the 1992 Husqvarna Owner's Service and Tuning manual in regard to the 1992 Showa forks. The manual says that the damper cartridges need to be removed to change the fork seals, but this isn't true. The damper cartridges can be left bolted to the lower fork legs without any difficulty for an oil change and fork seal change. This is significant because it is a bit difficult to get the lower bolt back in that holds the damper cartridge in. I did take one of those bolts out, which was no problem at all, but then going back in the damper cartridge spins and the bolt won't tighten down. I was able to get the bolt to start in using an impact wrench, and then once the bolt starts to tighten the damper cartridge holds tight and the bolt can be torqued down with a wrench. The 1992 Husqvarna Owner's Service and Tuning Manual makes no mention of any of this about how to get the damper cartridges bolted back in.
The only reason to take the damper cartridges out is to more easily pump the last ounce or so of fork oil out. Leaving the damper cartridges bolted in most of the fork oil pours right out, but the last little bit doesn't seem to want to come out. This is significant if the old oil is very black and dirty, as it then requires using a bit of extra fork oil to rinse the stubborn damper cartridges clean.
The 4W Motorex fork oil seemed pretty much exactly like the 5W oil that was in the 1992 Showa forks before. The old 5W oil was black and very nasty looking, but the viscosity seemed the same as new 5W oil and very nearly identical to the 4W oil. I ran a few drops of the new 4W oil down a sloping board next to a few drops of 5W oil and I couldn't really see any difference.
I put the 4W oil in the 1992 Showa forks anyway as it was all that I had that day. The result was pretty much the same. Perhaps there was a slight bit of a change, but not much at all. There was still a lot of excess harshness from the 1992 Showa forks, and the response of the compression clickers seemed pretty much the same. There was still a huge amount of compression damping with the compression clickers all the way in, so there seemed like room to go even thinner on the oil.
I ordered 2.5W Bel Ray oil and waited for it to show up. And waited, and waited and waited. When the first company didn't deliver I placed a second order, and a third order. Eventually the packages started showing up. It seemed like a 2.5W oil conspiracy, as all of the mail order companies had claimed to have the 2.5W Bel Ray oil in stock and ready to ship.
When I finally had a bottle of 2.5W Bel Ray fork oil in hand I took the 1992 Husqvarna out for a test ride on the 4W Motorex fork oil. It seemed the same. Still tons of compression damping with the compression clickers turned all the way in.
I pulled the front end apart, poured the 4W Motorex fork oil out and added the 2.5 Bel Ray fork oil. I did this without even taking the springs out this time, so I was just guessing about the fork oil level. Using just a single 1L bottle of 2.5W fork oil for 500ml in each leg resulted in a somewhat low oil level since the 1992 Showa 45mm USD forks hold as much as about 580ml up to the maximum recommended oil height.
The 1992 Showa forks were better with the thinner 2.5W oil. I started out with the compression clickers all the way in, and the compression damping was still very excessive. I did notice though that the forks were smoother and worked better with the 2.5W oil.
Then I turned the rebound clickers all the way in and turned the compression clickers all the way out. This was worse. The rebound damping was extremely excessive to the point where the front end didn't come up after compressing, and the forks were also very harsh through the entire stroke. Very harsh with the compression clickers all the way out? Strange.
Next I went all the way out on all the clickers. This was much better, but the rebound damping still felt excessive. It was that right hand rebound clicker. It wasn't working. I began to suspect that the rebound damping was stuck in the maximum position on the right hand side. When I had the forks apart the first time to install the new fork seals and the 4W Motorex fork oil I had noticed that the right hand side damper cartridge moved out on the rebound stroke a lot more slowly than the left hand side damper cartridge even with the clickers all the way out. This seemed very wrong, but I didn't know what to do about it so I ignored the problem.
With the 1992 Showa forks working noticeably better with the 2.5W oil I got motivated to try to figure out what was going on with the right side rebound clicker. I took the handle bars off and pulled the right side fork cap off with the forks still bolted on the bike. With the bike sitting on the left fork spring only there was a very substantial amount of bike sag so that the right side damper rod stuck up out of the fork tube.
The rebound clickers on the 1992 Showa forks are just clickers that bolt to the tops of the damper rods. When I unbolted the right hand side rebound clicker there wasn't anything wrong with it. The clicker moved freely and seemed perfectly new. The clicker has a "D" shaped lower end something like on a potentiometer or a gas valve on a cook stove. That "D" shaped lower end fits into a long "D" shaped steel tube that goes down through the damper rod and into the damper cartridge. When I pulled this "D" shaped tube out I found that the lower end was stripped out. Or rather that the lower end of the "D" shaped tube was deformed out into a round shape. This seemed like it might be the problem, the "D" shaped tube was being turned by the rebound clicker, but it wasn't actually turning the rebound adjuster in the damper cartridge.
I flipped the "D" shaped tube end for end so that the pristine end went down into the damper cartridge and the deformed end was up at the rebound clicker. The "D" shaped tube engages with the rebound clicker over a long half inch length so the approximately 1/8" long deformed end part didn't prevent the clicker from engaging with the tube.
This did the trick. When I turned the right side rebound clicker out counter clockwise the rebound damping did decrease. I could feel the rebound damping substantially decrease just pushing the damper rod in and out by hand.
With the forks back together I went out for another test ride. This time I rode gingerly and carefully at first with all the clickers all the way out. Sure enough the 1992 Showa forks felt floppy and under damped for the first time. I then turned the left hand side rebound clicker all the way in, and the forks got super harsh and stiff with tons of rebound damping.
What I noticed was that going in on the rebound clicker increased the rebound damping, but it also increased the harsh compression damping a whole lot. When I went all the way out on the rebound clickers again the front end was a lot smoother and more comfortable. Next I tried going in just a little bit on the rebound clickers, but this also drastically increased the harsh compression damping and the ride was noticeably a whole lot worse. So that's the real problem with the 1992 Showa forks; the rebound adjusters don't work. They do increase the rebound damping, but they also increase the harsh compression damping so much that they can't reasonably be turned in at all. That's very bad.
With all of the clickers all the way out with 2.5W fork oil the 1992 Showa forks were feeling somewhat close to reasonable in terms of comfort and plushness over sharp bumps, but clearly there wasn't enough of any sort of useful damping either.
Going in on the compression clickers did improve the action of the 1992 Showa forks. The increased hold up with more compression damping slowed the front end down and substantially made up for the lack of rebound damping. What was interesting was that going in on the compression clickers didn't increase harshness and discomfort as much as going in on the bad rebound adjusters.
With the bad rebound adjusters still all the way out I experimented with different settings on the compression clickers. First I went down from the maximum setting at 16 clicks in. With the compression clickers all the way in the front end was stiff and hardly moved. This worked well for aggressive high speed riding, but the front end was brutally harsh over all types of bumps. Going out to just 12 clicks in on the compression clickers improved performance drastically. The damping still felt very stiff and very substantial, but the ride was smoother and the bike actually seemed easier to control. I then went out to 10 clicks in on the compression clickers, and again the front end felt better. It was the same with going out to 8 clicks in, a noticeable increase in comfort and compliance.
Then I tried going from the other end of the range of adjustment. I started with the compression clickers all the way out, and the front end was just very floppy and under damped. I went in on the compression clickers to 6 clicks in, and this was a large improvement. The bike felt much more stable and in control, and the ride wasn't all that much harsher. I then went in to 8 clicks in on the compression clickers, and this also seemed like an improvement. Going in from 6 to 8 clicks in on the compression clickers made a noticeable improvement in hold up and impact absorption without the front end getting all that much harsher. Going from both directions I landed at 8 clicks in on the compression clickers, so that seemed like the best setting.
The problem is that the rebound damping still feels insufficient for very fast aggressive riding over uneven terrain. The 1992 Showa forks just don't quite work. The compression clickers do increase a good sort of compression damping that provides some hold up and impact absorption capability without increasing harshness all that much. The rebound adjusters though don't work, and that is a problem.
When I rode the improved and upgraded 1992 Husqvarna back to back with some of my other Husqvarnas it obviously wasn't as good. The 1992 is still the worst of the 1990's Husqvarnas I have. The 1992 Husqvarna is still harsher at both ends, and it just doesn't work as well. The flat 1999 TC 610 Sachs shock is harsher and more uncomfortable, but then it also lacks hold up and impact absorption capability with the low speed compression clicker not working. The 1992 Showa forks with 2.5W fork oil are still harsher than the 1991 Husqvarna WMX 610 White Power forks with 5W oil, and the 1992 Showa forks also lack rebound control which makes the bike slow and out of control when pushed hard. What really drives it home about the 1992 Showa forks is that even with 2.5W oil and all the clickers all the way out so that the front end flops around badly they are still harsher and more uncomfortable over sharp bumps than the 1991 Husqvarna WMX 610 White Power forks with 5W oil and the compression clickers in one click to provide very substantial hold up and impact absorption. That looks very bad for the 1992 Showa forks. Going in on the compression clickers on the 1992 Showa forks improves performance quite a lot, but then they do get even harsher and more uncomfortable also.
With the 1999 TC 610 Sachs shock the 1992 Husqvarna is drastically improved though. It might still be harsher and slower than the 1991 Husqvarna WMX 610 White Power suspension, but the gap has been narrowed considerably. The 1992 Husqvarna was seeming good enough that I did ride it around some, and it was fun to give the old 10.2:1 points ignition 610 motor a chance to run. It was running pretty well also. Particularly the torque down around 3,000 to 4,00RPM was fairly strong, very reliable and quite smooth. Quiet it is not with the loud 1992 exhaust system, but running 21 degree BTDC spark timing the torque remained quite smooth and very usable down to 3,500 and even down to around 3,000RPM. Up higher though the power was sort of hit and miss. Most of the time there was top end power, although not huge amounts of it. Sometimes the engine was also reluctant to get going around 4,500 to 6,000RPM, and this seemed to come at the same time as increased lag down at 3,000 to 4,000RPM. At one point there was tons of lag down low, and then there seemed to be a bit of a dead spot around 5,000RPM. I checked the static timing setting, and it was looking like it was down closer to 20 degrees BTDC again so I bumped it up to 22 degrees BTDC. This got the torque coming on much more instantly down low at 3,000 and 4,000RPM even after the engine had cooled off from coasting down hills. Going up to 22 degrees BTDC also improved the instant response around 5,000RPM, and the engine seemed to be working tolerably well. There was seemingly fairly good torque down low to 3,000RPM, torque increased nicely up from 3,000 to 5,000RPM and then the top end power took off around 6,000RPM and kept pulling reasonably long. The power and torque were reliable, instant and fairly strong and there wasn't any surging at any engine speed. Fairly good operation overall even if top end power was only sort of respectable.
The motor was running and pulling fairly well, but then the next day it was hard to start. It took lots of kicks with prodigious use of the choke to get it going when it was cold, and then even once fully warmed up there were problems. The low idle was down extremely low and somewhat unstable. It wasn't stalling, but it was idling down much lower than is usually possible with that much spark advance and there was also a very noticeable ethanol exhaust smell. Not the acrid alcohol exhaust smell that I associate with methanol, but the milder ethanol exhaust smell. It was the milder ethanol alcohol exhaust smell, but there was quite a lot of it. It seemed like a huge amount of ethanol in the gasoline. The engine was also popping out the exhaust a huge amount on deceleration. The really significant change though was that the stock 10.2:1 points ignition 610 motor was requiring the choke to restart even when the cooling jacket was still substantially warm to the touch. It was restarting without the choke immediately after being shut off, but if it sat for even a short 20 minutes it was then hard to start.
Drastically increased popping out the exhaust, a very low and unstable idle, an inability to start without the choke even with the cooling jacket substantially warm and there was a lot of ethanol exhaust smell. That all seems to add up to severely watered down ethanol laden gasoline. It seemed like more than 20% ethanol. It actually seemed like it was probably around 30 or 40% ethanol. Interestingly the power and torque were still respectable though. Not huge amounts of power, but substantial power none the less and still no surging and no cutting out. Fairly good torque and power once running, but way too hard to start and lots of annoying signs of severely watered down gasoline. The gasoline was also disappearing quickly. At one point I ran out of gas unexpectedly and had to push the bike back a half mile, which luckily was mostly down hill.
Even after putting more gasoline in the tank the hard starting, huge popping out the exhaust and strong ethanol exhaust smell persisted. The big problem was hard starting. It was taking lots of kicks to get the thing going first thing in the morning, and then if it sat for even a half hour during the day it was again requiring the choke and a bunch of kicks. It was firing easily every time without the choke after a single kick with the choke had gotten some gasoline flowing, but often it only fired low and weak for a few revolutions and then died. Over and over it would fire low and weak like that and die so that it was taking several times more kicks than normal to get it started from cold.
The cranking compression was seeming substantially high for that worn out old motor. Once up on compression it took a long few seconds of pushing down on the kick starter to bump past compression. Enough cranking compression that the manual compression release was a very useful feature for getting it past compression quickly and easily in setting up for a kick. The old worn out 610 motor with the incorrectly super wide gapped compression ring obviously didn't have as much cranking compression as my other nearly new 610 motors, but still substantially as much cranking compression as this old worn out 610 motor has ever had in the 20 years that I have owned it. It takes a much larger reduction in cranking compression than that to actually prevent starting. When the exhaust valves stopped sealing back in 2005 the kick starter was just falling right past compression with hardly any noticeable resistance before the engine wouldn't start anymore.
The fact that the old 10.2:1 points ignition 610 motor fires easily, but runs low and weak and dies also indicates that it isn't a problem with low cranking compression. When an engine won't fire because of low cranking compression it just won't fire at all with the kick starter, not even a weak low little pop. No, those are two totally different and unrelated phenomenon. Hard starting can be caused by lots of different things, but low cranking compression only causes hard starting once the cranking compression is so low that the compressed intake charge can substantially leak past the rings in the short time that the piston is comming up to top dead center. When it takes several long seconds to bump the engine past compression pushing down on the kick starter compared to the engine just falling effortessly past compression with the manual compression release pulled that really is very substantial cranking compression.
The first thing I messed around with was the idle mixture screw, but that didn't seem to do any good. No matter where I set the idle mixture screw there was still tons of popping out the exhaust and the bike wouldn't restart easily after sitting for just 30 minutes. With the engine low idling, turning the idle mixture screw in substantially caused the engine to stall as usual, but turning the idle mixture screw out didn't seem to help much with reducing popping out the exhaust. The idle mixture screws on the DellOrto carburetors are on a mixing type circuit where both gasoline and air flow past the idle mixture screw. This seems to work fine, and turning the idle mixture screw in usually does lean out the idle a small amount and turning the idle mixture screw out does usually richen the idle mixture a small amount. The total range of adjustment is however rather narrow, and on severely watered down 40% ethanol garbage gasoline it just didn't quite work. With that much ethanol in the gasoline turning the idle mixture screw out just didn't seem to do anything. It was still so lean that it would barely run, and it wouldn't start at all without the choke once it had cooled off even slightly.
Interestingly though the power and torque remained fairly strong. Not huge, but respectable strong without any surging or cutting out anywhere. It seemed like it was probably race gas mixed with the large quantity of ethanol. Even with 40% ethanol that remaining 60% race gas was able to make some fairly substantial torque and power with rich jetting.
Sort of on a witch hunt for the cause of the problem I decided to pull the main jet out. Sure enough there was a huge hole in it! I could see right away that the hole in the main jet was much larger than the last time I had seen it. When I measured that huge hole I found it to be well over 185, about the 190 size I would say. A 0.0727" diameter #49 drill stock just fell right through the giant hole, and there was quite a bit of side to side wiggle also. Certainly bigger than 188 size, and probably at least up at a 190 size. A huge oversized hole in the main jet! Someone had drilled out my original stock 180 size main jet, probably while the bike was sitting all those months and never being ridden. So that was the problem. The main jet had been drilled out, and that was somehow attracting gasoline with a huge amount of ethanol in it.
To repair the main jet I soldered it up and re-drilled it to the same 175 size main jet that I have in all my other Husqvarna 610 motors. With a fresh tank of 87 (RON+MON)/2 octane rating regular gasoline straight from the gas station the sock 10.2:1 points ignition 610 motor was still having some difficulties, but it did run better. That fresh 87 (RON+MON)/2 octane rating regular gasoline at first had a noticeably somewhat slower flame front travel speed, and it was still severely watered down with ethanol so that the exhaust smelled noticeably of ethanol exhaust smell. It wasn't however quite as severely watered down as the garbage that had been showing up in the tank with the 190 size main jet.
The slower flame front travel speed regular was providing very instant and very crisp torque down low at 3,000RPM and even slightly lower, and that torque was building nicely up to around 4,500RPM. Then there was a bit of reluctance to get going around 5,000RPM followed by some good top end power. With the needle clip in the second groove and with the 175 size main jet there wasn't much of any richening of the mixture from 1/2 throttle to 3/4 throttle. With the static timing setting still at 22 degrees BTDC the stock 10.2:1 points ignition 610 motor was able to take wide open throttle without any difficulty at lower engine speeds, and there was fairly good toque across all throttle openings at 3,000 and 4,000PM. Then up higher around 5,000RPM though the engine seemed to be having a bit of difficulty. It was running consistently and pulling fairly well at moderate throttle openings across the entire midrange, but then it seemed a bit slow to get going and make big power. If I rolled the throttle on just right the torque built fairly nicely, but it kept seeming like it needed a slightly wider throttle opening to really get going. If I opened the throttle too fast though nothing happened, it just stayed flat and dead feeling around 4,500 to 6,000RPM. Above 6,000RPM though the power was taking off substantially, and then the engine was taking wide open throttle without any difficulty and making some pretty good power without any surging. It was just too reluctant to get going. That was in the evening in the cold and dark without much of a headlight, and it seemed like I just wasn't getting the engine warmed up enough for it to pull consistently.
In the morning with the same 22 degree BTDC static timing setting the stock 10.2:1 points ignition 610 motor ran somewhat differently. The flame front travel speed of the gasoline had increased overnight, and the energy density seemed a bit higher also. The temperature of combustion potential was however lower. The flame front travel speed was obviously higher because the crispness down at 3,000RPM and 2,700RPM decreased. The torque was still very reliable and mostly instant at 3,000 and 4,000RPM, but the torque was more instant at 4,000RPM than at 3,000PRM. A clear indication of a rather fast flame front travel speed gasoline in the big bore 610 motor.
The reluctance to get going around 4,500 and 5,500RPM was different, and mostly gone. There didn't seem to be any problem with opening the throttle quickly, and the engine was very loud all the way through from 3,000 to 6,000RPM. Torque at around 5,000 to 6,000RPM was however slightly low at all throttle openings. There still wasn't any surging, but once I got the engine fully warmed up going through the gears there was a bit of a slipping feeling after up shifting. The little bit of slipping feeling was only present after accelerating hard and then up shifting so that the engine speed dropped down to around 5,500RPM. Just going up from 4,000RPM in one gear the slipping feeling wasn't there, and there wasn't any surging at all. That little slipping feeling was however an indication that the big 3" stroke length engine was getting close to some surging. It still wasn't surging at all, and it was running quite well, but it was lower temperature of combustion potential gasoline that was getting close to some surging around 5,000RPM.
The energy density of the gasoline was still low also. There was still quite a bit of popping out the exhaust on deceleration, but not as severe as it had been on previous days. The low idle was still way down unbelievably low, but the engine still wasn't staling. There was still some noticeable ethanol exhaust smell, but not as strong as it had been on previous days with the 190 size main jet. Most significantly though the engine was still having trouble restarting after sitting for just 30 minutes. It was still requiring the choke to get it to restart with the cooling jacket warm. In general the stock 10.2:1 points ignition 610 motor was just taking a lot more kicks to get started than it ever did in the past, and that was annoying. The combination of hard starting, still quite a bit of popping out the exhaust on deceleration and some noticeable ethanol exhaust smell all adds up to there still being a substantial quantity of ethanol in the gasoline. Less ethanol than the crazy 40% ethanol mixture that was showing up with the 190 size main jet, but still too much ethanol. Still certainly a lot more than the 10% maximum allowable ethanol content under California law.
The 190 size main jet seemed to be attracting 40% ethanol and 60% race gas. The repaired 175 size main jet still with the needle on the second needle clip position was attracting something entirely different. Less ethanol, but also a lot less race gas.
I suspect that the three inch stroke length Husqvarna 610 motor can be made to run strong without surging without any race gas at all, but that probably also requires either no ethanol or that the ethanol content be very low. Like less than 10%. There is probably some midgrade type of normal cheap gasoline that would at least sort of work in a three inch stroke length engine. Perhaps a mixture of half slow flame front travel speed cheap regular gasoline and 50% of a faster flame front travel speed premium gasoline. Or perhaps cheap fast flame front travel speed premium gasoline would work, as long as it's not some dramatically weak specialty fuel. The extremely dramatically lower temperature of combustion potential specialty gasoline seems to need a stroke length shorter than 2.5" to work at all, where cheap slow flame front travel speed regular gasoline has a sufficiently high temperature of combustion potential for the 3" stroke length to at least sort of work but then doesn't have a fast enough flame front travel speed for the 4" bore and no advance curve.
The stock 10.2:1 points ignition 610 motor has actually been running pretty strong. There hasn't been any surging, and torque and power have for the most part been very reliable and fairly instant at 21 and 22 degree BTDC static timing settings. In fact this stock 10.2:1 points ignition 610 motor has never exhibited any surging at any static timing setting at any time since 2002.
On the same cool early November day with the ambient temperature holding steady at 61 degrees Fahrenheit I tried two quarts of the day old 87 (RON+MON)/2 octane rating regular gasoline out of the 1992 Husqvarna with the 10.2:1 points ignition 610 motor in the 9.7:1 386 stroker motor, and it worked fairly well. Again there wasn't any surging at any engine speed, and both torque and power seemed fairly good. There were a few little slight problems, but overall the 386 stroker motor was running strong with good instant torque across all engine speeds. The main problem was just that sometimes it seemed like I had to twist the throttle more than usual to get the engine going around 5,000 and 6,000RPM. It seemed like the low energy density gasoline was favoring a big twist up onto the 149 size main jet to get going and really make torque around those 5,000 and 6,000RPM midrange engine speeds, but down lower at 3,000 and 4,000RPM the torque seemed strong at all normal throttle openings. The other slight problem was just that the 9.7:1 386 stroker motor seemed to be shutting off a bit early. The torque was increasing nicely up from 3,000 to 4,500RPM and the power seemed pretty good throughout the midrange from 4,000 to 6,500RPM. Then the power really took off right at 7,500RPM. That was interesting that the power took off at 7,500RPM, above the crankshaft wiggle advance at 7,000RPM and well below the intake stack boost at 8,000RPM. The power seemed quite strong for 7,500RPM, but then it only increased slightly out to 8,000 and 8,600RPM. The power was increasing up to 8,600RPM, but then that was it. The 386 stroker motor was able to pull to 8,800RPM, but there wasn't any more power up there and then it seemed totally done at 8,800RPM. That's a bit earlier than the 386 stroker motor usually ends, but the power down at 7,500RPM to 8,000RPM was substantially impressive. When I got back from that little test ride I checked the static timing setting on the 386 stroker motor, and it was down at about 21.5 degrees BTDC. Pretty much exactly the same static timing setting as I was running on the same gasoline in the stock 10.2:1 points ignition 610 motor.
When I tried the same gasoline in the Husqvarna 410 motor I was quite impressed with top end power. Torque down low was substantially strong for the 400cc motor, and then the power really took off above about 6,000RPM. Wow, lots of power. Way more power than from the 386 stroker motor on the same gasoline. Interestingly the 410 motor seemed to be ending early also, but before it shut off there was pretty big power over a range of upper midrange and top end engine speeds. Not quite as much power as from the stock 10.2:1 points ignition 610 motor, but substantially lots of power. I would have to say impressive power from the 400cc engine, and enough barking but smooth and usable torque down low to make it really seem like a highly functional dirt bike engine.
There were two other things that I was impressed with in riding the 1999 Husqvarna WMX 410. The suspension felt way better than the improved suspension on the 1992 Showa/Sachs Husqvarna. The 1999 TE 410 shock was considerably smoother and more comfortable, and also had much better hold up over rolling bumps and landing from jumps. At the front end the 1991 Husqvarna WMX 250 White Power forks were also a lot plusher and more comfortable than the 1992 Showa forks. I went back out to the #3 rebound position on the White Power forks on the 1991 Husqvarna WMX 410, and that seemed better than the #4 rebound position I had been running. Going out to the #3 rebound position increased the plushness and comfort, and the rebound damping still felt plenty substantial. Part of this was in comparison to the insufficient rebound damping on the 1992 Showa forks, but really the 1991 WMX 250 White Power forks seemed to be working fairly well on that short test ride.
The other thing that I noticed was that the 1999 Husqvarna 410 motor was loud. Especially once the power really took off around 6,000RPM the power came on with quite a scream. Not as loud as the thundering 1992 exhaust system and 610 motor, but still lots of noise. I guess all those "twice pipes" 1990's Husqvarna exhaust systems are just loud. Before I had thought that the 1999 exhaust system was fairly quiet, but running on the same gasoline out of the 10.2:1 610 motor the 1999 Husqvarna 410 was pretty darn loud.
For the next head to head comparison I picked up two gallons of 91 (RON+MON)/2 octane rating premium gasoline from a busy gas station near the interstate. I took the two gallons of fresh premium straight home, with no stops, and put it into the empty tank on the 1992 Husqvarna. First I drained the remaining quart of old gasoline, and I even rinsed the tank through with a few ounces of the fresh premium. A clean tank, a clean slate.
Wow, more top end power, and the slipping feeling after shifting was gone also. The popping out the exhaust was not however totally gone, and neither was the ethanol exhaust smell. What was different though was that there was somewhat less popping out the exhaust, and the ethanol exhaust smell was less noticeable. It did seem like slightly higher energy density gasoline, and power output was up noticeably.
Torque from the stock 10.2:1 points ignition 610 motor was seeming similar, and response down around 3,000 and 4,000RPM was seeming very much the same. There was some considerable lag sometimes after coasting, but most of the time the torque came on fairly instantly across all lower engine speeds. There wasn't any hesitation, and the torque was very reliable.
Again the 610 motor was delivering some substantial quiet and smooth torque at small throttle openings down around 3,000 to 4,000RPM, but just a bit wider throttle opening delivered a loud barking large increase in torque. After coasting it was all just very quiet and smooth torque for a long second after the throttle was opened down around 3,000RPM, where once the engine was fully warmed up on some big pulls the barking torque came on much more instantly so that there wasn't any problematic lag in delivery.
There wasn't any difficulty with twisting up to wide open throttle on the 175 size main jet at any engine speed either. Just reliable and instant torque building from 3,000 up to 6,000RPM. Top end power was somewhat stronger than on recent days on the 87 (RON+MON)/2 octane rating regular, and very loud. Loud everywhere really, across all the engine speeds. Up on the top end the loud exhaust note also had somewhat of an early sound to it. The engine sounded like it was hitting extremely early times of late compression ignition across all the upper engine speeds. The sound was a lot like a four stroke outboard motor with a big camshaft running above the normal maximum engine speed of 5,500RPM. It was that sort of a sea sawing top end scream as an engine falls over to early times of late compression ignition without actually making more power. There still wasn't any surging though at any engine speed. None of that slipping feeling anywhere, no surging, no whining and fairly good feeling power delivery. The scream on the top end was sounding very much like it was getting close to doing something very bad, like surging. There wasn't however any surging or any cutting out. The stock 10.2:1 points ignition motor was running fairly well at a static timing setting of about 21 degrees BTDC. Most importantly it was pulling consistently without doing anything horrible like hesitating, surging or cutting out.
With a bit more top end power the lack of rebound damping on the 1992 Showa forks with the rebound clickers all the way out on 2.5W oil was seeming like a problem that needed a solution. It wasn't like the bike was really out of control or doing anything horrible, it just felt like the front end lacked precision and control when really blasting the big rolling bumps. I went in on the rebound clickers just a few clicks, about 15% of the total range of adjustment. That made a huge difference. The ride got harsher at all normal speeds over sharp edged bumps, which was disappointing. The rebound damping did increase a lot also though, and that was very welcome. The increase in rebound damping delivered a drastically more confident and in control feel to the front end, and the bike seemed to work a whole lot better when pushed hard. Yes, it really did need more rebound damping at the front end.
For the first time the 1992 Husqvarna chassis was really seeming like a Husqvarna; that is fast and solidly in control under all conditions.
Next I took a gallon and a half of that fresh premium gasoline that I had just been riding with out of the tank on the 1992 Husqvarna and I put it in the empty tank on my 1991 Husqvarna WMX 610 with the bone stock 10.2:1 Czech Republic CDI ignition 610 motor. Again I drained the old gasoline out, and then I rinsed the tank through with a few ounces of the fresh premium just to make absolutely sure that I wasn't getting any mixing of the fresh pump gas with anything crazy that might have mysteriously showed up in the gas tank overnight.
While I was waiting for the gasoline to drain and while I was verifying the static timing setting post ride the motor had a chance to cool off a bit. Before I set out on the CDI bike I kick started the stock 10.2:1 points ignition 610 motor just to see how easily it would start after sitting for about 20 minutes. The ambient temperature was holding steady at 56 degrees Fahrenheit, which is about as cold an ambient temperature as I usually chose to ride at. It fired right up on the first kick without the choke without difficulty like it always did before. That had only been the extremely drastically watered down 40% ethanol garbage that the mysteriously drilled out 190 size main jet had been attracting that had been preventing easy restarting with a warm cooling jacket.
The big surprise with the bone stock 10.2:1 Czech Republic CDI 610 motor was that it was surging. There wasn't any surging from the stock 10.2:1 points ignition 610 motor on the same gasoline, but the stock 10.2:1 Czech Republic CDI 610 motor was exhibiting some very noticeable surging. Aside from the little bits of surging the bone stock 10.2:1 Czech Republic CDI 610 motor was running fairly well. Response was very instant across all lower engine speeds, and torque seemed quite good even with the leaner mixture from the needle clip at the first groove position. The leaner needle clip position was actually very noticeable, but it wasn't seeming problematic. With more spark advance and a leaner mixture the response across all lower engine speeds from 3,000 to about 5,000RPM was very instant, and modulation of torque delivery was actually improved. Torque was noticeably slightly lower at around 1/4 and 1/3 throttle compared to the richer mixture of the 2nd needle clip position, but the difference was only very slight. The lost torque was available just by twisting a little bit farther, and this seemed to be working really very well. With so much torque on tap from the big 610 motor this really seemed to be a great feature, and I was impressed with the overall usability of the bone stock 10.2:1 Czech Republic CDI Husqvarna 610 motor. All was not perfect though.
Down bellow 3,000RPM the torque was going a bit whacky. There was some torque down there, but it was alternately harsh or very delayed and the response seemed somewhat unpredictable. It just seemed like the advance curve shoulder was up at too high of an engine speed for this particular gasoline in the 3.0" stroke length engine.
The surging was also a bit annoying. The surging was avoidable by feathering the throttle, and there was pretty good torque all the way through by keeping the throttle just below where the surging set in. The precision of throttle adjustment required to get this nice torque without surging was however a bit of a task. It seemed like very narrow ranges of throttle settings that were actually delivering strong torque without surging. The result was that when I focused on riding the bike fast I just ended up twisting the throttle farther and the surging set in frequently. The surging was avoidable, but not easily. Not as easily avoidable as has often been the case with the stock 11:1 Swedish SEM ignition 610 motor when it has been running extremely excessively crisply with apparently the same 26 and 27 degree BTDC spark timing at 3,000 to 6,000RPM.
Then the top end power was inconsistent also. A lot of times the bone stock 10.2:1 Czech Republic CDI 610 motor seemed to be ending very early around 6,700RPM running this same fresh 91 (RON+MON)/2 octane rating premium that had been delivering strong reliable top end power in the stock 10.2:1 points ignition 610 motor with the same 175 main jet size. Power was pretty good from 5,000 up to around 6,500RPM from the bone stock 10.2:1 Czech Republic CDI 610, but then it didn't always willingly go farther. When fully warmed up on some big pulls it was revving out to 7,700RPM and there was somewhat more power up above 7,000RPM. For the most part though the top end power seemed very flat and lifeless from the bone stock 10.2:1 Czech Republic CDI 610 motor running this somewhat weak watered down gasoline. The best feeling and most reliable power was certainly down lower around 5,000 to 6,700RPM on the bone stock Husqvarna 610 motor.
Something else that really jumped out at me were the differences in exhaust sound and engine vibration between those two mostly stock 10.2:1 Husqvarna 610 motors. The 1992 exhaust system was a lot louder. So much louder that when I put the same gasoline in the 1991 chassis with the dual inline mufflers it sort of seemed like I couldn't even here it running. What I did hear though was lots of clatter from the engine. That heavy stock Mahle piston crashing around could very noticeably be felt through the foot pegs, and there was also just a lot of mechanical clanking noise from the engine. That's just the way that the bone stock Husqvarna 610 motors are, they make a lot of mechanical noise and vibration at all elevated engine speeds.
So what caused the surging in the bone stock 10.2:1 Czech Republic CDI 610 motor? I am sure that some people would be fairly well screaming out: "ITS THE LEANER MIXTURE!!!"
Well, maybe perhaps sort of a little bit. What was really causing the surging was excessive spark advance. The 10.2:1 motor was running excessively crisply with 26 degree BTDC spark timing at 3,000RPM and 27.5 degree BTDC spark timing at 5,000RPM. Excessive crispness can easily cause surging when the gasoline is just barely powerful enough for the stroke length being used, and on this fresh premium straight from the gas station the temperature of combustion potential seemed exactly just barely high enough for the 3.0" stroke length. Adding more spark advance delivered excessive crispness, and that excessive crispness made the engine too easily fall over to earlier times of late compression ignition down to excessively low engine speeds. There is more to it than just the excessive crispness though. The 26 and 27 degree BTDC spark timing is actually too early for any compression ratio, so early that the time of late compression ignition tends to fall over to earlier times excessively easily. Very early 26 and 27 degree BTDC spark timing like that can cause harshness down around 2,500 to 5,000RPM on more powerful gasoline or in shorter 2" stroke length engines. That wasn't exactly the problem on this somewhat weak watered down gasoline. The problem was that the early spark timing and huge amounts of crispness were causing the time of late compression ignition to easily fall WAY over to very early BTDC times which was causing difficult to avoid surging around 4,000 to 6,000RPM.
I knew that right away, although I was also a bit suspicious of the leaner needle clip position on still rather weak watered down gasoline. To sort out the differences I rolled out the big guns; the 12.2:1 hot rod 610 motor.
Again I drained the old gas out of the tank on the 1991 Husqvarna WMX 610 with the 12.2:1 hot rod 610 motor, and then I took a gallon of the fresh premium out of the tank on the 1991 Husqvarna WMX 610 with the bone stock 10.2:1 Czech Republic CDI 610 motor. I added the already twice tested fresh premium to the empty tank, and then I set the static timing setting.
I had been running a 22 degree BTDC static timing setting on the 12.2:1 hot rod 610 motor before, but I expected this obviously lower pressure gasoline to do better with a bit less spark advance. But how much less? That was a big question, and I didn't really expect to get it right on the first try. I knew it was going to need less spark advance than I had been running in the 10.2:1 points ignition 610 motor, but there were reasons to believe that this still rather weak and watered down gasoline was going to favor large amounts of spark advance and large amounts of crispness. I was going to aim for around 18 degrees BTDC with the aluminum points mounting plate cold, but when I landed on 19 degrees BTDC on the first try I decided to just give that static timing setting a try.
Wow, strong torque down very low and strong power up very high! There was a really big difference between the 12.2:1 hot rod 610 motor and the two stock 10.2:1 Husqvarna 610 motors. Perhaps the most significant thing that kept jumping out at me was that the torque felt flat and lifeless around 4,500 to 5,500RPM, but a big part of that was just relative to big increases down lower and up higher. Down low it was down extremely low that the difference was largest. Wow, tons more torque in the 2,500 to 3,000RPM range than I had gotten from either of the 10.2:1 motors on the same gasoline. It was somewhat harsh down there, but lots of torque for such a low engine speed. The torque was noticeably stronger than from the 10.2:1 610 motors all the way down to about 2,300RPM, but the dramatic difference was in the 2,600 to 3,200RPM range of engine speeds.
Then as the 12.2:1 hot rod 610 motor was revved up a bit more the torque remained perfectly instant and perfectly reliable, but it started to get very quiet and a bit disappointing sounding as low as around 4,000RPM. By 4,500RPM the torque seemed low, although it was still pulling substantially. Was that low compared to the 10.2:1 motors, or was it only low compared to what I expected from the much more powerful 12.2:1 hot rod 610 motor? That was a bit hard to sort out even when riding the bikes back to back. No, the torque was a bit low around 5,000RPM, although the bike was still scooting along substantially at all of those midrange engine speeds. It was just extremely quiet and mild sounding. It sounded and felt like it could have happily run there at 5,000RPM wide open for hours on end without any difficulty.
Then the power really came on around 5,500 or 6,000RPM depending on how heavily warmed up the engine was from big pulls. The torque actually increased at 5,500 to 6,000RPM accompanied by a large increase in noise. Wow, lots more top end power than from the stock 10.2:1 Husqvarna 610 motors. The power increased quite a lot as soon as the engine got louder around 5,500 or 6,000RPM, but then the power kept on increasing substantially up to higher engine speeds also. The crankshaft wiggle advance at 7,000RPM didn't seem all that noticeable, but the engine did keep right on revving without any cutting out and without any difficulty at all. At 7,500RPM the power did increase noticeably with the onset of the intake stack boost, but then up higher it went flat very quickly. Mostly I was revving out only to 7,900RPM. It was able to go to 8,400RPM fairly easily, but it was flat up there above 8,000RPM. Not as much power as the hot rod 610 motor often makes. A lot more than from the stock 10.2:1 motors on the same gasoline, but not as much as has often been available. Certainly rather weak watered down gasoline even if it was running tolerably well in all three of the 610 motors that I tried it in.
Something unusual I noticed was that the 12.2:1 hot rod 610 motor would only make top end power with the throttle wide open or rather close to wide open. It would rev up with smaller throttle openings without cutting out and without any stumbling, but there was hardly any power. Only at big throttle openings near wide open was there substantial top end power above 6,000RPM. Down lower around 3,000 and 5,000RPM there was substantial torque at all throttle openings, but the top end power was only available wide open on the big 175 main jet. At 4,500 and 5,000RPM twisting up onto the big 175 main jet did deliver slightly stronger torque, but the torque was fairly respectably strong at 1/2 throttle at 4,500 and 5,000RPM also. It was only up higher that the big 610 motor absolutely needed more gasoline dumped in to make power.
There was some substantial popping out the exhaust on deceleration also. Not huge amounts, but more than on gasoline of a normal energy density. The low idle was also unstable, although it wasn't stalling and it was restarting with the kick starter without all that much difficulty. The 12.2:1 hot rod 610 motor was reliably restarting with the kick starter, but sometimes it took a few kicks. It was often just a big lean sounding backfire on the first kick, and then it would fire up and run without difficulty on the second kick. Sometimes it even took a few more kicks, but it was always starting up with a few rather small kicks. The large amount of overlap on the 1994 camshaft has sometimes caused the points ignition hot rod 610 motor to need an extra kick now and again, but the trade off is that it also often starts right up very easily with rather small kicks both hot and cold. As with the other 610 motors I was smelling a little bit of ethanol exhaust smell this 56 degree Fahrenheit November day on the fresh 91 (RON+MON)/2 octane rating premium straight from the gas station. Not much, but I was noticing it. In my experience it takes more than about 20% ethanol to get any noticeable ethanol exhaust smell at all. This did seem like somewhat watered down low energy density gasoline, although not as bad as much of the garbage I had been suffering through in past weeks and past months.
What was really significant was that there wasn't any surging from the 12.2:1 hot rod 610 motor. Not the slightest bit of surging anywhere at any engine speed or at any throttle openings despite very crisp operation and rather large top end power output. Instead of surging there was just a bit of a flat spot around 5,000RPM. It wasn't much of a flat spot though, and torque was actually still fairly good throughout despite the low energy density and low temperature of combustion potential gasoline. It was weak watered down gasoline, but not all that extremely weak and not all that extremely watered down. At least by comparison to much worse and much more severely watered down gasoline.
No surging from the lean jetted 12.2:1 hot rod 610 motor and no surging from the richer jetted stock 10.2:1 points ignition 610 motor. The only surging was from the bone stock 10.2:1 Czech Republic CDI 610 motor running substantially more spark advance. Too much spark advance makes an engine more prone to suffering from surging. This is opposite from what some people might claim, but that is just the way it is. Too much spark advance is just too much, and it can cause a whole host of problems from a lack of low end torque to excessive harshness to a lack of top end and a lack of overrev. And added to that list of problems that too much spark advance can cause is surging. When the gasoline is very powerful then too much spark advance just causes a whole lot of extreme harshness and a lack of bottom end power. When the gasoline is so weak that it can barely be made to work with that long of a stroke length then too much spark advance tends to cause surging.
When I got back from a substantial little test ride on my favorite dirt bike (also the one that happened to have a set of nearly new modern tires on it just right for railing on the freshly rain wetted but not yet muddy ground) I checked the static timing setting on the 12.2:1 hot rod 610 motor. Not surprisingly I found the static timing setting up at 20 degrees BTDC with the points mounting plate hot. There was that one degree of additional advance again once the aluminum points mounting plate was up to operating temperature. Something interesting that I have been noticing again in recent weeks messing around with my old 10.2:1 points ignition 610 motor is that it's aluminum points mounting plate doesn't deform and change the static timing setting as much as it heats up. It's the same aluminum cut from the same piece of plate, but the shape is different. For the old worn out 10.2:1 motor going into the 1992 Showa chassis I had just hacked off an octagonal piece of the aluminum plate and stuffed it onto the engine. For the hot rod 1997 rebuild I had carefully cut a minimalistic triangular points mounting plate to be as light as possible. The minimalistic triangular plate that weighs half as much also deforms much more as it heats up. It's a noticeably smaller change in the static timing setting from cold to hot on that big octagonal points mounting plate. Perhaps like half as much change in spark timing with the same change in temperature. That's interesting.
So the static timing settings I was running on the stock 10.2:1 points ignition 610 motor and on the 12.2:1 hot rod 610 motor were extremely similar on that same gasoline. The higher compression ratio engine was running a bit less spark advance, but not much less. Was it really only one degree of crankshaft rotation of difference between the two motors? I don't know exactly, it's hard to get a good accurate read from my little marks on the 0.9" diameter crankshaft. It's close to that though. Perhaps the 10.2:1 motor was up at 21.5 degrees BTDC, and perhaps the 12.2:1 motor was down at 19.5 degrees BTDC. I don't know exactly, but they certainly were amazingly similar spark timing values for such largely different compression ratios. How could the 10:1 motor get away with such small amount of spark advance on the same gasoline that the 12:1 motor wasn't surging at all on? There are several different things going on that add up to this somewhat surprising similarity in spark timing values.
One thing is the actual amount of spark advance. That 20 degree BTDC spark timing is a very flexible and easy to tune spark timing value for a four inch bore engine. It's enough spark advance that full flame front travel mode operation works quite well at all lower engine speeds up to around 4,000RPM. More power in full flame front travel mode can be made with 25 degree BTDC spark timing at 4,000RPM, but 20 degrees BTDC up to 4,000RPM does work in a four inch bore engine running reasonably fast flame front travel speed gasoline. The 20 degree BTDC spark timing is also early enough that the engine can fairly easily fall over to earlier times of late compression ignition as required at higher engine speeds. The 20 degree BTDC spark timing is however also late enough that it is fairly easy to keep an engine running at the latest possible time of late compression ignition over a range of throttle openings and a range of operating conditions. Getting up towards 25 degrees BTDC an engine just extremely easily falls over to earlier times of late compression ignition, so there tends to be only a very narrow window of tuning available where the latest possible time of late compression ignition can be hit for smooth and powerful low end torque down very low. At just 20 degrees BTDC this tuning window where the latest possible time of late compression ignition can be obtained is substantially wide, and this makes for easy and flexible tuning.
So the spark timing value of 20 degrees BTDC itself provides some flexibility in the compression ratio. At 20 degrees BTDC the 12.2:1 hot rod 610 motor was extremely crisp down low, easily attaining late compression ignition at all lower engine speeds with a rather small twist of the throttle. At 21 degrees BTDC the 10.2:1 points ignition 610 motor was a bit reluctant to pop off on late compression ignition down low, and this resulted in a lot of lag sometimes. Despite the lag the 10.2:1 motor was able to reliably enter late compression ignition mode, it just took a bit more coaxing.
The point is that at a 20 degree BTDC spark timing value engines tend to stay at the latest possible time of late compression ignition over much wider ranges of compression ratios (and/or wider ranges of elevations and/or wider ranges of throttle openings) than at a 25 degree BTDC spark timing value. The 20 degree BTDC spark timing value is just inherently a much more stable and flexible spark timing value than 25 degrees BTDC or 30 degrees BTDC.
The more obvious difference between the 10.2:1 points ignition 610 motor and the 12.2:1 hot rod 610 motor is of course in the mixture ratio. The stock 10.2:1 points ignition motor was running the richer second needle clip position where the 12.2:1 hot rod 610 motor was running the leaner first needle clip position. Dumping more gasoline in does often help get an engine into late compression ignition mode more easily. That extra gasoline also results in more lag and a more pronounced difference between full flame front travel mode and late compression ignition mode, but it does get the engine into late compression ignition mode.
There are other things that conspire to allow the 10:1 and 12:1 Husqvarna 610 motors to run surprisingly similar static timing settings also. One is the engine speed where the crankshaft wiggle advance comes on. The heavier 368g cut down Mahle piston and the lower 10.2:1 compression ratio both tend to lower the engine speed where the crankshaft wiggle advance comes on where the lighter 332g cut down Woessner piston and the higher 12.2:1 compression ratio both tend to raise the engine speed where the crankshaft wiggle advance comes on. The result is that the 10.2:1 points ignition 610 motor gets wide open throttle crankshaft wiggle advance down perhaps as low as 6,300RPM where the 12.2:1 hot rod 610 motor has to rev to 7,000RPM before the wide open throttle crankshaft wiggle advance comes on. Between 6,300 and 7,000RPM the 10.2:1 stock 610 motor is running substantially more spark advance than the 12.2:1 hot rod 610 motor.
The camshafts are a bit different also. I don't have the 1994 camshaft advanced quite all the way to the stock setting in the 12.2:1 hot rod 610 motor. Even after adjusting the cam timing to get the 12.2:1 motor back close to the stock cam timing there is still perhaps a degree of crankshaft rotation of difference from stock. That's not much, but the intake valves do stay open that small one degree of crankshaft rotation longer on the 12.2:1 hot rod 610 motor than on the stock 10.2:1 motor. Even beyond just the intake valve closing time there are other camshaft differences also. The larger amount of overlap from the big 1994 camshaft does also make some small difference. Mostly what the larger amount of overlap does is make starting and low idling a bit more dicey, but there are some small performance differences also. The larger amount of overlap tends to put more emphasis on the exhaust system. More overlap can mean more exhaust scavenging and more torque if the exhaust system is up to the task. Few exhaust systems provide strong scavenging over wide ranges of engine speeds though, so more overlap has a tendency to make an engine either narrower or more finicky and more demanding. I don't know exactly what the dynamic of the 1991 exhaust system with the big 1994 camshaft means for engine performance at various engine speeds, but there certainly is the possibility of slightly different results compared to the big loud 1992 exhaust system with the smaller stock 1991 camshaft.
Mostly there seem to be things that offset each other. The 1991 exhaust system with the secondary Supertrapp muffler is very long, but it also has somewhat smaller tubes than the big 1992 exhaust system. The 1992 exhaust system seems to have a tendency to bluster torque in the 3,500 to 6,000RPM range of engine speeds, probably due to the larger total exhaust system volume and still very long total length out to the back of the rear fender. Interestingly though the 1992 exhaust system seems to have more of a tendency to cut the torque short bellow about 3,500RPM. The 1991 exhaust system on the other hand just seems very even and uniform all the way through from 2,500 to 10,000RPM without favoring any particular engine speeds. It seems that perhaps the 1992 exhaust system is giving the stock 10.2:1 points ignition 610 motor in the 1992 Husqvarna a bit of a boost around 4,000 and 5,000RPM to get it going with an unexpectedly small amount of spark advance.
The larger amount of top end power from the 12.2:1 hot rod 610 motor is very easy to explain. A lighter piston, bigger valves and a bigger camshaft. Yep, that all adds up to more top end yank.
When I first got my old 610 motor going in the 1992 Husqvarna chassis back in early 2015 I used the same old O-ring chain that had been on the motor the last time it had been running back in 2006. The old chain was rusty from sitting outside, but it had still been in fairly good condition because I hadn't gotten much riding in before the engine lost compression and wouldn't start anymore. I also used the 52 tooth steel sprocket that I had put on new with that chain, but I opted for a 13 tooth front sprocket instead of the stock 12 tooth I had been running eight years earlier.
I used to run the stock 12/52 gearing a lot in those early years of riding the 1991 Husqvarna WMX 610. In the very early years from 1998 through 2001 with the stock CDI ignition set at the stock 33 degree BTDC spark timing there was never much low end torque, so the very low gearing was required to keep the engine up higher around 5,000 and 6,000RPM where the power was found.
Then from 2002 through 2006 with the points ignition system the very low 12/52 gearing was favored for a completely different reason. With the points ignition set at a static timing setting of 23 degrees BTDC there was tons of torque everywhere from about 3,000RPM all the way up to 7,000RPM, so the gearing wasn't all that critical and I ran 13/51 sprockets with good overall results.
Then the engine started kicking back so bad at 23 degrees BTDC that the bike was basically useless. After destroying the soles of quite a few pairs of boots I had to give up on the 23 degree BTDC static timing setting. Going down to a 21 degree BTDC static timing setting nearly entirely eliminated the kicking back, but it also caused worse problems with hesitation. Even with the 23 degree BTDC static timing setting there had been times when the torque response was less than instant. Going down to a 21 degree BTDC static timing setting made the little bits of hesitation much more common, and sometimes the engine would hardly work at all at the 21 degree BTDC static timing setting. Particularly up at higher elevations above about 4,000 feet there was a lot of bad hesitation with a 21 degree BTDC static timing setting. Normally the torque was still perfectly reliable and fairly instant down at sea level, but not always. There were a few times when it was hesitating horribly and would hardly go even down at very low elevations with the 21 degree BTDC static timing setting.
The reason that the gearing was significant was the particular way that the hesitation often came on at higher elevations. I remember a lot of rides where I was getting very impressive torque way down low around 3,000RPM but only over a narrow range of engine speeds. It would chug powerfully down supper low and it would rev up just enough to smooth out above 3,000RPM but then there was a huge flat spot from 3,500 to 5,500RPM. Then there was always large power available from 5,500 to about 6,500RPM, but sometimes the power was very narrow. Often it was a situation where the 610 motor would only pull over two very narrow ranges of engine speeds, so the very low 12/52 gearing was required so that the jumps between the gears didn't seem as wide. I remember many times where the motor seemed to be pulling well enough down at 3,000 feet of elevation, but then climbing up just a small bit to 4,000RPM the low end torque around 3,000RPM nearly totally disappeared so that the only way to get any power was to keep the engine above 5,500RPM. This made the 13/51 and 14/51 gearing seem useless for trail riding, so I went back to 12/52 gearing which was much more workable in the narrow 5,500 to 6,500RPM power band. The reason that the power was coming on right at 5,500RPM was of course the crankshaft wiggle advance. With the heavy 406g stock Mahle piston the wide open throttle crankshaft wiggle advance came on somewhere just bellow 6,000RPM, and feathering the throttle delivered the crankshaft wiggle advance down even slightly lower. I didn't know about the crankshaft wiggle advance back then in 2002 through 2006, but I did know that lower gearing made the jumps between the gears seems smaller and this obviously smoothed out the problems and allowed the bike to work better on the trails.
By early 2015 when I put my old 610 motor in the 1992 Husqvarna chassis I had figured out a lot more about the points ignition system and about gasoline engines in general. Before I even got the motor installed I knew that I was going to be able to extract better and more reliable performance through targeted tuning. A big break I got in this regard was that the points ignition bikes weren't kicking back at all at any static timing setting at that time. That allowed me to just pile on as much spark advance as was required to eliminate the hesitation.
Knowing that I was going to be able to eliminate the hesitation I figured that I could easily make 13/52 gearing work well, so that was what I started out with on the 610 equipped 1992 Husqvarna. I used the rusty old O-ring chain, the rusty old 52 tooth steel sprocket and a new 13 tooth front sprocket to get the project going with minimal input of money and new parts. This worked great, but the 13/52 gearing felt extremely low on faster dirt roads compared to the 13/53 gearing I was running on the 386 stroker motor at that time. The shorter stroke length engine revved up with much less fuss, so low gearing worked fine for most purposes. Right away the 13/52 geared 610 felt short legged, so I went up to 13/50 gearing. Moving the rear axle back towards the end of the range of adjustment also seemed like a good match for the big torque of the monster 610 motor.
The 50 tooth aluminum rear sprocket I installed on the 1992 Husqvarna was itself a bit of an experiment also. I hadn't tried an aluminum rear sprocket on the 610 in a long time, and this 50 tooth French made AFAM brand sprocket that I happened to have is also an extremely light piece that is cut down to where there is hardly any sprocket remaining.
The light weight aluminum sprocket worked and held up to massive amounts of torque from the rich jetted points ignition 610 motor. The 13/50 gearing also worked very well much of the time. Going up to more than 26 degrees BTDC on the static timing setting did drastically cut into low end torque production, and this sometimes made the 13/50 gearing feel too tall. It was still workable, I just had to slip the clutch a bit more in first gear on tight trails when the static timing setting was up very high.
What was interesting was that the aluminum sprocket seemed to hold up very well. I checked the length of the chain often, and it was holding steady at around 12.53" for a 20 link length. What I realized was that the aluminum rear sprocket held up well as long as the chain didn't stretch much and I stayed out of the mud. I didn't ride the 1992 Husqvarna all that much, but over the years I have managed to burn up two rear tires. The first tire was the Kenda K270 that was on the 1992 Husqvarna when I got it, and that tire didn't last at all. It seemed a bit dry rotted from being many years old, and the knobs chunked off easily when subjected to the big 610 torque. The crappy Showa shock didn't help either, and the old tire was gone in basically just one ride. The next tire I tried was a slightly used 120/100-18 Dunlop D739 Desert AT that held up much better. The tread blocks were still chunking off a bit, but the Dunlop held up long enough that it actually got worn out from dozens of hours of trail riding.
As soon as the rusty old chain was out to 12.58" over a 20 link length the aluminum rear sprocket started to show signs of wear. It turns out that a lot of the problems I had had years ago with aluminum sprockets failing abruptly was that I was going by the chain maintenance recommendations in the 1991 Husqvarna Owner's Service and Tuning manual which claimed that the wear limit was 12.70" for a 20 link length. It turns out that the real wear limit for a 520 chain is about 12.60" for a 20 link length. Out to 12.54" and 12.56" for a 20 link length the rear sprocket holds up fairly well, but past 12.58" for a 20 link length an aluminum rear sprocket just falls apart very quickly. The KTM recommended wear limit for 520 chain is 12.60" for 20 links, which is a bit on the long side in the old Husqvarna tradition. Not as drastically far on the long side as the 1991 Cagiva recommendation of 12.70" though.
If the goal is reusing an aluminum rear sprocket then replacing the chain a bit before it is totally toast is a good idea. If a new rear sprocket is on the list anyway then there is little harm in letting the chain go somewhat longer. Once the chain starts stretching much more rapidly though and frequent chain tension adjustments are required there is little point in trying to run it longer. It's not going to go many more hours. All it's going to do is get more and more floppy and tear up the rollers and sliders more rapidly.
The other trick that I figured out many years ago is to replace the front sprocket before replacing the chain. Replacing the front sprocket once as soon as it starts to show substantial wear but before the chain is out past about 12.52" or 12.53" for a 20 link length does wonders for extending the life of the chain. Then if the chain is replaced before it goes past about 12.58" for a 20 link length the rear sprocket is likely to be in good condition and can also be reused for another new chain.
The summation of this information is very simple: A worn front sprocket ruins the chain, and a worn chain ruins the rear sprocket.
There is however a lot more to chains and sprockets than just these simple little wear relationships. There are lots of other things that can rapidly accelerate chain and sprocket wear also. Mud and water kills open chains very quickly. As soon as the chain lube in the rollers is replaced with abrasive dirt and mud the chain tends to wear a whole lot more quickly. Keeping an open chain working well for a long time requires either keeping it out of the mud or doing large amounts of chain maintenance. The sealed O-ring and X-ring chains tend to last a whole lot longer in conditions where there is water or mud.
A sealed O-ring chain just needs a little bit of dry chain lube to keep the chain rolling over the sprockets smoothly with low friction. The dry type chain lube doesn't attract dust as much either, so the entire chain and sprocket system tends to stay a lot cleaner. A clean chain looks good, and a clean chain and clean sprockets also tend to last well.
What kills O-ring chains is heat. Not a high ambient temperature, not a nearby hot exhaust system, not that kind of heat. Heat from going fast. More specifically heat from the chain moving fast. High speed highway cruising heats O-ring chains up and pushes the grease out more quickly. Once the grease is expended the O-ring chain begins to fail because it is no longer well lubricated. Compared to running an open chain in the mud the rate of wear from a poorly lubricated O-ring chain may still seem rather low, and O-ring chains generally have a reputation for dramatically outlasting open chains under all types of conditions. There is also the fact that the pins and rollers on O-ring chains are nearly always made out of high quality hard high alloy steel that is both strong and long wearing. Open chains are sometimes similarly well constructed, but not always. It is much more common to find open chains with defective soft pins and/or soft rollers that wear rapidly even when well lubricated.
The material that the plates of a chain are made out of is important for different reasons. Very soft plates may actually deform where the pins are pressed in causing genuine "chain stretch", but that isn't why chains usually "stretch". Chain stretch is normally caused by wear of the pins and rollers themselves. Very hard chain plates makes pressing a pin out to reduce the length more difficult, and some chains require more substantial chain breaking tools than others. How easily a pin can be pressed out is however not necessarily any indication of how hard and/or long wearing the pins and rollers actually are. Likewise the tensile strength rating of a chain isn't necessarily a good indication of how hard and/or long wearing the pin and rollers actually are either.
The longest wearing chains use hard and strong high alloy pins and rollers, and these chains tend to be rather strong also. A long wearing chain doesn't necessarily carry a high tensile strength rating though. Plates made out of cheaper carbon steel may reduce the tensile strength rating slightly without compromising the longevity of the pins and rollers at all. It's not that the carbon steel plates can't handle the load, it's just that the press fit of the pins and rollers into the plates isn't quite as stable when the plates themselves aren't as strong.
A cheap 120 link 520 O-ring chain that retails for under $40 may have very good high alloy pins and rollers and last spectacularly well even if it carries a slightly lower tensile strength rating than the more expensive $60 and $80 chains. Cheap carbon steel plates may slightly reduce tensile strength ratings without compromising longevity in the slightest.
I have actually had very good luck with the cheap $40 O-ring chains. One spectacularly long lasting one went for 170 hours on the hot rod 610 motor, although it was beyond toast when I finally took it off. That was also mostly on a 15 tooth front sprocket. It started out with a 14 tooth front sprocket, but when the 14 tooth was showing signs of heavy wear at 50 hours I replaced it with a 15 tooth and the chain kept going for another 100 hours. I have several other cheap $40 O-ring chains on my other 610 bikes, and they have been holding up quite well with very little stretch also. I do keep the chains and sprockets well lubricated, and I mostly stay out of thick mud. After 40 hours of hard riding the new O-ring chain on my 12.2:1 hot rod 610 motor is still in like new condition and doesn't appear to be out to even 12.51" for a 20" length. On my 1991 Husqvarna WMX 610 with the bone stock 10.2:1 Czech Republic CDI 610 motor I have managed to wear out just one rear tire, and that O-ring chain is also holding steady at 12.50" for a 20 link length after 15 hours even though it is running on a smaller 13 tooth front sprocket.
That 40 hours on the new chain on the 12.2:1 hot rod 610 motor without any noticeable chain stretch is really very impressive, and it hasn't been just slow casual cruising either. It has been a lot hard riding using full torque much of the time. It even includes a trip out to a sand dunes riding area where I spent several afternoons blasting wide open up the big hills at 7,000 to 8,000RPM. What I haven't been doing on this new O-ring chain is cruising for extended periods a high speeds on the highway. Before I put this new O-ring chain on I ran 16/50 gearing on an even cheaper open chain. With the super tall 16/50 gearing and old bald tires front and rear I spent some considerable time out on the highway just getting high speed cruising out of my system. With that really very tall gearing there seemed to be little danger of damaging the oil reed valve lubricated engine from running at high engine speeds, so I just let it loose for extended periods of time. With that itch scratched so to speak I put the much lower 14/48 gearing on with a new cheap $40 O-ring chain and that bike hasn't been out on the highway much at all since. Instead I have mostly been riding my 386 stroker motor for longer distance highway jaunts.
On smaller displacement engines 520 chains last even longer. I have over 150 hours on the chain on the 386 stroker motor, and it's still very close to new at 12.52" over a 20 link length. Again though that's with a few front sprocket changes. The 13 tooth front sprockets were showing signs of wear rather quickly, so I changed them twice. Now the 14 tooth is lasting much better. The smaller 386 stroker motor is also a lot easier on rear tires, and I got a big 90 hours out of a 110/100-18 Maxxis IT Desert by running it all the way down past bald. Of course the Maxxis IT Desert rear tires are just very long lasting, and I got 75 hours out of one on the hot rod 610 motor by running it all the way down to the cords.
There are quite a few ins and outs of what exactly makes chains last longest. Since O-ring chains are killed by heat from high speed riding smaller size rear sprockets can be an advantage. A 48 tooth rear sprocket runs the chain at the same speed for 70mph as a 52 tooth does for 65mph. A smaller rear sprocket does however increase the tension in the chain for the same torque transferred to the rear wheel. Interestingly though even on the big 610 motor smaller rear sprocket sizes don't appear to cause more rapid chain wear. What does cause very noticeably increased chain and sprocket wear is a lower tooth count on the front sprocket. A 14 tooth front sprocket on the big 610 seems to last better than a 13 tooth front sprocket on the smaller 386 stroker motor. That's just because the 13 tooth sprocket count is getting down to where a chain and sprocket system simply doesn't work well. It's too few teeth. A 12 tooth front sprocket really tears up chains, and even a 13 tooth is way on the small side.
The implication is simply that the 2.3:1 primary reduction on the three inch stroke length 1980's and 1990's Husqvarna motors isn't quite enough for most purposes. The only thing that such a small amount of primary reduction is good for is very high speed racing or casual highway cruising. For a dirt bike rather low gearing works best, and that tends to push the front sprocket tooth count down to 13 teeth. Especially with the five speed transmission it is difficult to run more than 13 teeth. The 14/52 gearing does work, but it is pretty tall for the five speed for most purposes. I am fairly sure that the vast majority of dirt bike riders would prefer 13/52 or 13/51 gearing on the five speed for any type of trail riding even with the 610 motor running well and pulling strong down to 3,000RPM and bellow. With the six speed it is much more reasonable to run 14/52 or even 14/50 gearing for general purpose trail riding, but the big jump between first and second does get a bit annoying. The only way to totally eliminate the problems with the over sized jump between first and second is to gear very nearly just as low as with the five speed and ignore first gear. Essentially turning the six speed back into a five speed. That low first gear can of course be useful no matter how adverse to the big jump from first to second someone happens to be, but it is a big jump down to 1st. Having a first gear down there lower is a good training tool for gaining confidence to run taller gearing and ride faster on single track. For one thing having that low first gear makes it easy to talk one's self (or someone else) into trying taller 14/52 or 14/50 gearing versus the 13/52 or 13/50 that seems mandatory on the five speed 610.
The low first gear itself is also confidence inspiring. It is nice to have a lower gear to fall back on when aggressive riding and clutch slipping becomes mentally overwhelming or physically exhausting. You won't make it up a tougher climb in first gear with 14/50 gearing versus second gear with 14/50 gearing, but it is still a big confidence booster to know that there is in fact another gear down there when the going gets tricky and the bike slows down too much.
Since the primary reduction on the 1980's and 1990's Husqvarnas is insufficient there are a few different strategies to get the desired gearing. One is to just bite the bullet and run 13 tooth front sprockets and replace them frequently. The other is to go large on the rear sprocket. The 14/52 gearing is already going pretty big on the rear sprocket, but 53 tooth rear sprockets are sometimes available also. The 14/53 gearing is getting close to reasonable for the five speed 610, but then the chain is going very fast at high speeds. That wears out chain sliders and rollers faster, and it just tends to sound louder also.
The chain going faster also heats and toasts sealed chains faster. It really does seem like overheating at high speeds is what most severely limits the longevity of sealed chains; and that seems true even on the big Husqvarna 610 which might be expected to be hard on 520 chain. The reality though is that 520 chain with hard high alloy rollers and pins is substantially burley for the 577cc engine. That same chain is what gets used on 650cc Japanese dirt bikes such as the air cooled dual sport Suzuki DR650, air cooled dual sport Honda XR650L, water cooled XR650R and the water cooled street/adventure Kawasaki KLR650, water cooled dual sport KLX650 and water cooled KLX650R.
It's also the same 520 chain that goes on the KTM690/Husqvarna 701. The 5/8" 520 chain also has the same diameter pins as the 5/8" 530 size chain used on big 1200 and 1300cc V-twin adventure bikes. It's about 650 to 700cc per cylinder that the 5/8" chain gets subjected to on a regular basis, so the 577cc Husqvarna 610 is only sort of close to the normal maximum. From another perspective though the same 520 chain is also on all of the smaller 450, 350, 300 and 250 dirt bikes. It's even on some of the 125 two strokes, which is obviously crazy.
Whatever the maximum displacement per cylinder might be for 520 chain the fact remains that high linear chain speeds heat the chain up and push the grease out of sealed chains. The smaller bikes could use smaller rear sprockets and more reduction in the gear box with 520 chain, but most use something close to a 45 tooth rear sprocket. If 520 chain holds up well enough to 650cc bikes with 45 tooth rear sprockets then a 450 could use a 32 tooth rear sprocket. That much smaller 32 tooth rear sprocket would then reduce the linear speed of the chain a whole lot.
As it is linear chain speeds are excessively high for sealed chains. Not on single track trail, and not even in most normal types of off road riding. Open a dirt bike up on some wide open terrain though and the chain gets hot and the grease gets pushed out of the sealed chain. The X-ring chains do a bit better with retaining the grease, but the same problem tends to exist. Too much linear chain speed heats the chain up and slowly pushes the grease out.
If heat is what causes sealed chains to fail, then there seems to be a way to use heat to repair sealed chains. Sealed chains work great for a while, but once the grease is gone they wear more rapidly and they also get stiff and inefficient because the O-rings bind without any grease behind them. At some point in the service life of a sealed chain it would be better if the O-rings could just be removed. This might be done with a solvent that would dissolve the rubber, but a solvent that will break down high quality O-rings designed to withstand a wide variety of solvents used in chain lube is likely to be some pretty nasty stuff that would best be avoided. The other way to get rid of unwanted old O-rings would be with heat. Toasting an old O-ring chain over an outdoor fire should be able to burn the O-rings out without overheating the steel to the point of modifying it's structural properties. The obvious problem with this is just the burning rubber air quality issue. Not quite as bad as burning old tires, but substantially of the same class of pollution hazards. Both as a localized health hazard and as a larger scale air quality hazard. It seems like there should be some safer and cleaner way to get unwanted old O-rings out of partially used up O-ring chain.
One option would be an outdoor oven that could be set at a critical baking temperature where the O-rings would harden and crack without actually catching fire and burning. That would seem to be a whole lot safer and cleaner than burning the O-rings out, but it would require an outdoor oven. There would be some VOC emissions from baking the O-rings, so it wouldn't be an indoor kitchen sort of a project.