When my original old 1991 Husqvarna WMX 610 motorcross bike lost compression back in 2006 I thought it was done for. At that time replacement pistons were not available, and the chassis was in very poor condition from many years of operation while hardly any replacement parts were available. When the bike would no longer start easily I figured it wasn't good for much of anything, and I parked it outside to make room for other projects in the shop building.
The Rusty Hulk
eBay Saves the Day
Engine Rebuild
Chassis Rebuild
Swedish and Italian CDI Ignitions
Suspension and Handling
After nearly ten years of sitting outside in the rain the old steed was looking even more dilapidated than it had when I had been riding it. The biggest eye sore were my custom mild steel parts that I had never painted but had not rusted while I was using the bike. After sitting outside in the rain for nearly a decade though my custom chain guide and custom muffler mounts were looking pretty shabby. Many of the original cast aluminum pieces that had always been shiny before also now had a dull powdery finish to them. All in all though the bike was essentially in exactly the same condition as when I parked it all those years earlier. Since I had always used a bit of waterproof grease on the threaded fasteners the bike was still for the most part easy to take apart. There were a few bolts that I had never had apart, such as the sub frame mounting bolts, which were rusty and a bit stuck.
My father had always been fond of the chassis and suspension on my Husqvarnas, and when I was not using the old WMX 610 he asked if he could use the chassis to put his 2002 Yamaha TTR225 motor in. I discouraged the idea on the grounds that the chassis was in very poor condition and replacement parts were not available, but I told him that he was welcome to use the chassis if he wanted to.
My father got as far as tearing the old WMX 610 all the way down to individual pieces and a bucket of fasteners, but he never got around to actually putting his TTR 225 motor in it. One of the stumbling blocks was that the rear wheel on the WMX 610 was in horrible condition with a bent rim and I was having trouble getting replacement spokes from local motorcycle shops. The logical thing to do for the TTR 225 engine swap was to use the entire Yamaha rear wheel. I thought this was a good idea as it would not only solve the problem of not being able to get the parts to rebuild the Husqvarna wheel, but it would also solve the problem of having to move the disk brake from the left of the bike to the right of the bike to use the Yamaha motor. As easy as it seemed to just drop the whole Yamaha motor and rear wheel into the Husky chassis my dad was opposed to this idea because the cheap chrome plated Yamaha wheel just did not look anywhere near as good as the original Husqvarna wheel. The very small drum rear brake on the TTR 225 also not only looked sub-par, but did not work all that well either.
What changed the fate of the 1991 WMX 610 was when I found a motor out of a 1997 Husqvarna TE 610 for sale on eBay. From the pictures it was obvious that the motor had been subjected to severely inadequate repair procedures, but the seller said it was complete and mostly in good condition. For just $650 it was impossible to pass up.
The 1997 motor was a disaster, with numerous horrendous problems. It did however have the six speed transmission I had so desperately wanted when I rode the five speed WMX 610, and the rod bearing appeared to be well within the Husqvarna specifications. The two really big problems with the 1997 motor were that half of the camshaft was missing and it had a 7:1 piston in it. I was steaming mad about the broken camshaft, because one of the pictures on the eBay listing actually showed the water pump drive protruding out from the side of the cylinder head. When I got the motor the whole part of the camshaft that sticks out past the left side bearing was broken off and missing. I could not even weld the broken pieces back together because the one piece was not in the box. When I complained to the seller he said that it must have gotten broken during shipment, but this seemed unlikely since the broken off piece was not even in the box when I got it. When I contacted eBay about the problem they agreed to force the seller to accept a return where he paid shipping in both directions. This was however not acceptable to me, because it would leave me without a new motor. In the end I was able to buy the entire cylinder head and camshaft off of a 1994 Husqvarna WXC 610 that was also listed on eBay, but I had to pay $300 to get it.
The 7:1 piston looked like it was from Vertex, and it was in pretty good condition. The compression ratio was however far too low, and it looked like someone had just stuffed a similar 98mm piston for a different application into the Husqvarna motor. Whoever had been running the 1997 TE 610 had probably been having a lot of trouble with the low compression ratio as they had the needle clip in the richest (fourth) position and they had the stock 175 main jet drilled out to 190. The carburetor also had some strange green colored fuel in it that smelled like something between Pinesol and cheap perfume. Whatever weird fuel they had been running had also clogged up the carburetor pretty bad with a crumbly brown material, and all of the jets and passages had to be cleaned out.
Other problems with the motor included an oil reed valve that was installed backwards and a missing plug for the hole between the crankcase and the clutch case. I once installed the oil reed valve backwards in my 1991 WMX 610 motor just to see what would happen, and the motor was loud and clunky with no oil pumping. After just one short test ride I took it apart again and put the oil reed valve back in so that the pedal falls slightly away from the engine case as the 1991 Husqvarna Owner's Service manual says is necessary. The combination of the backwards oil reed valve and the missing rubber plug would have meant that not much oil was being pumped, but the 1997 engine must not have been run long like this because the rod bearing seemed to be in decent condition.
Apparently this engine had dropped a valve which then broke the piston up at some earlier time as the seller also had the damaged head and the original broken cylinder. The engine had been repaired with a new cylinder head, new valves, new cylinder and the new 7:1 piston that had not worked out. When the engine was put back together though it had not been cleaned out very well, and a chunk of steel had gotten stuck in the cylinder wall and caused a deep gauge through the hard plating and into the aluminum. It looked like the engine had not been run much after having been put back together, because it was still full of aluminum chunks from the old piston that had disintegrated at high engine speed.
To repair the engine I sent the cylinder off to U.S. Chrome in Wisconsin to have it re-plated and I ordered a new piston from Woessner in Germany. It was just last year that Woessner started offering pistons for the early Italian Husqvarna motors, and this is a major boon for being able to use these motors. The new Woessner piston is lighter at 369g than the 406g Mahle piston that came stock in the 610 motors and it also has a higher compression height. The stock Mahle piston had a compression height of 1.15" for a 10.2:1 compression ratio, where the new Woessner piston has a 1.18" compression height for an 11.3:1 compression ratio. Woessner also has a top end gasket kit available for the 610 motors, and when I got the gaskets they were thicker than the old stock gaskets. It looked like these thicker gaskets were going to yield an actual compression ratio of 10.95:1, which was a significantly smaller bump in the compression ratio than going all the way up to 11.3:1.
Cutting the old stock Mahle piston in the 1991 WMX 610 motor down from 406g to 368g had worked out so well that I decided to see how far I could cut the new Woessner piston down. The Woessner piston is actually a better design, and it turned out to be fairly easy to get it down to an even lower weight. The 406g Mahle piston was a crude design, but because it was so radically overweight it was easy to take some material off of it. Going down even further on the piston weight tends to be difficult though. The taller compression height of the Woessner piston tends to make it heavier, and it also has large full size skirts. I left the skirts full size, and instead focused on removing extraneous material on the pin bosses and the webs that connect the skirts to the pin bosses. In the end I got the new Woessner piston down from 369g to a substantially lighter 332g.
The 1994 WXC camshaft is actually a bit bigger than the 1991 WMX camshaft. When I carefully measured both camshafts with a degree wheel bolted to the front of the camshaft I found that the 1991 camshaft is about 242 degrees at 1mm valve lift where the 1994 camshaft is very close to 250 degrees at 1mm valve lift. Even more significantly the 1994 camshaft has 130 degrees of duration at a quarter inch of valve lift, where the 1991 camshaft has only about 122 degrees of duration at a quarter inch of valve lift. Both camshafts have the same 0.40" maximum valve lift. The 1994 cylinder head is exactly like the 1991 cylinder head, but the 1994 cylinder head I got was not in all that great of condition. The 1997 TE 610 cylinder head was on the other hand in perfect condition other than an intake valve that had not been lapped in properly and was leaking a bit. It took just a small amount of lapping with valve lapping compound to get the intake valve to seat perfectly, and the head and valves were otherwise in perfect condition. The 1997 cylinder head does however use larger valves than the earlier cylinder head. The exhaust valves are significantly larger at 32mm than the original 30mm exhaust valves found on all earlier Husqvarna heads going back to 1983. The intake valves on the 1997 head are also slightly larger at 36mm than the 35mm intake valves on the earlier cylinder heads. Larger seat inserts were used for the 32mm exhaust valves, but the 36mm intake valves were just installed on the same seats that the old 35mm intake valves used. The old 35mm intake valves sat rather low in the head, where the 36mm intake valves sit higher for better flow. It is often said that the valves should sit low on the seats for best cooling, but this is most important on the exhaust valves, and it does not appear to cause any problems to allow the intake valves to sit a bit higher for better flow.
Just as with the earlier Husqvarnas the 1997 model has very fancy looking stainless steel valves. Those original Husqvarna stainless steel valves are however not all that great of a design. The problem is that they are heavy, with way too much material on the heads of the valves. All modern valves use much thinner heads for lighter weight, and these new thinner valves appear to work quite well. When I checked the volume of the 1997 cylinder head it was as near as I could tell identical to the 1991 cylinder head. The larger intake valves sitting higher on the seats would tend to reduce the volume of the head, but the shape of the casting on the sides of the valves was also changed both to improve flow and to get the volume to come out the same as the old heads. There is actually considerably more material that could be removed on the outboard sides of the valves to improve flow, and the small change in shape for the 1997 cylinder head is only a very slight improvement over the earlier heads. As with the earlier 610 heads the 1997 cylinder head is also reverse compatible with the original Swedish 510 motors. Even with the larger valves a 3.63" bore will still fit.
The big difference with the 1997 cylinder head is that the bores for the header pipes are larger at 1.49" than the 1.38" header pipe bores on the earlier Husqvarna cylinder heads. The ports themselves are the same diameter, it is just the mounting flange on the header pipes that is different. This seems like a significant obstacle to using the 1997 head on the 1991 bike, but the mounting bolt locations are the same. As it turned out it was very easy to mount the 1991 header pipes on the 1997 cylinder head just by cutting spacers out of sheet metal. I used two spacers for each header pipe cut out of 0.02" galvanized sheet metal. The pipes mounted up very securely and are essentially just as good as having the header pipes matched to the cylinder head.
To assure that the new larger (and heavier) valves would not cause any problems up to high engine speed with the rather aggressive 1994 camshaft I decided to cut the heads of the valves down to make them more like modern valves. I just chucked the valves in a drill, a then hit them with a hand grinder. In a short time I had the intake valves cut down from 55g to 49g, and I cut the exhaust valves down a bit from 50g to 47g.
For 1993 or 1994 (I am not entirely sure which) a centrifical de-compressor was added to the Husqvarna four stroke engines. This centrifical de-compressor is a weight and shaft that holds the exhaust valves open a bit just after the intake valves close. To make room for this weight and shaft on the end of the camshaft the bolt pattern of the timing sprocket was changed. The 1997 610 motor uses the same 5/16" pitch timing chain as the early 350 engines, but with 19/38 sprockets instead of the 17/34 sprockets on the earlier engine. I was thinking of using the really huge 1992 TE 350 camshaft in the 1997 engine, but this did not seem like a good idea considering how ridiculously big and aggressive that 1992 camshaft is. Not only does the 1992 camshaft have 260 degrees of duration at 1mm valve lift, but it also has such aggressive fast closing intake ramps that it actually cracked the intake valves in the stock 1992 TE 350 engine. I also could not get the crankshaft timing sprocket off of the 1991 WXE 350 crankshaft with my puller because the sprocket was pushed on a bit farther than it should have been. I could have used the 1992 camshaft if I had no other options, but luckily the 1994 camshaft became available.
When I installed the 1994 camshaft in the 1997 engine it came out timed four degrees of crankshaft rotation advanced from split overlap at top dead center. The 1991 and 1992 motors always came out with split overlap right at top dead center as has been typical for all camshafts. In recent years it has however become popular to install camshafts in performance engines that have to run down to 3,000RPM with a few degrees of advance for better low end torque. Installing the camshaft a bit advanced gets the all critical closing time of the intake valves a bit earlier, which is a substantial boon to low engine speed volumetric efficiency with a long duration camshaft. Even though the 1994 camshaft has about eight degrees of crankshaft rotation more duration than the 1991 camshaft both end up with very nearly the same closing time of the intake valves when the 1994 camshaft is installed four degrees advanced.
The 1997 610 motor is essentially identical to the original 1991 610 motor, the only differences being the centrifical de-compressor and the aluminum clutch added for the 1992 model year. I was already familiar with the new larger diameter clutch from the 1992 TE 350, and it is an interesting upgrade. The upgrade is that the larger diameter clutch requires less clamping force and allows a much lighter pull at the clutch lever. To get the weight of the significantly larger diameter clutch to remain the same the new basket is aluminum and the friction plates are aluminum. The intermediate plates remain steel though unlike some modern aluminum clutches which also use aluminum intermediate plates. The durability of the aluminum clutch basket is not great, as even the 350 motor had worn pits where the friction plates ride on the basket. I had temporarily fixed this problem and restored perfect clutch action on the TE 350 simply by filling down the arms of the clutch basket until they were once again flat. This is however only a temporary solution, and the longevity of the newer aluminum clutch is not nearly as spectacular as the original all steel basket and all steel plates. The aluminum clutch on the 1997 TE 610 motor I got is however in like new condition, so it should be OK for a while.
Before the final assembly of the 1997 motor I tried to rinse as much of the aluminum debris from the previous engine failure out of the cases as I could. Amazingly there was quite a large quantity of aluminum flakes in the gear case that just kept coming out rinse after rinse. I tried rinsing the cases with solvent, but this was not getting the aluminum flakes to come out. I then switched to flushing the gear case with lube oil, and this did a better job of suspending the aluminum flakes and carrying them out. After about a dozen rinses the number and size of aluminum flakes coming out with each rinse was much smaller, and I figured that the last little bit would just have to come out with the drain oil as the engine ran in.
When I torqued the cylinder head nuts down the Woessner head gasket and base gasket compressed down to the same thickness as the stock gaskets, so I did end up with an 11.3:1 compression ratio after all. I was kind of hoping to stay below 11:1 for the traditional ideas about the maximum compression ratio for premium fast flame front travel speed pump gas, but compared to other port injected vehicles these days even the 11.3:1 compression ratio is really extremely low. Nearly all the new 2015 model year port injected street legal motorcycles are up in the 12:1 to 13:1 range.
Upon final assembly I set the valve lash closer to the stock recommended 0.004" on the intakes and 0.006" on the exhausts than I usually do. Normally I have used about 0.006" on the intakes and 0.009" on the exhaust valves, which works pretty well. The reason for the tight valve lash is to reduce valve train wear with the aggressive fast opening roller camshaft. The Husqvarnas do tear up the tops of the valves and the valve adjusters being run up at high engine speed with those aggressive fast opening camshaft lobes, but excessively large valve lash exacerbates this wear. The aluminum cylinder head and overhead camshaft does not require large amounts of valve lash because the valves do not tighten up as the engine heats up. On an iron block and iron head push rod engine the cast iron does not expand with heat as much as the forged steel valves and pushrods do, so the valve lash actually closes up considerably as the engine heats up. Even going to aluminum heads on an iron block with push rods still typically causes some closing up of the valve lash as the engine heats up, although to a much smaller extent than with iron heads. With an overhead camshaft on an aluminum head the aluminum expands at a much higher rate than the steel valves, and the valve lash does not close up as the engine heats up.
It is often thought that the valve lash actually opens up on an aluminum head engine when it heats up, but this is typically not the case and is certainly not the case on the Husqvarnas. With a 2.5 inch thick cast aluminum head and four inch long valve stems the valve lash ends up being very close to the same over a wide range of engine temperatures. There is however still some reason to leave the exhaust valves a bit on the loose side, since the exhaust valve stems would tend to heat up more being directly in the path of the hot exhaust gasses. What I ended up setting on the 1997 engine was about 0.005" on the intakes and 0.008" on the exhaust valves. One valve setting is typically good for a hundred hours or more on the Husqvarnas, but more frequent checks are a good idea because it is hard to tell just when the wear of the valve tops and adjusters will not be well matched to the pounding in of the valves into the seats. Cutting the valves down to a lighter weight should help reduce pounding into the seats for longer overall valve and seat life at high engine speeds.
The biggest problems with my 1991 WMX 610 chassis were the rear wheel and the swing arm bearings. I actually replaced the needle bearings and outer races in the swing arm once, but this was only a partial solution because I could not get the replacement inner races which apparently are Husqvarna specific parts. Just replacing the needle bearings and outer races got the swing arm working smoothly again and removed most of the play, but the worn out inner races still allowed a bit of side to side wobble. The solution was to buy the swing arm off of the 1997 TE 610 bike as well since the seller was parting out the whole bike. I used the original 1991 WMX 610 swing arm and the new bearings I had installed in about 2004 with the 1997 inner races which yielded a smooth and tight swing arm. The original problem with the swing arm bearings on the 1991 WMX 610 was that they had not been lubricated enough before I got the bike, and the problem got worse the first year I rode the bike because I also did not have a grease gun to lube the bearings. By the time I got a grease gun the bearings were totally wasted. If the swing arm bearings are kept regularly lubricated with high quality water proof grease they appear to last forever.
To solve the rear wheel problem I bought the whole rear wheel off of the 1994 WXC 610 that also was being parted out. The spacers on the brake side of the Husqvarna rear wheels changed a number of times throughout the 1990's, and the 1994 wheel is not exactly like the 1992 rear wheel or the 1991 rear wheel which also are not interchangeable. All of the 1990's Husqvarna rear wheels take the same size axle and have the same sprocket and brake disk mounting patterns and locations. The difference is just in the bearings, seals and spacers on the brake side. To use the 1994 wheel on the 1991 chassis I just machined a custom spacer out of a piece of bronze pipe. This way the 1991 chassis is still entirely stock, it just has a new mostly compatible rear wheel installed on it. The 1994 rear wheel also uses the same Takasago Excel aluminum rim, and looks essentially identical to the 1991 rear wheel.
Another problem I had with the 1991 WMX 610 chassis was that I had cannibalized the rear brake system to fix the 1991 WXE 350 chassis when rear brake parts broke. The adjuster on the 1991 WXE 350 rear brake was rusted solid, and when I tried to adjust the rear brake it just broke. Since I had taken the adjuster off of the 1991 WMX 610 I was left with an incomplete rear brake system and no way to get the new adjuster I needed. To solve the problem I bought the entire rear brake system off of a 1999 Husqvarna TE 610 that was also being parted out. The 1992 and later Husqvarnas use a different mounting bolt pattern for the rear brake master cylinder, but I had already modified the 1991 WMX 610 chassis to take the newer rear brake master cylinders because replacement 1991 rear brake master cylinders had not been available. The whole 1999 rear brake system went onto the 1991 chassis by using the 1991 caliper and the 1991 brake line, but there was some modification required. The modified 1991 WMX 610 chassis that will take either the 1991 or the 1992 and later brake master cylinders actually mounts the master cylinder lower and closer to the brake pedal than the later chassis. When I had used the 1991 brake adjuster this was not a problem because the range of adjustment was wide enough to accommodate the longer 1992 and later rear brake master cylinder. When I tried to put the 1999 rear brake system on though the adjustment of the peddle came out far too high. To solve this problem I had to remove the hold down lock nut and cut some of the adjuster shaft off to clear the peddle. This means that the only way to adjust the height of the rear brake is by disassembling the whole system and installing an additional washer. Decidedly inconvenient, but I never make adjustments to the rear brake anyway once the proper height is attained.
Because I needed a new rear fender for the 1992 TE 350 chassis I bought the whole rear fender, rear sub frame and air box off of the 1997 TE 610 for about the cost of a new replacement rear fender. As it turned out it was a good thing that I got these parts as the air boot on the 1991 WMX 610 was in poor condition and not really usable. To use the 1997 air boot I also had to use the 1997 air box, which is not exactly the same as the 1991 air box. As it turned out though the 1997 air box bolted up to the 1991 frame just fine, and I only had to modify the 1997 air box slightly to take the 1991 air box cover. This way the 1991 WMX 610 looks entirely stock on the outside even though it now takes the slightly shorter 1992 and later air filters. Even though the 1992 and later air filter element is slightly shorter the 1992 and later air box is considered to flow better both because it has a less restrictive shape and also because it lacks the slightly restrictive intake screen of the 1991 and earlier Husqvarnas.
When the fork guards on the 1991 WXE 350 had gotten broken last fall I took the last of the good pieces off of the 1991 WMX 610 to make one good set. In the intervening months that one good set got broken again, and I was left with not even one complete set of fork guards for the 40mm White Power forks. As it turns out the 40mm White Power forks were used on a few other models over the years, and replacement UFO brand fork guards are now available from a retailer in Austria specializing in vintage KTMs. A version of the 40mm White Power forks was used on some mid 1980's KTM enduro bikes, and then again in the mid 1990's a version of the 40mm White Power forks was used on the American built ATK dirt bikes. Apparently the 40mm White Power forks were never as competent on these other models as they were on the 1991 Husqvarnas, but the fork guards are fully compatible.
The Supper Trap brand muffler that I had used on the 1991 WMX 610 before was moved to the 1991 WXE 350 last year, so I got another Supper Trap muffler to put on the 1991 WMX 610 with the rebuilt 1997 motor. The first Supper Trap muffler came with eleven diffuser disks, and I always ran all eleven of them. With just the Supper Trap on the 1991 WMX 610 and the stock motorcross muffler removed the bike was still really loud, just as loud as with just the stock motorcross muffler. I tried removing disks to quiet the bike down, but this did not really work. Removing just one, two or three disks did not hurt performance much, but it also did not quiet the bike down at all. Removing four disks was the magic number. With just seven disks the bike was somewhat quieter, but it also was quite a lot slower. The combination of the Supper Trap with all eleven disks behind the stock motorcross muffler was of course what actually quieted the bike down a whole lot without hurting performance at all. The Supper Trap muffler I put on the 1991 WMX 610 this year came with fourteen diffuser disks, and I left them all in place.
The 1997 TE 610 motor came with the stock Ducati ignition system already installed. As these ignition systems had a reputation for being much more usable than the older Swedish SEM ignition systems I decided I would give it a try instead of going directly to another points ignition system as I have done on all of my other Husqvarnas.
Assuming that the stock advance curve and timing setting were for the stock 10:1 motor I was somewhat worried that the new 11.3:1 compression ratio would not work. My 10:1 motors had however been requiring considerably more spark advance of late, and it certainly seemed that the pump gas was being supplied for much higher compression ratio engines. In the hopes of getting the 11.3:1 compression ratio to work with the all stock ignition settings I set the needle clip position at the leanest setting. The 40mm DellOrto carburetor on the 1997 TE 610 motor had a 45 pilot jet in it, which is a whole lot smaller than the stock 62 pilot jet that came in all of the 40mm DellOrtos on Husqvarnas. I switched to a 60 pilot jet I had thinking that I wanted to start with close to the stock pilot jet size to assure best possible starting and idling performance. I drilled out a smaller main jet I had to the 175 size to replace the 190 jet that came with the 1997 carburetor. The 175 main jet is really very close to the 180 main jet that was stock for the 1991 WMX 610. The idea behind all of this was to lean out the lower middle part of the throttle opening with the leaner needle clip position to compensate for the higher compression ratio while retaining a nearly stock main jet size for full power production up at higher engine speeds.
With the bike all put back together I tried to kick start it, and nothing happened. There was no spark, not even a faint light blue spark, just nothing. I went over all the wiring, and repaired one connector that was a bit loose. Still no spark, the stock 1997 ignition system appeared to be totally dead on arrival.
At this point I was sort of stuck as the 1997 flywheel took a different size puller, and an oddball size at that. I poked around a bit but I just could not find any reference to an inch and an eighth flywheel puller. Finally I decided to just drill and tap the flywheel to pull it off. I drilled four holes and tapped them for 1/4-20 threads, and I made up a plate out of a piece of half inch mild steel plate. To drive the plate away from the crankshaft I just put a large nut and bolt with a pipe nipple over the end of the bolt down in-between the plate and the crankshaft. I was just able to turn the nut and bolt with an open end wrench inserted between the four 1/4-20 bolts. Luckily someone had had the flywheel off recently, and it popped off with only a moderate amount of pulling force. The 1991 flywheels had been extremely difficult to get off the first time, pushing the well greased flywheel puller threads absolutely to the limit of their strength.
With the flywheel removed I found that the ignition system had been worked on. The windings looked anything but production, and the whole thing was sealed with large amounts of hot melt glue.
I was pleased to find at least that the mounting location for the stator on the side of the engine cases was the same as the earlier Husqvarna engines, and the end of the crankshaft also had the same tapper on it. In the interest of expedience to get a test ride I decided to give my stock 1992 Swedish SEM ignition system a try. That ignition system had been very hard starting on the 1992 TE 350, but it did start and run as long as the timing setting was set up close to the maximum advance.
Somewhat optimistically I set the timing in the middle of the ten degree of crankshaft range of adjustment, even though this was a considerably later timing setting than I had ever been able to get to start on the stock SEM ignition systems. I figured that this middle of the range setting was a spark timing of about 29 or 30 degrees BTDC with a starting and idling spark timing setting of about nine or ten degrees BTDC.
I was able to get the engine to start with the kick starter when I test fired it, but then after shutting down to change the lube oil it would not start again. I took the spark plug out and heated it up with a propane torch and the engine fired up again.
When I rode off I was amazed that the 11.3:1 compression ratio was actually sort of working with the 30 degree BTDC timing setting. I had a great little test ride five miles out a local trail, and I was amazed by how well the bike and engine were working. I however had some really poor performing old motorcross tires on front and rear, and I kept washing out the front end in the turns. Just after I turned around to come back home I washed out the front end badly and the engine stalled. It would not restart with the kick starter, and even rolling it down a hill it would not fire at all. I had brought tools with me, but only the one spark plug. When I first pulled the plug out it looked dry, like the engine was not getting fuel. I pulled the carburetor all apart thinking that a bit of the gunk that had been in the carburetor had plugged something up again. The carburetor however looked totally clean, and I blew through all of the passages. When I put the bike back together I was able to get the spark plug wet with fuel in kicking the bike over, but it still would not start. After considerably effort to get the bike going I had to give up and walk many miles back home to get the truck. Luckily the section of trail I was on was accessible in a roundabout way via a dirt road that was easy to get the truck down, and I had the bike back home before midnight.
The next day I put a new spark plug in, and the engine fired right up on the first kick. The plug that had been in the engine on the previous afternoon's botched test ride had just been some old plug that I had run for unknown hundreds of hours on the points ignition system. With a new plug the engine seemed to run even better, and did not at first have any stalling problem. The engine would however not kick start after that first time. I had to roll the bike down a hill to get it started, but it did start every time. After a few hours of operation the engine began to stall sometimes at low idle, and was harder to restart by roll starting.
In the early hours of operation I changed the lube oil very frequently. I used 10-40 conventional oil for the first three oil changes just because it was cheap. Then I switched to the 15-50 full synthetic I have been running lately in place of the 20-50 conventional oil I ran before. On the early oil changes the filter screen came out covered in aluminum flakes, and there was even a small amount of steel material on the magnet. The drain oil also had a pearly color to it from the aluminum flakes being pulverized in the bearings to form a fine aluminum suspension. On each oil change I ran the engine longer, and the oil continued to come out cleaner and cleaner.
I rode the bike around like this for several days, and it was usually running fairly well. I was amazed to find that the engine started quite easily each morning with the kick starter, but then would not restart after it was warmed up. Amazingly there was sometimes excess hesitation even with the 11.3:1 compression ratio and the 30 degrees of spark advance, but most of the time it ran well with hardly any hesitation and was able to rev out and make big power. What I noticed most was that the much lighter cut down piston was allowing the engine to rev up quickly and attain high engine speeds with little fuss. The engine also seemed considerably smoother than the old 1991 WMX 610 engine had been. It is hard to tell if this is a better balance job on the 1997 crankshaft, or if all of the difference is in the lighter piston. The 1991 WMX 610 motor had become somewhat smoother after I cut the piston down from 406g to 368g, but it remained more of a vibrator than the 1997 engine with the 332g cut down Woessner piston.
When I first tried to connect the SEM lighting coil to my lighting system it burned the headlight out. The headlight came on very bright for a second, and then went dead. The turn signals came on, but looked far too bright. The flasher I had on the turn signals was a solid state unit for use with LED turn signals even though I was using the old incandescent turn signals. I thought that the transistor switching in the LED flasher was cutting down the voltage just enough so that the bulbs did not burn out in short use.
I had been thinking that I was going to need a bridge rectifier anyway so that I could add a battery to the system to meet the California requirement of the headlight being able to be lit without the engine running. Since the bridge rectifier would reduce the voltage by going through two diodes I figured this might be enough to keep the bulbs from burning out. Since I did not have a large bridge rectifier, and did not know where to get one locally, I just made one up out of four 20A diodes.
With the bridge rectifier installed after the stock short circuit type voltage regulator and a new headlight bulb in place I fired the engine up for another test. This time the headlight came on and stayed on, and the turn signals also worked as normal.
I was not surprised to see that the headlight dimmed considerably at low idle speed, as this was the way most dirt bike lighting systems had always worked. What I was amazed by though was that the stock SEM lighting coil was powerful enough to run the headlight and two turn signals at the same time at about 2,500RPM. I also tried wiring both the high beam and the low beam to come on at the same time, and the lighting coil was able to power this larger load. I also tried both the high beam and low beam together and the turn signals at the same time, and the lighting coil was able to power this very substantial load as well as long as the engine speed was up a bit. A very powerful lighting coil indeed.
I renewed the registration on the bike and with a new sticker on the license plate I took a long ride way out around the west side of the county. I ran the low beam headlight the whole three hours of engine run time, and I turned the turn signals on often just to see them come on. The headlight and turn signals worked for the whole ride, but the next day when I fired the bike up the headlight would only come on very dimly and the turn signals did not work at all. At low idle the headlight blinks even dimmer when the turn signals are on, but the turn signals do not come on. The LED turn signal flasher emits an audible sound when it flashes, and this sound can still be heard with the engine low idling but the turn signals themselves do not light up. The headlight comes on very dimly at low engine speed, but it does not get brighter when the engine is revved up. It appears that the voltage regulator has somehow mysteriously reset itself to a lower voltage that will not illuminate the 12V bulbs. I have two more of the stock SEM voltage regulators to try, but so far I have not tried to deal with the very mysterious and unexpected failure of that first voltage regulator that presumably worked uninterrupted for the first 25 years it was in service.
After the first 15 hours of engine operation on that first new spark plug I put in the stalling and hard starting problems were getting much worse. I was having to turn the idle speed all the way up to 2,400RPM to keep the engine from stalling, and then with the idle stop turned in that far the bike was hard to roll start. Eventually I had to turn the idle up to keep the engine from stalling, and then I had to turn the idle back down to roll start the engine.
Finally I gave in and put another new spark plug in, and the engine fired up on the first kick even though it was warm. After just a few minutes of operation the engine would not start again when hot. The engine did however continue to start very easily when rolled in third gear. The new spark plug totally solved the stalling problem as well, and the bike once again could be idled down to 1400RPM with ease. I took one long ride on the new spark plug, and then I put another new spark plug in. Again the new spark plug got the engine to fire up with the kick starter warm, but only twice. The third time after just a few minutes of run time it would not fire with the kick starter. Very frustrated by having a dirt bike that absolutely would not start with the kick starter I decided to try something different.
Back in the 1990's when I ran the 1991 WMX 610 on it's original stock SEM ignition it was usually hard to start. A new spark plug made starting easier, and I also found that if the plug gap was wider than the stock recommended 0.023" the bike was much harder to start. Since the 0.027" gap that came on the new NGK C7E spark plugs caused harder starting than the factory recommended 0.023" gap I thought that an even smaller gap might make starting even easier. I tried smaller plug gaps, but this did not help with starting and that ignition system eventually failed so that nothing would get it to fire, not starting fluid or towing the bike behind a car with the engine spinning over at 4,000RPM. It just flat out failed even though a weak blue spark could still be seen when kicking the bike over.
On the new rebuilt 1997 610 motor with the 1992 stock SEM ignition I was pretty sure that the hard starting was due to the timing being backed off so far. I had always assumed that the root problem with the stock SEM ignition systems was that the spark was too weak at cranking speed. A very weak spark that will barely fire the plug will not be able to fire through a denser intake charge. Advancing the timing so that the spark plug fires earlier means that it does not have to fire through such a dense intake charge before the piston has come all the way up to top dead center. It does not seem like changing the timing a few degrees on either side of the 13 degree BTDC factory stock starting and idling spark timing would make much of a difference in the density of the intake charge, but the observed reality was always that the SEM ignitions started more easily with more advance.
With this weak spark problem in mind I decided that I would again try reducing the spark plug gap. While out on the trail I took the spark plug out and pushed the side electrode down much closer to the center electrode. It looked like about 0.012" of plug gap to me just by eye, certainly a very dramatic plug gap reduction. Amazingly the engine then fired up warm on the first kick. I rode the bike for a while and each time I shut it off it started with the kick starter, although not always on the first kick. Sometimes it took two or four kicks to get going but it always started. I then tried gapping a plug down just half as far, but the engine would not start at this plug gap. I gapped my other plug down to the same radically reduced plug gap, and the engine fired on this plug as well.
This then was the solution to the hard starting and timing problems of the stock SEM ignition systems. The weak spark was just not powerful enough to bridge the stock 0.023" gap when the timing was backed off so that the plug fired closer to top dead center when starting. Reducing the plug gap a bit did no good, it took a dramatic plug gap reduction to get the engine to continue to start on an old spark plug.
When I got home I measured the gap of the plugs that I had radically reduced the gap on, and they were both at 0.009". I then tried gapping a brand new plug down to 0.016". Unsurprisingly the engine fired up hot with the new plug, and ran well without any stalling. When I shut the engine off again after a few minutes of operation it absolutely would not start with the kick starter. I rode for about an hour on this new plug with the 0.016" gap and stopped many times to see if it would kick start. Not once was I able to get it to fire with the kick starter. When I put one of the 0.009" gap plugs back in though the engine fired right up with the kick starter.
The engine did run a bit differently with such a dramatically reduced plug gap. One day when the fuel was seeming like a very slow flame front travel speed fuel there were some strange bobbles and stumbles at low engine speed as soon as the throttle was cracked. Switching back to a wider plug gap made these strange stumbles go away, but the fuel was obviously a slow flame front travel speed fuel as the engine would not rev all the way out and there was considerable hesitation up at higher engine speeds. I rode to town and filled up with a fresh tank of 91 (RON+MON)/2 octane rating premium gasoline and the engine once again revved all the way out and made full power. The stumbling problem also went away on the fresh fuel with the 0.009" gap plug reinstalled.
I had actually been having considerable trouble with the flame front travel speed of the fuel seeming unusually slow, but each time I got fresh premium gasoline the fuel seemed more like it always had in the past. The stock SEM ignition is actually much more sensitive to problems with the flame front travel speed of the fuel being unusually low because it does not get the additional crankshaft wiggle advance up at 5,500 or 6,500RPM the way that the points ignition system does. There is in fact a very slight observable bump in the timing at the same engine speed as the crankshaft wiggle advance. This can be seen with an inductive pickup timing light on the stock SEM ignition, but the advance provided is extremely small compared to the substantial 4 degrees of crankshaft rotation that the points ignition system gets from the crankshaft wiggle advance.
Because the stock SEM ignition essentially provides just a fixed spark timing from 2,000RPM all the way up slower flame front travel speed fuel tends to cap the maximum engine speed. On faster flame front travel speed premium gasoline like has always been available at the pumps as 91 (RON+MON)/2 octane rating premium gasoline the Husqvarna engines rev all the way out and make big power up to very high engine speed just with the single fixed spark timing value.
With the dramatically reduced plug gap seeming to totally solve the hard starting and stalling problems of the stock SEM ignition system I thought that perhaps I could back off on the timing even more. Since I had always run 21 or 23 degrees BTDC on the static timing setting on the 10.2:1 1991 WMX 610 motor with the points ignition I figured that 29 or 30 degrees really was far too much advance for the 11.3:1 engine.
There are actually a few reasons why the stock SEM ignition would tend to require slightly more spark advance than the points ignition system, but this would be just a small difference. The narrow plug gap dictated by the weak spark means that there is a slightly longer initial delay in the building of the flame front after the spark plug fires. The stock SEM ignition required a 0.023" or smaller plug gap, but I often ran the plug gap up at 0.030" or even 0.035" with the points ignition. With the points ignition the plug gap is not critical, and the engine also seemed to run about the same with the plug gapped all the way down to 0.023". The difference between 0.023 and 0.035" is really not all that huge. With the plug gapped all the way down to just 0.009" though the spark is dramatically smaller, and this does introduce an additional small delay as the flame front builds. The smaller spark can make the engine seem like it is running with a slightly slower flame front travel speed fuel or that the engine has a slightly larger bore. The reality though is that this additional delay caused by the extremely small spark is not of the same nature as slower flame front travel speed fuel or a larger bore. With slower flame front travel speed fuel or a larger bore the intake charge takes longer to burn throughout the flame front travel period. When it takes longer to burn the amount of fuel required to get the engine to light off on late compression ignition this is going to cause somewhat harsher and less efficient engine operation. The delay caused by a very small spark is however only an initial delay that requires a bit more spark advance. The amount of fuel burned during this initial time when the flame front is small in the area of the spark plug electrodes is in fact very small. Once the flame front builds to a slightly larger diameter the rate of fuel consumption is the same as for any larger spark plug gap. The one thing that the very small plug gap does however do is exacerbate problems with a fixed spark timing since the small additional delay is more significant up at higher engine speeds. The additional delay caused by a small plug gap can be totally compensated for with a different advance curve with essentially no penalty in engine performance or efficiency, where slower flame front travel speed fuel will always reduce the range of engine speeds over which good performance and efficiency can be obtained no matter how competent the ignition system is.
The fact that the stock SEM ignition does not get the additional crankshaft wiggle advance also means that the timing setting would tend to need to be a bit earlier to get the same high engine speed performance. There is however another factor that tends to counter these tendencies, and that is the fact that the actual spark energy may increase dramatically at higher engine speeds on the magneto driven CDI type ignition system. With the points ignition on the other hand the spark energy is at a maximum at low engine speeds, and drops off somewhat at higher engine speeds. The fact that the spark energy drops off somewhat at higher engine speeds with the points ignition system while the spark energy increases at higher engine speed on the stock SEM ignition system does somewhat make up for the lack of a crankshaft wiggle advance on the stock SEM ignition system. All in all the stock SEM ignition system seems very similar to the points ignition system in terms of high engine speed performance. All of the spark size and spark energy differences end up being quite small factors in overall engine performance. It is down at low engine speed that the two ignition systems are radically different.
Of course the biggest practical difference is that the stock SEM ignition stays at the timing value that is set, where the points ignition system is constantly on the move and spark timing can be hard to maintain at a constant value. The big reason that the spark timing changes on the points ignition is that the cam follower wears, constantly yielding a later and later spark timing. If the cam is well polished and the cam follower is well lubricated the ignition system can stay set to within one degree for dozens of hours of operation even on the very high revving short stroke 350 engine, but there are other reasons that the points ignition on the Husqvarnas ends up not staying set at the same value. One is that the points slip sometimes and jump to a different timing setting, typically a later timing value but sometimes they will jump to an earlier timing value shortly after being set. This particular problem of the points jumping to a different timing value has to do with just what hardware is used to hold the points down on their plate, and is much less of a problem if a substantial plate is used instead of a flimsy sheet metal timing plate out of a distributor like I used on my first Husqvarna points ignition system. The other reason that the timing does not stay at the same value on the points ignition is that timing changes can be made with just a few small and light tools that are easy to carry on a trail ride. With the ability to easily change the timing it is inevitable that small adjustments will be made to attain best possible performance in light of small variations in fuel properties. With the stock SEM ignition not only can adjustments not be made on the trail, but even at the truck or shop where the tools are available it is a somewhat more involved process to pull the flywheel off than to simply bump the points a bit. When the fuel changes and the bike does not run as well all that is usually done with the stock ignition is to complain about crappy fuel and hope for better performance on the next tank.
The stock SEM ignition system really does perform much differently down at low engine speed because it advances from the starting and idling spark timing up to the running spark timing at 1600RPM. The stock SEM ignition provides 20 degrees of crankshaft rotation of additional advance at 1600RPM, although all of that advance does not actually come right at 1600RPM. Right at 1600RPM about 15 degrees of the additional advance comes on all at once, and then the remaining five degrees of additional advance comes gradually as the engine speed is increased up to about 2,000RPM. The result of this low engine speed advance is that the engine runs more smoothly and powerfully down at extremely low engine speeds than is possible with the points ignition with no centrifical advance mechanism. Since all of this advancing comes down at such a low engine speed, well below the normal operational engine speed range, it is easy to overlook as insignificant. The performance of the engine way down low is however significant for preventing stalling on tight and difficult trails. If the stock SEM ignition is stalling like it often does with the stock recommended 0.023" plug gap then all of this low engine speed competence really is insignificant. If on the other hand the stock ignition happens to not be stalling with the dramatically reduced plug gap then very low engine speed performance can be quite impressive.
After I reduced the plug gap all the way down to 0.009" I was amazed at the way that the big 577cc engine could tractor along in difficult situations without stalling. All the way down to 1200RPM very small amounts of power could be delivered in low gears to get through tight maneuvering situations without having to resort to slipping the clutch. With the points ignition the absolute minimum engine speed was usually about 2,000RPM, although slightly lower engine speeds were sort of possible. At the low end around 1500RPM the points ignition system with it's dramatically reduced flywheel weight just coughs and dies. The combination of much later spark timing at very low engine speeds and a substantial flywheel weight means that the engine can just be lugged down lower and lower without stalling, even if it does become quite harsh. It should be noted though that the extra flywheel weight is not really desirable in and of itself. The clutch action when delivering power at a more normal 2,000 to 3,000RPM feels much better without the flywheel. It is only way down at the lowest engine speeds below about 1,500RPM that the extra flywheel weight is any real benefit, and even at that it is mostly the later spark timing that prevents stalling way down low.
Even though the stock SEM ignition might tend to require slightly more spark advance than the points ignition system I still thought that the 29 or 30 degree BTDC timing value was far too early for the 11.3:1 engine. I took the flywheel off and turned the stator all the way forward for the latest spark timing value. All the way at the late end of the ten degree of crankshaft rotation range of adjustment I believe the spark timing is close to 25 degrees BTDC. With the 20 degrees of additional advance at 1600 to 2000RPM this would yield a starting and idling spark timing value of 5 degrees BTDC. Quite late, but not unheard of.
After I backed off all the way on the timing the engine fired right up with the kick starter, but then stalled. It took another kick to get it to start and run. The idle speed had dropped off a bit, but the idle was stable and it did not stall. When I blipped the throttle it sounded healthy, and when I rode off I was amazed that hesitation was not much of a problem.
With a fresh tank of 91 (RON+MON)/2 octane rating premium gasoline the engine ran great with the new later spark timing, really much more similarly to the way that the original 10.2:1 WMX 610 motor had run with 23 degrees BTDC on the static timing setting on the points ignition system. The rebuilt 1997 motor is however capable of making more power than the bone stock 1991 WMX 610 motor for a variety of reasons.
The increased compression ratio is inevitably going to lead to a boost in performance, but the boost in performance from an increase in the compression ratio is especially dramatic when the compression ratio is so low that spark timing earlier than 20 degrees BTDC is being required to get the engine to run. The longer 250 degree at 1mm valve lift of the 1994 camshaft installed four degrees advanced also seems to be a significant performance increase over the 242 degree at 1mm valve lift 1991 camshaft installed straight up. With the same intake valve closing time low engine speed performance way down at 3,000RPM is just as good, but the longer overall duration means that the engine can be a bit more efficient up at high engine speed. Particularly the longer exhaust duration reduces the pressure in the cylinder on the exhaust stroke, increasing efficiency at maximum power output. The larger valves also inevitably lead to better flow at high engine speed and increased engine performance. Of course the really big dramatic difference is with the lighter piston. The dramatically lightened 332g piston just frees up a whole lot of power up at the top of the engine speed range. Not only does the engine make more power up at high engine speed, but it is also smoother and quieter. The stock 406g piston revving out to 8,000RPM always made a horrible racket, and the engine just felt heavier and less willing to rev even if it did sometimes rev well over 8,000RPM making large amounts of power. With the cut down piston high engine speed is much less drama and a whole lot more power. The power does not level off as much at the top end either with the cut down piston, instead it just makes more and more power as high as the fuel will allow the engine to rev.
Just how high the three inch stroke engine can rev ultimately depends on the temperature of combustion potential of the fuel. If the flame front travel speed of the fuel is extremely slow then the fixed spark timing does not allow the engine to rev all the way out, but this is not an ultimate engine speed limitation it is just a mismatch between the fuel being used and the advance curve (fixed spark timing in the case of the SEM ignition). The real engine speed limitation has to do with the stroke of the engine and the temperature of combustion potential of the fuel. On the low end pretty much any fuel does better and better up to a mean piston speed of a six inch stroke diesel engine running at 2,500RPM, which would be equivalent to 5,000RPM in the three inch stroke Husqvarna. For gasoline engines though all normal fuel is usually considered to be good for considerably higher mean piston speeds.
A common example of elevated mean piston speed would be the ubiquitous four inch stroke big block Chevy automotive and racing engines that often make power all the way up to 7,000RPM even in rather mild street and performance builds such as the factory performance "crate engines". In terms of mean piston speed the four inch stroke big block Chevy revving to 7,000RPM is equivalent to the three inch stroke Husqvarna revving to 9,300RPM. The difference though is that the Husqvarna with it's much lighter rod and piston along with four valves per cylinder running on an aggressive roller camshaft is going to have a much easier time attaining this elevated piston speed, and is also going to be much more efficient up at that high mean piston speed as well. Getting the big block Chevy to rev out to 7,000RPM with it's two valve per cylinder push rod valvetrain and heavy rods with rod bolts is not so easy, and efficiency at that high piston speed is typically very poor.
This same mean piston speed is also attained on shorter stroke engines, particularly dirt bikes without rod bolts. A 250F class dirt bike with a 2.07 inch stroke revving out to 13,500RPM, or a 450F class dirt bike with a 2.5 inch stroke revving out to 11,000RPM are both operating at this same mean piston speed as a four inch stroke big block Chevy revving to 7,000RPM. It is of course more difficult to get a small engine to run at this high of a mean piston speed when it requires 13,500PRM, but the 250F class dirt bikes have extremely radically oversquare configurations with really huge valves for good flow at those very high engine speeds.
An example of this same high mean piston speed in a three inch stroke engine would be the V12 Ferrari engines that rev out to about 9,000RPM in stock form. And that is with rod bolts!
As soon as I rode the 1991 WMX 610 for the first time in nearly a decade I realize that the suspension is somewhat different than the externally nearly identical 1991 WXE 350 suspension. Both bikes use the 40mm White Power 4054 Multi Adjuster forks, labeled "Husqvarna Special Fork by WP" on the 1991 WXE 350. Both bikes also use the same White Power shock and the exact same linkage on identical frames and swingarms. The external difference is simply that the WXE version of the forks is 9/16" shorter and has 0.2" less travel. The shorter fork allows the front end to sit lower for the slightly shorter wheelbase and slightly steeper head angle of the enduro model. Otherwise the bikes appear to be identical, and the spring rates even appear to be identical. The valving on the two 1991 Husqvarna models is however not identical. As similar as the two models are they ride and handle somewhat differently. The biggest difference is that the compression damping is more progressive on the enduro model, which results in somewhat harsh and uncomfortable landings from big jumps. The WXE does not appear to be particularly prone to bottoming, it just gets harsher down near the bottom of the stroke and is therefore not able to handle as large of impacts with ease.
There are some other subtle differences with the valving also. The WXE 350 tends to get a bit harsh feeling when delivering big power at high speeds over rough terrain, where the WMX feels much more able to handle big power and high speeds. The WMX can feel a bit more harsh and unwilling to follow the terrain at low speeds, but this problem is easily solved by going faster. At higher speeds the WMX suspension comes alive, and even on the roughest terrain it is able to go very fast and stay well in control. The rebound damping also feels a bit more competent on the WMX, although this difference is extremely subtle.
I pretty much run all stock settings on the suspension on both of the 1991 Husqvarnas, but there are some significant differences that are a big performance improvement on both the WXE enduro bike and the WMX motorcross bike. The big change is going down to 5W fork oil from the stock recommended 10W. This change was recommended by Racetech, and is in essence a revalve of the suspension. I believe that the 40mm White Power forks were in fact designed for 5W oil, but for some unknown reason Cagiva specified 10W in the 1991 Owner's Service and Tuning Manual. Going down to the 5W oil dramatically improves comfort and makes the bike significantly faster under all conditions. Even out on a motorcross track the 5W oil just works a whole lot better. Even after going down to the 5W oil I pretty much always stick with the stock compression and rebound settings, except that I often run one click less (out of seven) on the compression damping to give an even smoother ride. Even with the lighter 5W fork oil and one less click of compression damping the forks are able to handle big impacts with ease and do not dive or wallow under heavy braking in rough terrain.
Out back the setup is a bit trickier. The stock shock is supple and comfortable on both the WXE and WMX models, but it is tricky to get the action to feel just right. The main problem is that the rebound damping always seems a bit too light. I always run the rebound damping turned in to seven on a scale of eleven, where the stock setting is just three. If I try to use more rebound damping than seven then drive traction is severely compromised, so this is the maximum rebound damping setting for all normal off road riding. The stock spring also tends to feel a bit too soft, and bottoming of the rear end is a constant problem. I used to go with more pre-load to reduce bottoming, but I found that this also causes traction problems and interferes with good handling over bumps. The reality is that the shock spring is just on the soft side. This is good for lighter riders, and is great for really rough trails. The trade off is that heavier riders always feel that the rear end bottoms too easily even if the bike is still extremely fast. The compression damping is quite competent on the shock, and big landings on a motocross track are handled with ease. When the big 13 inch travel rear suspension extends all the way while in the air there is a whole lot of energy absorption that can take place upon landing.
I had become so fond of the overall handling of the WXE 350 that the first thing I changed on the WMX when I started riding it again was to raise the fork tubes in the triple clamps as far as they would go. It turned out that I was only able to raise the tubes 3/8", but this was enough to bring the front end down closer to the way the WXE sits and handling under all conditions seemed to improve. At first I had a bit of trouble re-adjusting to the feel of the WMX in turns, although switching to a Maxxis Maxxcross IT front tire that I had never used before was part of this. Interestingly the Maxxis tire has appeared to work better as it wears in, on the first ride I thought it was so twitchy that I swore I would never buy another Maxxis front tire. The problem is that the Maxxis IT front tire does not have much in the way of side knobs. Once traction has been lost there is just not much there to fall back on. The Kenda K760 front tires I usually run are much more forgiving, and ultimately quite a bit faster as well.
For the six speed WMX 610 I chose 50/14 gearing, and this has turned out to be really quite good. In fact the wider spacing between the gears is so good on the big 577cc motor that I am thinking about trying 49/15 gearing also. Having a low first gear makes running taller gearing much less of a chore. I still usually stay in second gear even on tight single track trails, but having a much lower gear to drop into for casual maneuvering at low speed is a huge labor saver. The jump between first and second is large, but this is surprisingly not much of a problem. A big part of why the wide spacing between the gears is working so well is that the engine is running over a very wide range of engine speeds. The higher 11.3:1 compression ratio seems to do wonders for low engine speed torque down to 3,000RPM and even 2,500RPM in emergencies, and I find that I rarely have to slip the clutch even on the toughest trails.
Fuel consumption is also lower on the higher compression ratio engine, and this is in large part due to the leaner needle clip position that the higher compression ratio makes possible. On one trail ride the bike burned only 0.55GPH, which is significantly less than I had ever seen the big 577cc engine to burn at the 10.2:1 compression ratio. And that was a mixed ride with considerable dirt road cruising and quite a bit of big power acceleration. Another benefit of the leaner needle clip position is that altitude changes are not as much of a problem. With all of the fuel being burned up to high elevation at small to medium throttle openings it is only the lower compression pressure caused by the thinner air that causes worse hesitation at high elevation. And as it turns out this compression pressure difference between sea level and 6,000 feet is a much smaller difference than the dramatic changes in engine performance seen when a rich mixture prevents all the fuel from burning at high elevation.
Thinking that the new centrifical de-compressor would be sufficient for all purposes I did not even bother to install the manual de-compressor leaver and cable which is so indispensable on the early Husqvarnas. As it turns out the centrifical de-compressor indeed does do everything that the manual de-compressor did, and more. Kick starting is easy in that the kick starter can just be stabbed, with no elaborate setup procedure for bumping past top dead center as is normally required to start a four stroke dirt bike. The centrifical de-compressor also does a fairly good job with roll starting, although it is still possible for the engine to end up stuck on the small amount of compression still remaining past the de-compressor cam. About 19 times out of 20 the engine does not end up in this precise location, and roll starting is just as easy as with the manual de-compressor. The final use of the manual de-compressor was to restart the engine after aggressive rear brake use down a hill caused it to stall. The centrifical de-compressor handles this task even better than the manual de-compressor, and it is essentially no longer necessary to even worry about stalling the engine with the rear brake. As long as the rear wheel is not brought to a complete stop in first gear on very loose terrain the engine will simply restart as soon as the rear brake is released.