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A Period Correct Big Cam 1991 Husqvarna WMX 610

Amazingly another 1991 Husqvarna WMX 610 showed up on the used bike market at a rather low price. In many ways this one turned out to be the best deal yet. Not only was it in fairly good overall condition and nearly ridable just as it was, but it came with two boxes of very interesting spare parts as well for a total purchase price of just $1000.

Purchase and First Inspection
Suspension Service and Inspection
Gasoline Failure
Plans for the Future
Box of Goodies
Maximum Torque Point

Purchase and First Inspection

The story was that it had been a one owner bike and the guys selling it had only had it for a short time. It looked like it hadn't been ridden much in quite a long time, but they said it did start and run. Sure enough it fired up on the third kick. One kick without the choke, nothing. One kick with the choke, again nothing. Then on the third kick it fired up and ran. It wouldn't low idle, and even at small cracks of the throttle it was a bit unstable. Once riding along though it responded fairly well at all smaller throttle openings. At wider throttle openings it felt like it was drowning in an extremely overly rich mixture. They said they had just gone through the carburetor, but this turned out to be a bit of misrepresentation of the facts. In any case it certainly was a running bike, so the $1000 asking price seemed entirely reasonable. Low even. They said they had been asking $1500, but even at the lower advertised price of $1000 I was the first taker.

The forks were covered with thick and sticky dried up old fork oil and as soon as I rode the bike more oil started leaking out of the seals. The shock also started to leak very slightly as soon as I rode the bike, and although the rebound damping still worked I could hear air squishing through the valving near the top of the stroke. It really seemed like this bike had not been ridden in a very long time. The tires also looked very old. Both because they were old fashioned Mezler tires that haven't been popular since the early 1990's and also because they were old and somewhat deteriorated like they had actually been on the bike since the 1990's. Both brakes worked though, and it really was close to being ridable just like it was. Both the forks and the shock had stickers from a race shop on them. Both were stamped "TC" to indicate the WMX motocross model and not "TE" for the WXE enduro model, so at least it was the correct suspension. I just rode it around a small bit to make sure the gears were all there and that the rod bearing wasn't totally toasted. I probably would have bought it anyway whatever was wrong with it, but it is generally good practice to get some idea about a bike with a little test ride if possible.

That first day I fired it up both for the test ride and again once I got it back home. It ran smoothly at all lower engine speeds around 2,000 to 3,500RPM at very small cracks of the throttle, and then at only very slightly wider throttle openings it got extremely harsh at all lower engine speeds bellow about 5,000RPM or so. The fact that it had started repeately on an old spark plug and the harshness at lower engine speeds both told me that the spark timing was set somewhere close to the factory stock 33 degrees BTDC on the stock SEM ignition. It seemed to rev up and pull fairly well at fairly small throttle openings around 1/4 throttle, but then it was sluggish at wider throttle openings. I pulled the carburetor off that same night to see what was going on with it. Sure enough the needle clip was in the factory stock 3rd groove on the stock K32 needle. The main jet was labeled as a 190 size, but it was drilled out to a bit bigger than the 200 size. That is a lot bigger than the stock 180 size main jet that is already rather fat. No wonder it felt like it was drowning in gasoline. The pilot jet was the stock 62 size, and the choke jet was the stock 60 size.

The old Twin Air foam filter element had seemed to work fine on those two short little test rides, but it turned out to be old and in very poor condition. It was clean and perfectly oiled, but it was also deteriorated and fell apart when I pushed on it. The intake screen had been removed from this bike, so that falling apart old filter could have resulted in some bent intake valves if substantial chunks of it had managed to make it through the carburetor under wide open conditions. There were two air filters in the boxes of spare parts. An old one still dirty and oily on a spare filter cage, and a new unused element from a company I hadn't previously known of. Holland Air, somehow very appropriate for a bike with Dutch made WP suspension. Instead of using this unusual filter I decided to clean up one of my old ones. Somewhat ironically this spare filter element that I had was the dirtiest and greasiest artifact from my 1991 Husqvarna WMX 610 riding more than ten years ago. It was the Uni brand filter element that was on my 1991 Husqvarna WMX 610 when it got parked outside under a tarp in 2007. It had already been on the bike for a while at that point, and was plenty dirty.

I set about cleaning the element using a recently purchased 14.5 ounce can of Uni brand foam filter solvent. It wouldn't cut the old Bel Ray brand Foam Filter Oil. This was the first time I had tried to use the new solvent, and it didn't work. I had been using my old can of PJ1 foam filter cleaning solvent from 20 years ago. That can of solvent had done probably a dozen filter cleanings and had seemed to work very well. It was the water activated type. The instructions were to wet the dirty filter with water and then spray the solvent on both sides, let sit for 15 minutes and then rinse out with water. This worked very well, and I used to save solvent by starting out with a pot of boiling hot soapy water to get the big chunks of dirt off. The new Uni brand solvent didn't work like that. It didn't seem to do anything to the Bel Ray brand Foam Filter oil.

I boiled the dirty element in hot soapy water for hours, but that wouldn't cut the Bel Ray Foam Filter oil either. I eventually sprayed most of the can of Uni foam filter cleaner on the element and rinsed it with what seemed like hundreds of gallons of pressurized water but the oil wouldn't budge. Eventually after all of that laboring most of the dirt did make it's way out of the filter. When I was done though I didn't even need to re-apply more Bel Ray Foam Filter oil. It was still substantially well oiled after all of that boiling and solvent spraying. The Bel Ray Foam Filter oil really doesn't dissolve in soap or in the wrong type of solvent. Looks like they are trying for larger sales volumes of new foam filter elements at Uni. I buy new ones all the time, but I also like to be able to clean the old ones when new ones are a whopping $23 just for a few little pieces of foam and not available from local stores.

What was most interesting about this whole laborious process though was that the old Uni brand foam filter element was in substantially good condition even after sitting all those years covered in oil and dirt. It held up amazingly well to all that boiling and rinsing and still seemed perfectly serviceable. The glue holding the pieces of foam together had weakened a bit and all that rinsing had opened up some small separations, but the foam itself still seemed substantially strong. In any case I got a clean filer installed, and I was also pleased to see that the original intake boot was in rather good condition.

The next day I soldered up the main jet and re-drilled it to the 175 size and I set the needle clip on the first groove. This got the engine pulling and I got an idea of what it was. Again it fired on the third kick, but it stalled. It fired right back up, but it was more unstable at small cracks of the throttle at first when cold than it had been the day before. It was still smooth, quite and rather powerful at very small throttle openings at all lower engine speeds around 2,000 to 3,500RPM and then it still got very harsh as soon as the throttle was cracked open farther at 3,000 to 5,000RPM. When I revved it up a bit farther though the power seemed fairly good for a stock 610 engine. With close to workable jetting it pulled at all throttle openings, and wide open on the top end it seemed to run stronger than my other bone stock 610 motor with the heavy 406g Mahle piston.

Something else I had noticed right away was that the 13/52 gearing on the bike felt strangely very tall. Once I got the motor pulling fairly well with reasonable jetting this strangely tall geared feeling persisted. I have my other five speed 1991 Husqvarna WMX 610 bikes geared much taller with 14/52 gearing on one and 13/48 gearing on the other. Strangely this stock 1991 Husqvarna WMX 610 had a very tall geared feeling in all gears with the 13/52 gearing.

I pulled the ignition cover off, which was poorly installed and had been leaking anyway. I found top dead center through the spark plug hole, made some marks on the flywheel and with a stroboscopic timing light I checked the spark timing. Sure enough it was about 32 or 33 degree BTDC running spark timing at all engine speeds above about 2,000RPM. My Actron L100 timing light was acting up very bad, so I had a bit of trouble with the reading but it was clearly above my 30 degree BTDC mark. At first my timing light was only firing every once in a while, mostly it was seeming dead. It had done that a few times in the past, so I banged it against my leg which had usually got it going. This time though it just completely stopped working. Absolutely nothing, and I could hear something rattling around inside. I pulled it apart and a large plastic piece with some windings on it to form an inductor was loose and bouncing around. This inductor had three terminals that were unevenly spaced; so there was only way that it would go back on the circuit board. I soldered it back in position and the timing light again worked poorly as it had before that thing had fallen off. Just a few flashes here and there and then up to 10 seconds with not a single flash. It was very hard to use, but I did see that the engine was firing well before 30 degrees BTDC. It looked like it was probably at 32 or 33 degrees BTDC and not out at 35 degrees BTDC.

The spark plug was very old looking, and the electrodes were actually worn quite a bit. It looked like the gap had been set close to the wide end of the factory stock recommended 0.023 to 0.027" range, but the electrodes had worn back to a 0.030" gap. That in itself was sort of amazing; that one spark plug had actually continued to work sufficiently long on a stock 1991 Husqvarna SEM ignition that the electrodes had actually had a chance to wear out. I gapped that lucky spark plug back down to the stock 0.025" gap and put it back in the engine and it continued to fire up fairly easily in just a few kicks every time.

While I had the ignition cover off was also an opportunity to check on the valve timing. There were some clues that something was far from stock about the camshaft in this motor. The unstable low idle seemed a lot like a big camshaft to me, and also the way that the torque felt as the engine speeds increased seemed to scream big camshaft. A big clue was that there was a nearly new looking and apparently mostly unused stock 1991 Husqvarna WMX 610 camshaft in the box of spare parts. As strange as this might sound the fact that the engine was starting so reliably on the stock SEM ignition also seemed to indicate to me that the camshaft wasn't stock. A later intake valve closing time reduces cylinder filling at cranking and idling speeds, and this can under some conditions allow a weak spark magneto ignition system to start more easily. And finally the harshness down at 3,000 to 4,000RPM had a unique character to it. It was obviously the extreme harshness of very early spark timing, but it was dulled a bit compared to what I had experienced on other Husqvarna 610 motors running similar spark timing. It was like the lower cylinder filling caused by the big camshaft was knocking the edge off of the harshness down low.

The best way to describe this is that the excessively early spark timing results in a certain pitch of sound and a certain feel to the harshness at around 3,000 to 5,000RPM. Different stroke lengths make this excess harshness feel and sound different and obviously dramatically different combustion properties of the gasoline make this excess low end harshness feel and sound different. On normal gasoline in an engine of about a three inch stroke length though there is a characteristic sound and feel to the excess harshness, and I am particularly familiar with how this sounds and feels on a Husqvarna 610 motor. This big cam 610 motor had that exact feel and sound to the excess harshness, there just wasn't as much of it. It seemed like the lower cylinder filling bellow about 5,000RPM was just reducing the amplitude of the harshness even though the sound and feel was very much the same.

I decided to pull the valve covers off to check the lash and also to make an estimate of the cam timing. The lash was set rather wide at 0.009 on the exhaust side and 0.008 on the intake side. I left the lash fairly wide, but I did tightened up the intake side to 0.005" and I set the exhaust side just a small bit tighter at 0.008".

When I measured the intake valve opening and closing times to 1mm valve lift I found that it is indeed a bigger camshaft, and it's installed straight up with split overlap right at top dead center. It seemed to be about a 252 degree at 1mm valve lift camshaft with the usual-for-Husqvarna 106 degree lobe center. Rather similar to a 1994 and later Husqvarna 610 camshaft but slightly bigger and installed straight up for a later intake valve closing time. The total valve lift was also a bit larger than on the stock Husqvarna 610 camshafts. Certainly a bigger camshaft but not radically huge, essentially just what I had expected from the observed performance of the engine.

I also pulled the clutch cover off, mostly to inspect the oil reed valve. I had already changed the oil once, and all seemed well and good. There was quite a bit of black material stuck to the magnet like it hadn't been cleaned in a very long time, but there weren't any metal flakes or shavings. A very good sign that the oil reed valve was working and that the rod bearing was in good condition.

There were several other reasons to pull the clutch cover off though. One was that it was leaking all around the gasket. Another was that I wanted to get a look at the clutch itself. The clutch was working, but it was dragging worse than on my other 1991 Husqvarnas.

Sure enough the oil reed valve was in good condition and installed correctly. The primary reduction gear showed hardly any signs of wear, and the clutch basket and clutch disks also were in like new condition. It looked like this engine had not actually been run all that much, or not all that hard. Overall the bike showed some signs of substantial use, but it appeared to be very light use. The foot pegs were worn a bit, but the wear was in a strange sort of a pattern. It was all on the fronts of the pegs. Like the rider had always favored the front of the seat and didn't stand up much. Which interestingly is what I have seen a few light weight novice level riders do when I let them ride my 1991 Husqvarna WMX 610. They just stayed way far forward on the bike and let the rear tire spin wildly. They didn't seem to care much about how fast they were going or trying to do anything particular on the bike, they just seemed to be interested in the sensation of tire shredding acceleration on an open class four stroke. The Metzler rear tire was a bit shredded on this lightly used bike when I got it. Not so much worn out as just lightly shredded from some wild tire spinning with a light weight throttle happy rider onboard.

With the clutch cover off I tried to figure out why the clutch had been dragging so severely. A common problem on the 1991 Husqvarna clutches is that the pressure plate lifts somewhat unevenly as the clutch lever is pulled, and this one also had that problem. It seemed much worse than on the other 1991 Husqvarna clutches I have though. I tried swapping the springs from side to side as sometimes helps on the 1991 Husqvarna clutches, but this seemed to make very little difference. The pressure plate was still pulling severely unevenly.

To figure out just how uneven the clamping force was I backed out on the bolt on the side that wasn't lifting. I kept turning the bolt out until the pressure plate started to lift more evenly. It took a massive 1-1/4 turns, or about 1.25mm of movement on the standard pitch 6mm bolt. Wow, that seemed like a huge amount. To set that amount of a decrease in pre-load with the bolt tightened down I made up a spacer by cutting down a washer. The finished thickness I ended up with was 0.055", which seemed to work well enough. The clutch was lifting much more evenly, and there was still plenty of room under the clutch cover for the 0.055" higher bolt head location.

With the partially broken up old clutch cover gasket glued in place with RTV silicone and the engine refilled with 15-50 oil I fired it up and the clutch certainly did disengage much more evenly. Not as good as a 1992 and later Husqvarna clutch, but at least as good as my other 1991 Husqvarna clutches. Being able to stop in gear is a nice feature, and being able to roll the bike back without too much difficulty while the engine idles in gear is also a big plus.

Suspension Service and Inspection

I scrubbed the old dried up fork oil off of the fork legs with some solvent, and this seemed to slow down the oil leaks considerably. The right side wasn't leaking at all anymore, and the left side was leaking only a small bit. I pulled the dust boots off and the seals were not the crappy stock type seals that seem to give so much trouble. They were more like the modern Italian made seals, and blue instead of the red stock seals. The left fork tube looked like it was scored a bit by the bushing, so I wiped over the slightly damaged area with some 600 grit silicon carbide paper. This got the oil leak slowed down even more, so the bike was fully ridable.

The forks were harsh feeling and felt a bit strange. With the rebound clickers at the stock #3 position there was way too much rebound damping and the front end stayed low over repeated bumps. I turned the rebound clickers all the way out, but then the rebound damping was insufficient and the front end lifted up too much and the bike was in danger of going out of control at higher speeds. At the #2 rebound clicker position the forks sort of worked, but they didn't feel like my other 1991 Husqvarna WMX 610 forks. The big difference was just a lot of extra harshness over all types of bumps at all speeds. I was hoping that it was just the factory stock 10W oil and that they would work better with the 5W oil I always run in the 1991 White Power forks. I pulled the forks apart and changed the oil. It all looked stock inside, but they had the slightly stiffer WXE springs.

Included with the spare parts was a White Power box with the stock WMX fork springs, clean and apparently unused. To set the spring rate the same as on my other 1991 Husqvarna WMX 610 bikes I replaced one of the slightly stiffer WXE springs with one of the new WMX springs. I also set the pre-load at the same level as on my other 1991 Husqvarna WMX 610 White Power forks. With the springs setup exactly the same it would be easier to determine if there were any variations in the valving. While I had the forks apart I also saw why they had been leaking. There was some thick brown residue stuck to the seals near the sealing lips. I tried to wipe this off, but it was hard and very stuck. Instead I wiped the sealing lips lightly with the 600 grit paper working the paper down between the multiple sealing lips the best I could. This didn't make much of a visual difference, but I figured it might be enough to stop the leak.

With the forks back together and filled with 5W oil to my usual 5-1/4" air chamber height I headed out for some more little test rides. Right away I noticed a dramatic difference in the feel and response of the forks. Most of the excess harshness was gone and they seemed to work a lot more like my other 1991 Husqvarna WMX 610 forks. With the rebound clickers on the stock #3 position the front end no longer stayed too low, but it wasn't excessively bouncy either. Much more like my other 1991 Husqvarna WMX 610 forks. Another big difference I noticed with the 5W oil was that the front end had better traction over sharp ruts. I had thought before that the trouble I was having with the little early spring ruts was probably just the inferior old fashioned Metzler tires. With the 5W oil though I was pleased to find that the old fashioned Mezler "MOTO CROSS" front tire actually did sort of work.

The leaks had also completely stopped. I had been correct that the fork seals were actually still good, it was just the hard brown residue on the sealing lips that had been the problem.

With the front end much more normal feeling the rear end then felt overly harsh and strange. The compression clicker wasn't working at all, but the rear end felt even worse than just that. It was harsh and I thought also a bit bouncy. I thought the front end also felt perhaps a bit bouncy. Sort of a bit more like the 1999 Husqvarna TE 410.

The rear shock spring was not the stock "58" rate. I thought it said "60" on it, but it actually seemed a lot stiffer than the stock "58" spring. I thought perhaps it might be a "68" with the blue paint for the middle part of the "8" missing. I backed off on the pre-load to set the bike sag closer to where I usually run it on the 1991 Husqvarna WMX 610 bikes, and this helped a lot with compliance. The valving did however still feel a bit harsh. One possibility is thickened old oil like I have run into in other 1991 White Power shocks. I am however still a bit worried that the valving was actually changed by the company that put their stickers on it. There does seem to be a bit of a bouncy feel that my other 1991 Husqvarna WMX 610 bikes don't have. Part of this is of course the stiffer shock spring. I do happen to have a spare stock "58" shock spring, and this bike also came with a spare "62" shock spring. When I get around to buying one of those Race Tech brand shock spring compressors I can put the stock spring on to get a better idea of the valving. I can also replace the oil, but I am still not sure what I am going to have to do to replace seals and get gas pressure back in the shocks. For the time being I was happy to have the suspension sort of working if not yet able to handle aggressive riding and landings from big jumps. As I have said before, even a flat 1991 Husqvarna WMX 610 White Power shock is a far sight faster than just about anything else around.

Gasoline Failure

Then the next day the gasoline had disappeared. The first gasoline failure was that the stock engine started to surge. Actually the first problem was that the bike wouldn't start. It was firing easily with the kick starter, but then it stalled each time. Each time it would fire, but it would only run for a short time before dying. Often it would run for a half second and then die. Then sometimes though it would run for a substantial five seconds or so and would even rev up. It would rev up rather high, but then it would start to cut out very bad and it would eventually stall.

Even once the engine warmed up enough from these false starts that I was able to pull out it still wasn't running. It would go for 50 feet or so, and then it would cut out hard and the engine speed would drop. The clue I got about what was going on was that it continued to get worse as I rode the bike around. It seemed like the gasoline in the tank was much worse than the gasoline in the carburetor bowel, and as the gasoline in the carburetor bowel was replaced with the gasoline from the tank the engine was less and less able to run.

It would be easy to misinterpret this problem as a problem with the float or a plugged up fuel filter, but there were obvious differences. It was the gasoline itself that was cutting out, not the carburetor. The engine would run at all throttle openings for a few seconds, and then when it started to cut out it was just cutting out all over the place at all throttle openings. That is not at all like a float problem or a plugged fuel filter. It was obviously some strange problem with the gasoline. Eventually it wouldn't start anymore, so I gave up for the day.

Then the next day the engine was again harder to start, but instead of continuing to run poorly it only had trouble for a few seconds until it started to warm up a bit. When I rode off there was a dramatic difference. Gone was the smooth, quite and powerful torque at small throttle openings. Instead it was loud and somewhat harsh right from the first small crack of the throttle, and when the throttle was opened farther there was nothing but a severe lack of power and some rather severe surging. The gasoline seemed extremely weak and it wouldn't rev out much despite extreme levels of crispness and quite a bit of surging across the entire midrange. Again I just gave up and parked the bike.

Then the following day the engine was a bit easier to start again, but the surging was even worse. Now the bike would rev out some and the midrange power was back substantially. Both low end and top end power were however very low and the surging was extremely severe. There was some power across the midrange engine speeds, but it was somewhat weak and the delivery was extremely unpredictable and difficult to control. The harshness at 3,000 to 5,000RPM was totally gone, but so was all of the torque down lower at 2,000 to 3,000RPM. It was very weak feeling at all lower engine speeds, and then the power picked up with an abrupt and unpredictable flurry of surging in the midrange. The engine was seeming to rev out quite a bit, but power was flat at the higher engine speeds despite the surging continuing up into very high engine speeds.

What was different about the big cam engine on low temperature of combustion potential gasoline was that the surging continued up to much higher engine speeds. With the stock camshafts low temperature of combustion potential gasoline causes lots of midrange surging around 4,500 to 6,000RPM in the 610 engines but then the surging drops off as the power drops off at higher engine speeds. With the bigger camshaft the engine was attaining higher cylinder filling to higher engine speeds, and this caused the surging to continue up higher. Essentially it was surging severely all the way up as high as it would rev. Interestingly though the top end power was still seeming very disappointing on the low temperature of combustion potential gasoline. The engine was surging over a wider range of engine speeds, but the power was still flat and dead up top. Part of this is the heavy 406g stock Mahle piston. As the engine revs past about 7,000RPM that heavy piston extracts more and more of a toll on efficiency and performance. When top end power output is already dramatically reduced by low temperature of combustion potential gasoline that big chunk of power required to throw the heavy piston back and forth represents an much larger and much more significant loss of output.

Then the next time I rode the big cam 1991 Husqvarna WMX 610 the surging was gone, but in it's place was huge amounts of hesitation and a severe loss of torque at all engine speeds. The hesitation was worst after coasting or cruising, and once warmed up on some big pulls I was able to get the motor going some. Even once it was running and pulling though torque was low and it wouldn't rev up much. The harshness was still totally absent, but so was most of the torque. Worst of all though the engine wasn't really usable anymore as the very severe hesitation just completely stopped the bike under most real conditions. Again I just gave up and parked the bike.

When I rode it again a few days later the harshness was back at 3,000 to 5,000RPM, and so was the surging. The surging wasn't as severe as it had been earlier, but it was still pretty bad. The power was low, but there was some power. The bike worked this way, but it wasn't good. Low end torque was pathetic and there was harshness, and then when revved higher there was bad surging that caused a rather unpredictable and difficult to use power delivery. The harshness was certainly smaller in magnitude than it had been, but it was also coming at smaller throttle openings. Instead of the smooth, quiet and rather powerful torque at small throttle openings from 2,000 to 3,500RPM there was just harshness and not much torque. The harshness didn't continue up as high in the engine speed range either, but in it's place was severe weakness at small throttle openings and then a bunch of surging all over the place at wider throttle openings.

As bad as this sounds it might have been the best overall performance of the big cam engine with stock 33 degree BTDC spark timing. The extremely excessively early 33 degree BTDC spark timing causes such severe harshness across a very wide range of lower and midrange engine speeds that overall performance tends to seem very bad. The weaker and lower temperature of combustion potential gasoline that causes surging is itself a lot worse, but it does substantially hide the excessive spark timing.

From a riders perspective the higher temperature of combustion potential gasoline and it's even and predictable power delivery is clearly far better. Regardless of the spark timing value the hotter burning gasoline delivers better power and more of it, a clear winner.

In terms of getting the engine to run well though hiding the severe problems associated with 33 degree BTDC spark timing is very compelling however it is accomplished. It's not that the engine runs better on the lower temperature of combustion potential gasoline, it's just that the excessive spark advance is no longer such an obvious issue. When riding the bike with the weaker gasoline the impression is always just that the gasoline is horrible, or that the stroke length is far too long. The lower temperature of combustion potential gasoline simply makes it easier to ignore the extremely excessive spark advance.

Of course it is also easy to blame the lower temperature of combustion potential on the excessive spark advance. The logic is very clear here. The extremely excessively early 33 degree BTDC spark timing is a huge problem. The lower temperature of combustion potential gasoline tends to hide the extremely excessively early spark timing by replacing the harshness with surging. In a way the extremely excessively early spark timing then requires the lower temperature of combustion potential gasoline. Another way of looking at this is that the lower temperature of combustion potential gasoline performs essentially the same regardless of the spark timing value. This very low temperature of combustion potential gasoline causes severe surging in three inch stroke length engines running any spark timing value from 10 degrees BTDC to 40 degrees BTDC. It simply doesn't matter much what the spark timing value is if the temperature of combustion potential of the gasoline is too low for the stroke length being used.

If anything, less spark advance does a better job of mitigating the problems of an excessively low temperature of combustion potential or an excessively long stroke length. I have noticed over and over again that my Czech Republic CDI equipped 10.2:1 bone stock 1991 Husqvarna WMX 610 surges much more severely than my 12.2:1 hot rod 610 motor on the same gasoline. On somewhat weak gasoline the 26 degree BTDC spark timing of the 10.2:1 motor can be useful for extracting good lower midrange torque around 4,000 to 5,500RPM where on the same gasoline the 12.2:1 hot rod 610 motor running 16 or 18 degree BTDC spark timing can easily seem a bit sluggish at those engine speeds. On extremely low temperature of combustion potential gasoline that causes huge levels of horrible surging in the stock 10.2:1 motor running 26 degree BTDC spark timing though the 12.2:1 hot rod 610 motor can do something entirely different. Sometimes it can do an amazing job of imitating a shorter stroke length engine by tuning the spark timing just right so that power comes on in one even surge from 6,000 to 8,000RPM. That is not really such a great setup for a Husqvarna 610 dirt bike when torque everywhere bellow about 5,000RPM is rather low, but it is possible to get some amazingly even and predictable power at 6,000 to 8,000RPM on gasoline that won't do anything but surge horribly at 4,500 to 6,000RPM with 26 degree BTDC spark timing in the 10.2:1 motor. The higher compression ratio and less spark advance can provide a very real means of mitigating extremely low temperature of combustion potential gasoline in a long stroke length engine.

Likewise the 33 degree BTDC spark timing on the big cam Husqvarna 610 motor seems to actually make the surging worse. The big camshaft stretches the surging over a wider range of engine speeds which is good for keeping acceleration high, but the hugely excessive 33 degree BTDC spark timing actually appears to make the surging more severe with a more difficult to control choppy power delivery.

Something interesting was that through all of these dramatic overnight gasoline changes the 13/52 gearing always managed to seem strangly tall. Even when the harshness was completely gone in either a sea of surging or a tons of hesitation the gearing continued to feel very tall. What the 33 degree BTDC engine also did was get very hot high idling at around 2,000RPM. Once moving along it cooled right off, but when high idling at a stop there was just tons of heat comming off. What ever was going on with the gasoline from day to day the huge 33 degree BTDC spark timing continued to cause noticeably much hotter operation than on my other stock 1991 Husqvarna 610 running 26 degree BTDC spark timing. Of course the stock camshaft and Czech Republic CDI ignition combination also idles down to around 1,200RPM at 11 degree BTDC spark timing which is obviously a milder low idle than 33 degrees BTDC at 2,000RPM.

I did have the big cam 33 degree BTDC Husqvarna 610 motor idling smooth and stable down at 13 degrees BTDC at 1,200 and 1,300RPM sometimes, but it didn't seem to wan't to do that. The bigger camshaft seemed to cause a rather unstable low idle that required very precise tuning of the idle mixture screw and the throttle stop. The easier strategy was to just turn the throttle stop in and let it high idle up at around 2,200RPM where it was much more stable and much less sensitive to mixture changes. Back in 2002 through 2006 I usually let my points ignition Husqvarna 610 idle up around 2,000RPM also. Back then 23 degree BTDC spark timing on the four inch bore seemed to want to idle up pretty high.

Plans for the Future

More than once I have been very tempted lay into this big cam, but otherwise entirely stock, Husqvarna 610 motor and fix it once and for all. I would like very much to just back off on the spark timing down to less than 27 degrees BTDC to get rid of the low end and midrange harshness. Probably even just going down to 29 degrees BTDC would substantially do the trick on the big 610 motor these days. I haven't done that though for a couple of reasons. Most importantly I am quite impressed with the repair that someone came up with all those years ago to simply put a big camshaft in the motor. I have myself speculated that a bigger camshaft would be good for using the stock SEM ignition on the 610 motor, and here is an example of someone else having come up with that same idea decades earlier.

A big part of what I like about this particular bike is that it is an example of how someone else actually did get a mostly stock 1991 Husqvarna WMX 610 to work fairly well as a dirt bike. It is almost like the stock SEM ignition is setup so that it will only work with a big camshaft. Why didn't it come with this bigger camshaft as stock equipment? It sort of seems like it should have.

Of course there is also the simple fact that it is too big of a camshaft for the application. With a functional ignition system the stock camshaft delivers much better low end and midrange torque and also a broader and more powerful feeling power band. The stock camshaft was correct, it was the stock ignition system that was incorrect. Despite this obvious reality the big cam engine is interesting. It does deliver more peak power, and moving the power up to higher engine speeds sounds impressive. Probably not a good idea for the oil reed valve lubricated rod bearing, but it does sound and feel impressive on normal gasoline. Power continuing to build to higher engine speeds feels like a shorter stroke length, and this can substantially bolster the thrill factor on a dirt bike.

In the end it is just the simple fact that it starts like it is that is most significant. A reliably starting stock SEM ignition is quite a treat after so many years of repeated frustrating failures of those stock 1991 Swedish SEM ignition systems. If it were my only dirt bike I would tend to get a lot more frustrated by the extreme levels of low end and midrange harshness. Since I have other bikes to ride though I can just switch to one of them to get away from the harshness when it is too annoying.

Just backing off on the spark timing while retaining the big camshaft probably wouldn't be a good idea. I have already tried the 250 degree at 1mm valve lift 1994 camshaft installed straight up with split overlap at top dead center, and that seemed like too late of an intake valve closing time for the Husqvarna 610 motor. It worked for delivering some additional top end power, and it also worked well for dealing with extremely slow flame front travel speed gasoline. Ultimately though it was just too late of an intake valve closing time that cut into lower midrange torque rather noticeably even though it was only a very small difference in valve timing.

It is specifically the huge 33 degree BTDC stock spark timing that works well with the bigger camshaft. Not only because of the weak spark starting issues of the stock SEM ignition and not only because the lower cylinder filling at 3,000 to 5,000RPM knocks the edge off of the extreme harshness. The big camshaft is also better matched to the 33 degree BTDC spark timing simply because the very early 33 degree BTDC spark timing tends to make the engine run as if the flame front travel speed is slower than it actually is. The huge 33 degree BTDC spark timing tends to reduce top end power output and make the engine reluctant to rev all the way out. The big camshaft counters these tendencies and provides a reliable top end pull despite the hugely excessive 33 degree BTDC spark timing. The 33 degree BTDC spark timing and the big camshaft do go together. If I backed off on the spark timing I would want to put the stock camshaft back in, or at least advance this bigger camshaft by four or five degrees to get substantially closer to the stock intake valve closing time.

It is very tempting to make both the spark timing and the valve timing modification to bring the bike back to an entirely stock 1991 Husqvarna WMX 610, and I might do that sometime. For now though I just like that it starts and runs. At least it appears to start and run when I can manage to get some sort of reasonably normal gasoline in the tank.

Then the next time I rode the big cam 1991 Husqvarna WMX 610 it was back to hotter burning gasoline that pulled hard on the top end without the slightest bit of surging. There was some considerable harshness down low and the torque at 3,000 to 4,000RPM was not spectacular. The torque did however build very nicely across the entire midrange, and then there was a fairly big top end pull.

This might have been reasonably hot burning gasoline, but the energy density was low. I had a hard time getting the big cam motor started, or rather I had a hard time keeping it running. It was actually starting very easily, but then it would run for only a few seconds and then it would die. I checked the float level, and I even raised it a bit. This wasn't the problem though. I also replaced the spark plug, and this helped but again it wasn't the real problem. On the new spark plug the engine was firing very easily just about every time I kicked it, but then it wouldn't run for long. Sometimes it just popped a few times and died, and quite often it would fire up and run for a substantial few seconds before stalling. I even got it going several times where I was able to ride it for a ways. It would pull for a few seconds, then completely cut out, and then sometimes it would get going and pull again for a second before stalling. Sometimes I even got a few hundred feet up the road shifting up through the gears before it conked out. It just wouldn't stay running. It seemed like specialty gasoline that just wasn't compatible with the somewhat larger amount of overlap.

When I finally got the big cam 1991 Husqvarna WMX 610 to continue to run it was then running without difficulty at all engine speeds. It was even low idling reasonably well. I rode around a bit, and it was actually running pretty well. It felt and sounded extremely lean, with unusual amounts of popping out the exhaust and even some slight lean missing and popping out the exhaust around 2,500 to 3,500RPM at all smaller throttle openings. Mostly though it was running consistently at all engine speeds, and the torque was building nicely all the way through the midrange and up to a substantial top end pull. The top end power was however extremely narrow. It pulled hard, and then it was done seemingly right away. Above the top end power was more lean cutting out. The power just ended very abruptly, but it was up pretty high around 8,000RPM and the power was strong before it cut out.

After riding around for a few miles using lots of wide open power up into the big top end pull I stopped and took the new spark plug out. It was white. Not light brown as is often the case with a rather lean mixture. No, actually white. The side electrode had a light coating of powdery white residue without the slightest hint of brown. The ceramic insulator also was just as white as a new plug. Obviously extremely lean. Amazingly though it had been making some power and torque. Certainly much more normal gasoline in terms of the temperature of combustion potential and the power production potential, just not much of it.

The very narrow top end pull was not a great feature, and the cutting out up above the power was so severe that it caused other problems. If I let the engine speed go up into that cutting out above the power then it got unstable and wouldn't low idle right after that. A few more minutes of riding around up into the top end power, but not beyond, and again the engine was mostly perfectly consistent and stable and was able to low idle without difficulty. Not only was this gasoline very low energy density, but it also appeared to very easily leave some sort of residue on the spark plug that caused unstable operation. The white powdery build up on the side electrode is probably a clue about this, and the very hard starting was probably related to this also. Some sort of a specialty non-starting additive I think. Once the engine was warmed up it was able to restart with the kick starter, but not exactly easily. It was taking a few kicks to get it going even when warmed up, but it was starting. As usual it was firing pretty much every kick, but it just took a few attempts to get the big cam motor to actually take off and run.

Probably the most interesting thing about this little ride though was that I hardly noticed the harshness down low. The engine was somewhat harsh down low, but the nicely building torque all through the midrange and the good top end yank substantially distracted me away from that low end harshness. With such nice response through the entire midrange and top end I hardly cared that it was harsh and weak down low. It was fun to ride. The down side though was that the combination of weak low end torque and a brutally narrow top end pull was obviously not a great combination for practical dirt bike propulsion. Fun as a thrill ride to be sure, but not exactly the broad and strong torque that makes the Husqvarna 610 truly fast. A bit more of that hot burning and powerful gasoline finding it's way into the combustion chamber surely would have broadened the peak pull and eliminated the cutting out.

Box of Goodies

The boxes that came with this big cam 1991 Husqvarna WMX 610 contain quite a variety of interesting spare parts. One item was the European version of the Shop Manual. The main difference is that it is in five languages, Italian, English, German, French, and Spanish. It seems to be in that order of significance also. It was obviously originally written in Italian with the intention of selling it mostly in English. The German also is a very careful and colorful translation. I never had one of these before. I tried to buy one at the local Husqvarna dealer, but they told me it was just the same information as in the Cagiva North America English only 1991 Husqvarna Owner's Service and Tuning Manual and quoted me a ridiculous high price if I really wanted to buy one. They even flipped through a few pages of the five language big book to try to demonstrate that it was all just the same information in five languages. I asked why they even had the five language book if all the information was exactly the same, and the answers I got were like asking why they only stocked a 12 tooth front sprocket when everyone ran 13 tooth front sprockets on Husqvarnas. Since the dealers were the only access to genuine parts or accessories at that time I had no choice but to just leave it at that. I could get sprockets from mail order companies, but not the Husqvarna Shop Manual.

So now I have one of those coveted five language Shop Manuals, and it is indeed mostly just the same information in five languages. In the intervening years I have learned German, which makes at least one fifth of it a whole lot more interesting to read. I haven't gotten all that much better at Spanish since then, but I do have a new appreciation for reading things in multiple languages. The Italian and French are of course the most flamboyant and exiting versions of the 1991 Shop Manual, but it takes me forever with a dictionary and grammar references to get anywhere in those languages that I have such little practical experience with. They are of course very similar to Spanish in some ways, but then both the vocabulary and the grammar of Italian and French take some much different roads sometimes. In fact even with a four sheet synopsis of French grammar I still can't seem to make sense of much of it. I know it is all based on German grammar and Latin grammar, but I don't know all of the Latin grammar either since I have never studied it formally and I don't own any reference works on it.

I know how the German grammar works though, and it is actually very entertaining to read an Italian's attempt to write good German about an important subject. With all those languages, that one short book could make for years of entertaining reading.

What I found though was that there are really only a few important things that are different about the five language version of the 1991 Shop Manual. Most significantly the valve timing specs were omitted from the Cagiva North American English only version. Reading the five language version provides lots of indirect information about why this omission was made. First of all the wording on the specs is wrong in English. Not just a bit awkward like much of the rest of the book, but completely incorrect both grammatically and logically. The wording is utter non-sense in English, but the wording doesn't matter if you know what the numbers refer to. The numbers are there, and they do make checking camshaft setup a bit easier if you have access to that information. The wording on the German version is a bit awkward, but both grammatically and logically correct. It is just the intake valve opening and closing times at the stock valve lash settings. Not all that meaningful, but a useful piece of information to have when looking over a mostly unfamiliar engine.

The five language version also has a lot more information about the street legal European only 1991 Husqvarna WXE dual sport models. That's pretty interesting to look over, but not really of any practical use. Much of the equipment that was required on street legal motorcycles in California in the 1990's was not required on motorcycles in Europe. Most notably a horn and a battery.

In reading over the five language version of the 1991 Husqvarna Shop Manual there are a few discrepencies that stand out as very different from the North American English only 1991 Husqvarna Owner's Service and Tuning Manual. In the five language version the introduction has a very different logical meaning in English than in German on the next sheet. In English the introduction says that the information in the book is for "...proffesional training of the operator.", which seems like an odd introduction to a shop manual. On the next sheet in German though the introduction says that the information in the book is for the "...proffessional Ausbildung des Fachpersonels.", which has a very different meaning. Basically it says the information is for the enlightenment/education of the commercial repair facilty employees. These are very different ideas. The former in English sounds like some sort of bazar fantasy idea about a motorcycle dealer training buyers of Husqvarna dirt bikes in racing and mantanence skills. The later in German is a simple statement about the information in the book being useful for dealer employees. Both versions of this introduction are entirly missing from the North American English only version.

Farther into the 1991 Husqvarna five language shop manual there are other incongruities that seem rather suspicious as well. The front suspension is for example described rather differently in English than in German on the next sheet. In English the front suspension is very confusingly described with the giberish phrase "...fork with turned rods 1.57" dia." This makes absolutely no sense other than listing the 40mm diameter in inches. In German the description is also very awkward with a lot of non-standard terminology, but again it makes logical sense and is writen with correct grammar. Most importantly the German version actually says something meaningfull. The phrase is "...Teleskopgabel mit umgekehrten Holmen 40mm.". Literaly telescopic fork with turned over columns 40mm. The word "Holmen" also is entirly giberish. It is probably a corrupted attempt at a plural form of "der Holm", which is a general term for the trunk or side rail of a ladder structure. The actual plural of "der Holm" is "die Holme". Interestinly though this incorrect use of "Holmen" as the plural of "der Holm" is not isolated to the 1991 Husqvarna five language shop manual. It appears be some sort of an antiquated form that is no longer part of what is considered correct modern German. In any case the correct German word for a 40mm fork tube would be "das Rohr", which might be considered a bit confusing in a multi-language publication. The roar of a three inch stroke length dirt bike is a somewhat different concept. In any case the German description does refer to the forks as being "umgekehrt", inverted, reveresed or flipped over. The English description makes no reference to the 40mm White Power 4054 forks being upside down forks.

Then there is the, perhaps more significant than intended, discrepency in fuel reccomendations. The North American English only version of the 1991 Husqvarna Owner's Service and Tuning Manual reccomends "PREMIUM GASOLINE with an octane rating equal to or higher than RM 98." This is all fine and good, except that octane ratings in America are "(RON+MON)/2", an average of both the 600RPM and the 900RPM traditional variable compression ratio motor tests. The "RM" term referes to only the 600RPM test. Obviously a single number to describe the combustion properties of gasoline at 600RPM or even 900RPM is extremely limited. A comparison of the 600RPM test results and the 900RPM test results could potentially yield some meaninful information, but this is not how the traditional tests have been used. In any case the 1991 Husqvarna shop manuals are vague and inconsistent in regard to gasoline terminology.

The North America English only version reffers to "PREMIUM GASOLINE", where the English description in the five language 1991 Husqvarna shop manual recomends "Super fuel 98-100 NO (min)." First of all "Super fuel" sounds much different than "Premium Gasoline". I havn't been to England myself, but I somehow doubt that they reffer to normal gasoline as "super fuel". The Italian, French and Spanish descriptions in the five language 1991 Husqvarna shop manual refer to "Supercarburant", which in Europe is a common description of premium gasoline. The term "supercarburant" has very strong conotations of a highly volotile premium gasoline with a fast flame front travel speed. The "NO" is also from the Latin derivative languages (numero ottanizzato, numero octane, numero octano), and was rather confusingly included in the English language description.

The rest of the inconsistoncies in the 1991 Husqvarna five language shop manual are mostly just funny little mixups of terminology that don't mean much of anything. The valve adjustment covers are reffered to as "[the] covers of registers", which is total nonsense. Calling the valve adjustment covers "covers of registers" is utter nonsense, but it does bring up some highly ambiguous shades of meaning. One is the rarely true but often quoted problem of a loud tapping sound comming from the valvetrain. This sound might rather poetically be reffered to as a "register" in the sence of the register of a musical instrument. Alternately the setting of the valve lash might be considered to be a register of the operating conditions of the engine. If the intake lash is found to be very loose and the exhaust lash is found near zero then it might be surmized that the valve lash adjustments have been overlooked for quite some time. If the intake and the exhaust lash are both found much wider than the recommended specification then it might be surmized that the lash was intentionally set wide by a mechanic most familiar with working on iron head pushrod engines.

The German description in the 1991 Husqvarna five language shop manual calls the valve covers "..die Deckel der Einstellvorrichtungen...". Literaly "the covers of the adjustment provisions". It just says that there is some way to adjust the valve lash under those covers.

Probably the most interesting thing in the box of spare parts I got with the big cam 1991 Husqvarna WMX 610 was the ring set. With it was also the 1993 sales receipt showing the retail price of $12.60 for the compression ring and $14.50 for the oil control ring. Wow, that's a lot different than the $100 and $80 that I was quoted just a few years later. I have been trying to buy that ring set for decades now, but I simply refused to pay $180 for a standard type ring set for a single cylinder engine. Even $27 for two rings seems steep compared to about that same price for a 24 ring aftermarket performance ring set for a Chevy V8, but $180 was just too much. At that time a full engine rebuild kit for a Chevy 350 including new pistons, rings, piston pins, bearing inserts, complete gasket set, oil pump and timing set was only $200. That made $180 for two rings of exactly the same type seem very hard to swallow.

Arguably the most functionally significant spare parts included with the big cam 1991 Husqvarna WMX 610 were the two Uptight Racing brand exhaust systems. One was installed on the bike, and the entire unused stock exhaust system was in the box including the race tip. Yahoo, another race tip. I almost never use those, but I only had one for my three 1991 Husqvarna WMX 610 bikes.

It was the Uptight Racing exhaust systems that were so interesting though. I have heard of those being used on later Husqvarna 610 bikes, but I never seriously considered trying to put one on a 1991 Husqvarna. The fact that they weren't for sale new anymore after a few years was part of it also.

There were two Uptight racing header pipe setups with the bike, but just the one muffler. It is a Supertrapp muffler with a long megaphone front section. I bought a mangled used one of those off of a Honda once just to get the extra diffuser disks, so I was familiar with the general shape. I had been wondering if they actually work better than just the traditional short Supertrapp mufflers that I have been using.

The mounts for the Uptight Racing Supertrapp muffler were very funky on this big cam 1991 Husqvarna WMX 610 and the whole exhaust system was just hanging lose and wobbling all over the place. The front rubber mount was broken through, and the front mount on the muffler itself was also beginning to separate from the body of the muffler. The rubber inserts in the stock Husqvarna muffler mount that had been used as the rear mount for the Uptight system were also very loose. I replaced broken front mount with a solid bolt, and I tightened up the stock Husqvarna mount by cutting down the steel inserts to squish the rubber down tight. This worked well, and the muffler mounted up tight and secure.

It turns out that the Uptight Racing exhaust system does work very well, but not so much because of the Supertrapp muffler. The single Supertrapp muffler is still LOUD. Very loud, probably just as loud as the stock motocross muffler with the race tip. The only real advantage of the single Supertrapp muffler over the stock exhaust system is that it provides a good spark arrestor that doesn't restrict flow as much as the popular Cobra Sparky spark arrestors. This big cam 1991 Husqvarna WMX 610 came with one of those also, but like the entire stock exhaust system it looked mostly unused.

The Uptight Racing header pipe setup is however an improvement over the stock 1991 Husqvarna exhaust system. It is more like the 1992 and later Husqvarna exhaust systems in that the individual header pipes are shorter and dump into a single pipe farther forward on the bike. This does work better overall, and the Uptight Racing system is carefully designed to maintain a near constant interior volume from the two pipes to the single pipes. The increasing volume into the very large single pipe on the 1992 and later Husqvarna exhaust system has a bit of a flow advantage, but mostly it is an expansion chamber of sorts that gives up a very small amount of low end torque for a boost in midrange and top end power. I like the 1992 Husqvarna exhaust system fine functionally as it seems flow slightly better everywhere above about 3,500RPM. The only problem is that it is loud, very loud. Painfully loud, and there isn't any easy way to quiet it down.

The Uptight Racing exhaust system is a compromise between the 1991 and 1992 Husqvarna exhaust systems. It moves the single pipe farther forward, but it maintains a constant interior volume for an even spread of torque all the way across like the stock 1991 Husqvarna exhaust system. This is a good match for the big camshaft, but so is the stock 1991 Husqvarna exhaust system. Both an upper midrange tuned expansion chamber type exhaust system and a big camshaft is certainly way too much top end oriented tuning for a dirt bike that is most loved for its huge torque everywhere down supper low to 3,500RPM or even slightly lower. Yes the Husqvarna 610 motor can be made to make even more power, but any of those power gains that give up torque at 3,500 to 5,000RPM seem to be barking up the wrong tree in my neck of the woods.

Maximum Torque Point

I was just looking over the published specs on the largely new 2017 Suzuki GSX-R1000R, which is of interest mostly because of the adoption of a finger follower valve train. Honda "Kicked the Bucket(s)" in 2017 on it's finger follower CRF450R and Suzuki did the same thing with it's flagship super bike. That's big news, to the tune of 1.43 foot pounds per cubic inch at 10,800RPM on the 2.25" stroke length Suzuki GSX-R1000R. That's lots of torque up at a very high engine speed and also a very high mean piston speed. Sort of unheard of levels of torque production to insanely high mean piston speeds really.

I personally believe that absolute maximum torque would be found down lower around 7,000 to 8,000RPM with a slightly lower mean piston speed associated with about a 2.5 to 2.7 inch stroke length engine even on that same powerful hot burning race gas. If that's true, then substantially more than 1.43 foot pounds per cubic inch is possible at around 7,000 or 9,000RPM instead of way up at 10,000 or 11,000RPM. The Suzuki specs seem somewhat contrary to this idea though as the new GSX-R liter bike is said to make 199hp at 13,200RPM which is still 1.30 foot pounds per cubic inch at that even more insanely high mean piston speed. And that's with rod bolts!

In any case, 1.35 foot pounds per cubic inch is routinely produced way down at around 4,500 to 5,500RPM in 3.75 inch stroke length pushrod engines. Sport bikes making 1.35 and now 1.43 foot pounds per cubic inch at 10,000 and 11,000RPM with 1.95 to 2.25" stroke lengths and the pushrod torque monsters pulling about the same specific torque down as low as 4,500RPM seems to fairly well bookend my suspicions that gasoline engines actually run best somewhere in the middle around 7,000 to 8,000RPM on race gas. I also believe that normal cheap types of premium and regular gasoline would have a somewhat lower temperature of combustion potential that would work best down slightly lower perhaps as low as about 6,000RPM with a substantially lower mean piston speed.

If this is all mostly correct guessing and assuming then substantially more than 1.43 foot pounds per cubic inch would be possible on a race engine tuned to pull hard over a broad range of engine speeds in the 6,000 to 9,000RPM range. This conclusion might however be irrelevant. Race engines are not typically about making absolute peak torque or about absolute maximum efficiency but rather are setup to be usable and win races. And winning races typically means pushing power to higher than ideal mean piston speeds simply because more revs can rather easily be converted into more power.

What might be more interesting and practical is enquiry into where more normal types of cheap gasoline attain peak efficiency, and that is probably both at slightly lower engine speeds and substantially lower mean piston speeds. Most likely though it is still substantially above the 5,000RPM level where parallel valve pushrod engines run best and even more substantially below the mean piston speed of a 3.75 inch stroke length engine twisting 5,000RPM.

As far as the three inch stroke length Husqvarna 610 motor is concerned though none of this speculation maters. Whatever the realities of where gasoline engines run best the Husqvarna 610 motor spends a lot of time at 3,500 to 5,000RPM under all normal conditions. Sure it makes more power up higher, but it lives at 3,500 to 5,000RPM when going fast in lower gears on anything smaller than a road race circuit, a giant sand dune or the open highway. Sure I rev it out insanely to 8,000RPM even on single track sometimes, but when I am actually trying to go fast the engine speed usually pops above 6,000RPM only occasionally to stretch a shift or blast certain types of straights or hills. Most of the time it's torquing down at 3,500 to 5,000RPM. I have gone on whole rides through the woods thinking that the bike was running great only to find that when I came out on a bigger road it wouldn't rev past 6,000RPM for anything due to slow flame front travel speed gasoline.

The point is that the 1991 Husqvarna WMX 610 exhaust system is correct for the way that the bike actually gets used. The Uptight Racing exhaust system is an improvement on the same concept, where the 1992 and later exhaust system is more about emotional response than throttle response. I think the 1992 Husqvarna exhaust system is an amazing piece of Italian art, and I have often considered trying to stuff one onto a 1991 Husqvarna WMX 610 just for the cool factor. Especially on a dual sport bike the flashy and impressive sounding 1992 Husqvarna exhaust system is way cool, and it is also just right for improving a certain type of thrill ride sort of cruising on bigger roads. More capacity for more power at higher engine speeds is hard to argue with when an engine is being used simply for the excitement of it. From that perspective the 1992 Husqvarna exhaust system is grand. On single track though the steady pull of the stock 1991 Husqvarna exhaust system wins out. When the engine is just a means of moving the bike and the riding of the bike is the main thing then it is practicality of power delivery that is of most importance. Yes, lots of power is important to go fast on a motorcycle under any conditions, but even at 4,000RPM the Husqvarna 610 motor certainly delivers the more power part of going fast on small trails.

If the 1.43 foot pounds per cubic inch at 10,800RPM and 1.30 foot pounds per cubic inch at 13,200RPM of the 2.25 inch stroke length finger follower equipped 2017 Suzuki GSX-R1000R are translated directly to the bore and stroke dimensions of the 577cc Husqvarna 610 motor it's more than 77hp at 8,100RPM and more than 86hp at 9,900RPM. To keep that in perspective a 95mm bore KTM/Husqvarna 450 motor is only down 6% on power production potential versus the Husqvarna 610 motor in the same comparison to the GSX-R1000R specs. Since it doesn't appear to be possible to get the full 94% power production out of a 95mm bore KTM/Husqvarna motor relative to the 98mm bore Husqvarna 610 motor though it can be concluded that 10,000RPM is too fast for a gasoline engine to pull absolute maximum torque. And this in turn means that the Husqvarna 610 motor could actually potentially do substantially more than 86hp at 9,900RPM and the KTM/Husqvarna 450 motor could actually do at least a bit more than 81hp at 11,900RPM.

These comparisons of maximum power production potential are of course rather ridiculous for dirt bike motors that are designed through and through to be broad and usable above all else. They can be modified with different camshafts, different intakes and different exhausts to do anything under the sun. But it is still entirely ridiculous to consider 86hp at 9,900RPM, it's exactly twice as much power as dirt bike riders have agreed for many decades is way plenty.

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