Home        The Boat        Photo Essay        Contact

2017 Updates on Various 1991 Husqvarnas

The really big news has to be that my 11:1 but otherwise bone stock SEM CDI ignition 1990 Husqvarna WMX 610 has been kick starting reliably every time both hot and cold. There have also been some interesting things happening with my low compression ratio 9.7:1 1991 Husqvarna WMX 386, most notably improved gas mileage along with strong and reliable low end torque.

Stock SEM 610 Kick Starts
High Gas Mileage from the 9.7:1 386 Stroker Motor
1991 Cylinder Head on 386 Stroker Motor
Better Shock, No Spoke Problems and Suspension Performance Rankings
Engine Performance Rankings
Easy Starting and More Power from 4-3/8" Stroke Jeep

Stock SEM 610 Kick Starts

The 11:1 but otherwise bone stock SEM ignition 610 motor kick starting might seem to have something to do with the warmer summer weather versus last winter when the bike setup exactly the same way was never able to kick start from cold. The ambient temperature is significant, but starting versus not starting with this particular stock 1990 SEM ignition seems to be mostly dependant on the properties of the gasoline. When the stock 11:1 1990 Husqvarna WMX 610 was failing to kick start from cold it always failed to start, and now that the same engine is kick starting from cold it is always kick starting from cold every single time. Sometimes it starts on the very first kick, but often it fires and doesn't start and then fires up and runs after a few more kicks. Often it won't fire at all without the choke, and then it doesn't fire with the choke on either but when I take the choke off it fires up on the next kick and runs. Sometimes though it only fires and then dies on that first kick after taking the choke off. That is what it was doing when it wasn't able to kick start from cold last winter. It would just fire low and weak, and then nothing on subsequent kicks. Not even a pop. Now though it does fire again on subsequent kicks. Sometimes it fires up and runs on the next kick without the choke, but sometimes it has required putting the choke on for one kick and then on the next kick without the choke it fires up and runs.

Then there is also the fact that now that the 11:1 but otherwise bone stock SEM ignition 610 motor is kick starting from cold it has done it even on unusually chilly mornings. Last winter when it was never able to kick start from cold it wouldn't kick start from cold even on balmy warm afternoons when the ambient temperature was substantially high.

For whatever reason the 11:1 but otherwise bone stock 1990 Husqvarna WMX 610 motor has been firing up with the kick starter from cold every time, and it has continued to start easily, usually on the first kick, once the engine is warmed up also. The low idle has also remained rock solid stable. The low idle has sometimes been frighteningly low with just 6 degree BTDC spark timing, but the idle has been stable and the engine never stalls. And this is all on the same C7E spark plug that I first put in last year. This particular 1990 SEM ignition system does appear to work, which is pretty amazing.

How has the 11:1 but otherwise bone stock SEM ignition 610 motor been running? Very inconsistently, but also reliably. Paradoxically the torque has been very reliably available every time I ride the 11:1 Husqvarna 610 bike, but the nature of the torque and the feel of the power delivery has been highly variable. Even at the same elevation and similar ambient temperatures with exactly the same 26 degree BTDC spark timing above 2,000RPM and the exact same jetting on the carburetor the 11:1 610 motor has run drastically differently from day to day. Most common has been lots of excess crispness down at around 1,000 feet of elevation, and then some surging at wide throttle openings from about 4,500 to 6,000RPM. The level of excess crispness has however been highly variable. Some days it has seemed like the 11:1 motor is just drastically overly crisp down at 1,000 feet of elevation to the point of being unusable, where other days at the same elevation the 11:1 motor has only seemed slightly overly crisp with exactly the same ignition and carburetor setup. Often the excess crispness has not been all that much of a problem, and fairly strong torque was able to build from 3,000 to 4,500RPM without surging when the throttle was feathered just right. Then there have also been a few days when the surging was much worse; where it was taking a big twist of the throttle to get torque and then as soon as the torque arrived it was accompanied by huge amounts of bad surging everywhere from 4,000 to 6,000RPM. The worse the surging generally the earlier the power signs off. On the days when there was very bad surging the power both had an unpredictable surging feel to it and the power was also seeming very narrow. The narrowing of the power also has resulted in somewhat low torque generation everywhere from 3,000 to about 4,500RPM.

When the 11:1 motor is running better with less surging the torque both has a much better feel to it, and is also stronger across the entire lower midrange. When the surging can be avoided by feathering the throttle then the torque builds nicely from 3,000 to 4,500RPM, and the torque is also stronger everywhere. When the 11:1 motor isn't surging as bad is also when torque seems most impressively strong down around 2,600 to 3,000RPM. It gets harsh down there and the torque is always a lot lower than up at 3,200RPM but some pretty impressive and very usable torque has often been available down amazingly low with the spark timing backed off all the way to 26 degrees BTDC. That 26 degree BTDC spark timing seems like it would be too early for really good torque down very low, but the reality is that the weak spark ignition can in fact tolerate a bit more spark advance.

The 26 degree BTDC spark timing on the weak spark SEM CDI ignition seems equivalent to about 24 degree BTDC spark timing with a powerful points ignition spark in terms of how well the 610 motor can run down to low engine speeds. The weak spark and associated very small 0.017" spark plug gap introduces an additional initial delay, and that initial delay allows a bit more spark advance to be used without the torque down bellow 5,000RPM being ruined. That extra spark advance doesn't really do anything beneficial though since the innitialfor delay also means that the engine runs in full flame front travel mode at small throttle openings as if the spark timing were 24 degrees BTDC not 26 degrees BTDC.

It is just a tricky little detail about these weak spark ignitions. Someone who wants huge 26 or 28 degree spark timing gets what they are looking for when they hook up a timing light, but that excessive spark advance doesn't do as much harm to torque generation bellow 5,000RPM as would be the case with a powerful spark. The trade off is that the weak spark can also cause severe hard starting and even stalling problems under some conditions. The paradox is that the widespread desire for excessive amounts of spark advance has indirectly brought about an ignition system that is so weak that starting and idling performance can seem like there isn't enough spark advance. Part of this is the huge 20 degrees of additional advance from 1,400 to 2,000RPM, but part of it is in fact the weak spark itself.

When I switched from the race tip to a USFS approved spark arrestor equipped Supertrapp secondary muffler before the spring dry out I didn't notice much of any change in performance. The more restrictive muffler must cut into top end power at least a bit, but I haven't noticed much of any real difference. Down low below about 4,000RPM the longer total exhaust system length is even perhaps somewhat of an advantage. Even after installing the Supertrapp muffler with 13 diffuser disks the 11:1 stock SEM ignition 610 motor has remained very crisp, excessively crisp much of the time.

Lately all of my Husqvarna four stroke motors have often been running amazingly strong at very low engine speeds around 2,500 to 3,000RPM and also impressively strong around 3,500 to 4,500RPM. Considerably more torque building from 3,200 to 4,500RPM than down at 2,800RPM, but still amazingly large amounts of very usable torque somewhat bellow 3,000RPM. Is the very low engine speed torque stronger on the lower temperature of combustion potential gasoline that causes severe surging around 4,000 to 6,000RPM in the 2.68" and 3" stroke length engines? It seems like it should, but that hasn't really been the case. The weaker lower temperature of combustion potential gasoline that causes very severe surging is smoother way down low around 2,400 to 2,800RPM. But stronger torque? It doesn't actually seem that way, but appearances can be deceiving. The Czech Republic CDI stock 10.2:1 610 motor sometimes seems to do a bit better than the 11:1 stock SEM ignition 610 motor way down very low on very weak surging prone gasoline. Backing off on the spark timing bellow 3,000RPM is a very tricky thing. What often happens is just a total loss of torque bellow 3,000RPM, or a surging unpredictable feeling to the torque at and just bellow 3,000RPM.

Sometimes though when the engines are running extremely crisply on very weak gasoline that is causing huge amounts of surging down to ridiculously low engine speeds around 3,500RPM then backing off on the spark timing bellow 3,000RPM can seem like something of an advantage. A bit less spark advance helps with being able to hit the latest possible time of late compression ignition down to lower engine speeds, and sometimes less spark advance and less crispness way down low at 2,400 to 2,800RPM can result in more torque at those very low engine speeds. Sometimes backing off on the spark advance to keep the engine in full flame front travel mode results in more torque at 2,300 to 2,700RPM also. Especially on more powerful gasoline keeping the engine in full flame front travel mode at 2,300 to 2,900RPM certainly results in much smoother and quieter operation.

Perhaps the best way to describe the torque difference would be that the weaker surging gasoline is perhaps a bit better over a narrow range of engine speeds around 2,300 to 2,700RPM, but that slight advantage doesn't translate into an impression of better torque because the torque then doesn't build as well from 3,200 to 4,500RPM. A lack of torque at 3,200 to 3,700RPM is a severe problem because that is right where the 3" stroke length engine most effortlessly runs, where down at 2,400 to 2,800RPM gasoline engines never run anywhere near as well. A slight bit of an improvement over lousy operation at 2,500RPM is a very poor substitute for good operation at 3,200 to 4,500RPM.

An interesting difference I have noticed on the 14/52 geared 11:1 stock SEM ignition 610 is that the weaker severely surging gasoline makes the gearing feel wrong; or more precisely it makes the five speed transmission feel very wrong. Unsurprisingly first gear feels too tall pulling out with the weaker severely surging gasoline and the very tall 14/52 gearing on the five speed. What I also notice on the same weaker severely surging gasoline though is that fifth gear then doesn't feel anywhere near high enough. When the 11:1 stock SEM ignition 610 motor is surging severely and lacks low end torque around 3,000 to 5,000RPM I also find that I am constantly looking for a sixth gear cruising along at rather low speeds. On the more powerful higher temperature of combustion potential gasoline that reduces surging the torque is stronger over most of the lower engine speeds, and pulling out in first gear with the tall 14/52 gearing for the most part works great. Likewise with the more powerful higher temperature of combustion potential gasoline that reduces surging the big 610 motor is more willing to rev out at small throttle openings, and fifth gear with the 14/52 gearing seems plenty high for most purposes.

High Gas Mileage from the 9.7:1 386 Stroker Motor

The actual change that seemed to spark higher gas mileage in the 386 stroker motor was getting the pilot jet back to close to the stock size. When I first bought the 1991 Husqvarna WXE 350 back in June of 2014 I noticed that the pilot jet was larger than stock. The 1991 350 motor seemed to run much better with the 65 size pilot jet that was in it than the 1992 350 motor had with the stock 58 pilot jet though, so I left the 65 pilot jet. I also left the stock 145 size main jet, but that turned out to not stick so well. I knew that the stock 145 main jet was on the small side in the 34mm DellOrtos compared to the stock 180 and even 175 main jet sizes in the 40mm DellOrtos on the 610 motors. The stock 145 main jets had however been working well on both the stock 1992 TE 350 motor and the stock 1991 WXE 350 motor.

Back in 2014 I had been able to confirm that the 145 marked main jets in the 34mm DellOrtos on both of my 350 motors were in fact very close to the 145 size. A #54 drill stock dropped through loosly, but a 0.0593" diameter #53 drill stock wouldn't start in at all. The 65 marked pilot jet in the 1991 350 and the stock 58 marked pilot jet in the 1992 350 I just had to assume were the marked sizes, since I didn't have much in the way of small drills.

Since the 145 jet was on the small side I did several times temporarily change it out for a larger size just to see what would happen. This didn't improve performance, but it was a good way to prove that the problem of excessive 28 and 33 degree BTDC spark timing wasn't that the 145 main jet was too small. First it was the 180 main jet out of the 40mm DellOrto that I tried in the 34mm DellOrto back in 2014, but that was obviously drastically oversized and the engine ran very poorly and blew lots of black smoke at throttle openings over about 1/2. Then in 2015 I tried a 160 size main jet, and that also was drastically too large. With the 160 size main jet the 386 stroker motor was able to take full throttle down at low elevation, but the exhaust got severely blackened and performance was noticeably worse than with the stock 145 main jet.

The problem that occurred was that the last time that the stock 145 jet was temporarily out of the bike someone drilled it out to the 155 size. Left with no stock main jet I ran the 155 size main jet for a while, but it also was far too big. This was at the time in 2015 when extremely high pressure gasoline wasn't working in any low compression ratio engines. The 11:1 hot rod 610 motor was requiring huge 29 and sometimes even 31 degree BTDC static timing settings to make power, and the 9.7:1 386 stroker motor was requiring 31 to 33 degree BTDC static timing settings to make power. Sometimes the 9.7:1 386 stroker motor just wouldn't make torque or power at all even with up to 40 degree BTDC spark timing. With that huge 40 degree BTDC spark timing the engine actually would rev out to about 8,000RPM, but there just wasn't any torque anywhere.

One day late in 2015 the 11:1 hot rod 610 motor wouldn't make power at a 27 degree BTDC static timing setting down at 1,000 feet of elevation. There was a huge amount of hesitation everywhere, and it just wouldn't get going to make power. I bumped the static timing setting up to 29 degrees BTDC, and this did deliver substantial power but there was still quite a bit of annoying hesitation down bellow 4,000RPM. The 11:1 hot rod 610 motor pulled pretty hard up above 5,000RPM, but there was still way too much hesitation even with that huge 29 degree BTDC static timing setting. I then drained the gasoline out of the tank on the hot rod 610 and put it in the empty tank on 1991 Husqvarna WXE 350 with the 386 stroker motor.

With 31 degrees BTDC on the static timing setting the 9.7:1 386 stroker motor wouldn't make torque or power, it would rev up past 7,000RPM but there wasn't any torque anywhere. I then bumped the static timing setting all the way up to 37 degrees BTDC, and again there was no torque and hardly any power. With the huge 37 degree BTDC static timing setting the engine was able to rev all the way out to 8,000RPM, but there wasn't any power up there at all. Just very flat and extremely weak. After getting no power with 31 and 37 degree BTDC static timing settings I backed off to 23 degrees BTDC, and it was pretty much exactly the same. The big difference backing off to 23 degrees BTDC was that the engine was actually a bit stronger and a lot quieter at all lower engine speeds up to about 6,000RPM. Only when I went all the way down to 10 degrees BTDC did the engine run a lot weaker. Even though there wasn't any late compression ignition on that very high pressure gasoline there still didn't seem to be much need for more than about 20 degree BTDC spark timing in the 85mm bore four valve per cylinder engine. At a 37 degree BTDC static timing setting and even down at a 31 degree BTDC static timing setting the fast flame front travel speed gasoline was actually reducing torque all the way up as high as 6,000RPM just in full flame front travel mode with no late compression ignition at all. I then set the static timing setting at 18 degrees BTDC. That seemed to be the best spark timing, as the engine was very smooth and quiet way down low, and the power up to about 6,000RPM was very nearly just as strong as it had been with 31 and 37 degree BTDC on the static timing settings. The huge amounts of spark advance had delivered a bit more power up at 6,000 to 8,000RPM, but it was still such a miniscule amount of power that it wasn't even close to worth revving the engine out that far. The engine just wouldn't make power with no late compression ignition. With the static timing setting at 18 degrees BTDC I headed up into the hills, and the performance of the engine didn't change at all gaining 3,000 feet of elevation. It was still just the same very smooth and very easily modulated but also very weak torque at 2,500 to 5,500RPM. I did notice a very slight reduction in power output at the higher elevations, but it was extremely slight and the engine still ran exactly the same with nothing but perfectly smooth and perfectly instant small amounts of easily modulated torque across all engine speeds with that 18 degree BTDC spark timing on the 85mm bore engine.

When I got back from that little full flame front travel mode ride up into the hills I drained the gasoline and put it back into the empty tank on the 1991 Husqvarna WMX 610 with the 11:1 hot rod 610 motor. The 11:1 610 motor ran just as it had on the same gasoline earlier in the day. Way too much hesitation, especially down at 3,000 to 4,000RPM, and then fairly big power above 5,000RPM once the engine finally got going. After the short test ride I checked the static timing setting again, and it was still at the same 29 degrees BTDC. Some very high pressure gasoline, that's for sure. And, no the 11:1 610 motor wasn't surging at all that day. There was never any surging back in 2015 from either the 610 motors or the 386 stroker motor. Just very harsh and rather weak operation at 3,000 to 5,000RPM when more than about 25 degree BTDC spark timing was required. Sometimes the 610 motors were able to tolerate up to 27 degree BTDC spark timing without losing all that much torque at 3,500 to 5,000RPM, but the smaller 386 stroker motor always needed to be at 25 degrees BTDC or less to work bellow 6,000RPM. Up at 28 degree BTDC or earlier static timing settings even the 610 motors tended to loose torque all the way up to 6,000RPM.

After running the 155 main jet on the 386 stroker motor for just a few rides I tried to get it back to the stock size, but the closest I could manage was about a 149 size with a 0.0593" diameter #53 drill that I ran just through and left as tight as reasonably possible. Going back closer to the stock main jet size did help a bit with getting the low 9.7:1 compression ratio motor to make power on the extremely high pressure gasoline that was prevalent at the time. The day that I made the modification I rode the bike first with the 155 size main jet, and it was barely getting going up on the top end at a 31 degree BTDC static timing setting. With the smaller 149 size main jet it was able to get going much more easily to make top end power with the same 31 degree BTDC static timing setting, and I was even able to back off to 28 degrees BTDC and still get some power.

For many months the 9.7:1 386 stroker motor was still requiring extremely excessive 28 and 30 degree BTDC static timing settings all the time, but top end power was certainly stronger and more reliable than it had been with the 155 size main jet. The stock 145 main jet in the 34mm DellOrto might be a bit on the small side compared to how the stock 610 40mm DellOrto is jetted, but the 149 size main jet seems plenty big.

It wasn't until May of 2017 that I got around to fixing the oversize 65 pilot jet. I had known that it was too big, but I didn't have anything else to put in it's place. It was actually when I was having a huge amount of trouble with unusually low energy density watered down gasoline that I decided to get the pilot jet on the 386 stroker motor back to stock. When I took the 65 marked pilot jet out I was able to confirm that it was actually very close to a 65 size jet. A 0.0257" diameter #70 drill stock went in cleanly with no resistance, but also with zero side to side wiggle.

First I went all the way down to the 56 size with a 0.0215" diameter #74 drill, but this didn't work at all on the low energy density watered down gasoline. The engine wouldn't run at all at small throttle openings. I then went up to a 59 size with a 0.0231" diameter #73 drill. The 59 size seemed to be well matched to the first needle clip position on the 34mm DellOrto, providing a fairly even and slightly richening mixture from the first crack of the throttle up to 1/8 and 1/4 throttle openings.

The big surprise was a large increase in gas mileage, even at higher highway speeds. The first long ride I went on with the repaired pilot jet I got 60mpg. Wow, that's the best mileage I have ever gotten with the 1991 350/386. I did get 60mpg a few times with the stock 1992 Husqvarna TE350, and that was with the needle clip in the stock second groove. With the 1991 Husqvarna WXE 350 I never got better than about 55mpg under any conditions with the needle clip in either the first or second grooves. The 386 stroker motor had been doing about the same, but often even a bit lower around 50 and 53mpg under conditions where the stock 350 motor had done 55mpg. Even going up to 14/53 gearing on the 386 stroker motor hadn't cracked 55mpg with that oversized pilot jet. Only after getting the pilot jet size back close to stock did the mileage improve. On that first long ride with the 59 pilot jet it was mostly larger dirt roads cruising in sixth gear, but a large portion of the ride was also on smaller dirt roads where I was often down in lower gears. I even hit a few sections of single track. When I drained the tank after the 3.2 hour and at least 95 mile ride I got 0.83 gallons out.

Then I got 57mpg out on the highway cruising continuously at around 50 to 60mph, again very impressive mileage and that time it had absolutely nothing to do with the pilot jet size. What was the same on both of these rides was very crisp operation and strong reliable torque without much of any surging with static timing settings of around 21 to 23 degrees BTDC.

So how has the 9.7:1 386 stroker motor been running? Strong and reliable torque, but inconsistent top end power and occasional severe surging. Over the past several months I have taken quite a few longer, multi-day rides on the 1991 Husqvarna WMX 386, and the one thing that really sticks out in my memory is strong and reliable torque at 3,200 to 7,500RPM along with unbelievably strong and very reliable torque down lower at 2,600 to 3,000RPM. On a few different trail rides I was constantly amazed at how much torque I was getting down to amazingly low engine speeds. The 2.68" stroke length 386 stroker motor was harsh bellow 3,000RPM and the torque certainly was a lot lower than the amount available at 3,200RPM, but the amount of torque available down to around 2,700RPM was still very impressive. Several times I was amazed at how well the torque was staying strong down to what seemed like very low engine speeds, and then when I looked at the tachometer I found that what seemed like extremely low engine speed was actually 3,200RPM.

On many different days at elevations up to about 5,000 feet the torque was remaining very usable and amazingly strong at all lower engine speeds down to around 2,500RPM with static timing settings usually right at 22 to 23 degrees BTDC. What was also usually true though was that the torque did increase a whole lot at about 3,200RPM and continued to increase steadily up to around 6,000RPM. The torque was amazingly strong way down low at 3,000RPM and slightly lower, but the torque was even more impressive up in the 3,500 to 6,500RPM range of engine speeds.

This very reliable and generally amazingly strong torque was not however accompanied by entirely perfect operation. Excess crispness down low around 1,000 feet of elevation also showed up sometimes. Sometimes the excess crispness was so severe as to actually reduce torque at 2,800 to 4,000RPM. Amazingly though torque usually stayed at least sort of fairly strong all the way down to 3,200RPM even when the engine got extremely overly crisp down at low elevation. Surging was also sometimes a problem. Extreme levels of crispness with slightly weak gasoline can cause surging at low elevation that then goes away when climbing up just a bit higher. Sometimes there was seemingly pretty bad surging at 4,000 to 6,000RPM at wide throttle openings, but feathering the throttle allowed still fairly strong torque without any surging at all. Then there were a few times when the surging was really bad, like it had often been through 2016 and through the middle of 2017. The very severe surging is the surging that is unavoidable; where the engine won't make power at all until the surging sets in. The 9.7:1 386 stroker motor was still doing this occasionally, and it was always associated with a severe loss of torque at 3,000 to 5,000RPM followed by a narrow peaky power band lacking top end power and lacking over rev. That's bad surging, and it takes spectacularly weak gasoline to get that bad of surging from the 2.68" stroke length 386 stroker motor.

The strange thing that has been happening in recent weeks though is that this very severe surging in the 2.68" stroke length 386 stroker motor has only occurred immediately after mixing fresh gasoline with some old gasoline in the tank. Several times I pulled into a gas station with the engine running strong with great torque down low and big top end power to 9,000RPM. Then after taking on a gallon and a half of fresh 91 (RON+MON)/2 octane rating premium gasoline the torque at 3,000 to 5,000RPM dropped off quite a lot and severe surging set in from 4,000 to 6,000RPM. The really funny thing though is that this lack of bottom end torque and severe surging caused by the fresh gasoline only lasted for an hour or so. Then the gasoline mixture broke down into something much more like normal gasoline, with the crisp and reliable torque coming back and the top end power improving with much less surging. That happened several times recently at gas stations, and it has also happened when I topped up the gas tank from a gas can before heading out on a ride. It appears that the dramatically weaker lower temperature of combustion potential gasoline that causes such extreme surging and narrow peaky power in the 2.68" stroke length engine is actually an unstable specialty fuel that breaks down easily into something more normal in the presence of a catalyst. I don't have any idea whether this is how normal types of gasoline would always tend to behave, but lately that is what has been happening.

Throughout these dramatic changes in gasoline properties the torque down low at 2,500 to 4,000RPM has however always remained fairly reliable. Not always quite so impressively strong, but very reliable low end torque has remained. Always very crisp operation with instant reliable torque and no hesitation. Only way up at around 5,000 to 6,000 feet of elevation was the torque becoming at all reluctant, and even at that it was usually just a bit of lag. The engine would still reliably make torque up to 6,000 feet, it just needed to be fully warmed up on some big pulls and wasn't always exactly instant.

The top end power has also been somewhat inconsistent. Up to 7,500RPM the power was usually reliable and fairly strong, dropping off very early above 6,500RPM only during the extremely severe surging. From 7,500 to 9,000RPM though the top end power hasn't been so reliable. Most of the time I was getting strong power to at least 8,600RPM, and the power was often staying strong to 9,200RPM. Sometimes though the engine would barely hit 8,000RPM, and the big power was ending early at 7,500RPM. When the torque was still fairly strong from 3,000 to 7,500RPM without surging, and then just ended abruptly at 7,500RPM this seemed like slower flame front travel speed gasoline. Usually the small 85mm bore 386 stroker motor doesn't have any trouble dealing with the slowest flame front travel speed gasoline commonly available, but when the power just turns off like that at 7,500RPM something seems very different.

The intake stack boost on the 386 stroker motor with the 34mm DellOrto carburetor comes on at 8,000RPM. Even with the cam timing back to stock the power nearly always increases at least somewhat right at 8,000RPM. That is why the 386 stroker motor does it's best pulling from 8,000 to 9,000RPM, that's where the intake stack boost hits. On the stock 350 motor with the same 34mm DellOrto carburetor the intake stack boost seemed to come up at about 8,500RPM, so the strongest power was from 8,500RPM up to about 9,500 or 10,500RPM depending on just how powerful the gasoline happened to be. The 9.7:1 386 stroker motor with the slightly later intake valve closing times caused by removing the base gasket always pulled very hard from 8,000 to about 9,500RPM. Now with the cam timing back to stock, and even perhaps a degree of crankshaft rotation advanced from the stock setting, the power isn't usually as strong at 9,000 to 9,500RPM. There might even be some reduction in power everywhere from 7,500 to 9,00RPM, although really it is mostly only above about 9,000RPM that there is an obvious reduction in cylinder filling. Power is still usually quite strong out to around 8,900 or 9,200RPM, but that seems to be about it for the stock cam timing on the 386 stroker motor. If I stay on it I can often get power to 9,600 or even 9,800RPM, but when the power drops significantly above 9,200RPM like that there just isn't any reason to rev higher on a regular basis. This is perfect for the oil reed valve lubricated rod bearing. It really should stay down around 6,000 to 8,500RPM most of the time, so the power dropping off severely above 9,200RPM really doesn't seem like any kind of a problem. When the power ends at 7,500RPM and the 386 stroker motor can't even make it up to the 8,000RPM intake stack boost though that just seems totally ridiculous.

Even when power is substantially strong everywhere from 5,500 to 7,500RPM it is just a huge disappointment to not have the even stronger power at 8,000 to 9,000RPM. The 386 stroker motor now is susceptible to losing power above 7,500RPM, but this seems to only actually occur on either extremely weak gasoline that causes huge amounts of surging or on unusually slow flame front travel speed gasoline. It probably takes some really very extremely slow flame front travel speed gasoline to get the 85mm bore 386 stroker motor to sign off way down at 7,500RPM.

The crankshaft wiggle advance on the 9.7:1 386 stroker motor seems to come right at 7,000RPM, and that is low enough that it is usually impossible for the power to end before 7,500RPM.

1991 Cylinder Head on 386 Stroker Motor

Back in May of this year I finally had to swap cylinder heads on the 386 stroker motor. The cheap Italian Mazda intake valves were mushrooming over so rapidly that I had to adjust the valve lash every 20 hours, and that was getting very annoying very fast. When I finally pulled that 1992 cylinder head off of the 386 stroker motor the intake valves looked like they had been used to stake out the big top for a few too many seasons, and the adjusters were perilously close to the end of their adjustment range. Clearly those cheap $8 auto parts store valves had not been up to the task. They were in fact so soft that I suspect they would have failed very early in a Mazda Miata. They seemed like defective parts. In any case I had no choice but to pull the cylinder head off again with only 105 hours on the new 386 stroker motor.

I didn't know quite what I would find. I thought I was hearing some additional clanking around in the engine at high engine speeds, so I thought I might find another toasted rod bearing. I had been getting some small amounts of metallic shavings out with the drain oil, so it seemed to look bad for the oil reed valve lubricated rod bearing.

The big surprise was to find the XR400 rod bearing still very tight. Even with the lube oil washed out with solvent I couldn't feel any radial play in the rod bearing. Wow, still brand new after 105 hours. That was pretty amazing, and I hadn't exactly been going easy on the 386 stroker motor either. With weak and highly variable gasoline over the preceding year I had been wringing the 386 stroker motor out as far as it would go every time I rode it just to see what it was capable of doing, and that usually meant full power to at least 9,000RPM.

What I did find was that the steel on steel piston pin was wearing faster than expected. The 20mm pin is probably plenty big for the 85mm bore engine, but the steel on steel bearing seemed like it might have been a bit too tight to begin with and it was gauling and scoring some. The clearance hadn't opened up much though, so that probably wasn't the source of the additional clanking around. The skirt clearance had opened up a bit also, but only by a few ten thousandths. I couldn't really find anything actually wrong with the motor other than those defective soft valves.

To replace the defective soft Italian Mazda Miata valves I just swapped the original 1991 Husqvarna WXE 350 cylinder head back onto the bike it had come off of. Those original Husqvarna valves were showing some signs of wear on the stems also, but only because I had been lazy about valve lash adjustment back in 2014 and I had let the clearance open up more than I should have. Running the engine up to around 10,000RPM with about 0.020" lash had smashed the stems and adjusters quite severely. Those original Husqvarna stainless steel valves are however very tough, and despite the damage I was quite sure that they would still work just fine. The lash adjusters were well within their range, so all looked well.

I checked the cam timing again when I put the 386 stroker motor back together, and I made sure that it was still all the way towards the advanced end of the range of adjustment I had created by elongating the sprocket mounting bolts.

What I was surprised by when I took the reassembled 386 stroker motor out after putting the 1991 cylinder head on was that power at 7,500 to 8,500RPM wasn't seeming quite as reliable as it had been in the previous months. It really seemed like going back to the stock 33mm intake valves had reduced the flow capacity of the engine by a significant amount. The oversize 34mm intake valves really had been a great idea on the slightly overbore 386 stroker motor. Still though, the stock 33mm valves seem like they should be good enough. The all stock 1991 Husqvarna WXE 350 engine was after all able to pull big power to 10,500RPM on that same cylinder head with the same camshaft. Yes, I had advanced the cam timing about a degree of crankshaft rotation past stock, but that's not much. The 386 stroker motor running at 7,500 to 9,200RPM would seem to have a much easier time flowing through the small valves than the stock 350 motor running at 8,500 to 10,500RPM. Sure it's 11% more displacement pumping through the same valves and the same carburetor, but it's also a 14% reduction in engine speeds. I guess what it comes down to is that when the valves are on the small side any little disruption to flow can make a pretty noticeable difference in top end performance. The 33mm valves are pretty small for the 386 stroker motor. The KTM 350 motor for example has much larger 36mm intake valves, about a 1/4 increase in maximum flow capability.

Despite that seemingly huge discrepancy in flow capability the 386 stroker motor doesn't actually seem all that much down on power compared to a KTM 350. Sure the KTM 350 motors do pull quite a bit more top end power all the way out to as high as 13,000RPM, but part of that is also the shorter stroke length. Down in the most usable 3,500 to 7,500RPM trail riding engine speeds the 386 stroker motor running at it's best is very strong, perhaps even a bit stronger than a KTM 350 motor. The 386 stroker motor does have a 10% displacement advantage, and the aggressive 1991 Husqvarna SOHC roller cam does seem to provide somewhat broader power than the stock finger follower KTM motors. What the KTM 350 motors also have though are huge 12.3:1 to 14.0:1 compression ratios for running high pressure race gas. The 9.7:1 386 stroker motor falls flat on its face when subjected to high pressure race gas; suffering a near total loss of bottom end torque everywhere bellow 6,000RPM when excessive 28 and 33 degree BTDC spark timing is required.

Better Shock, No Spoke Problems and Suspension Performance Rankings

All through 2014 I had a huge amount of trouble with the spokes on the rear wheel of the stock 1991 Husqvarna WXE 350. The spokes kept loosening up, and spokes broke very frequently also. I had to continually torque the spokes, and over tightening caused more spokes to break. When I put the 1994 rear wheel on my original 1991 Husqvarna WMX 610 in April of 2015 I then had a large supply of replacement spokes for my problematic 1991 WXE rear wheel, but it was a lot of work keeping up with the loosening and breaking spokes. All through 2016 I didn't ride the 1991 Husqvarna WXE 350 chassis much, but I still had to attend to the spokes on the rear wheel after nearly every long ride. I did get better and better at evenly torquing the spokes. The tighter I made them the less they loosened up, and the more even I got the tension the fewer broken spokes I ended up with. It still seemed like about two tanks of gas in off road riding was all I could manage between major spoke maintenance sessions though, and it was beginning to look like I was going to need another 1994 rear wheel.

Then after I put the 1991 Husqvarna WMX 610 shock on the 1991 Husqvarna WXE 350 chassis early in 2017 the rear wheel spoke problems vanished. In over 35 hours of riding the upgraded 1991 Husqvarna WMX 386 I haven't had the slightest bit of trouble from the rear spokes. Not one broken spoke yet, and I haven't had to tighten the spokes at all. They are just holding solid like on all of my 1991 Husqvarna WMX 610 bikes. It turns out that the 1991 Husqvarna WXE 350 shock is so much harsher over sharp bumps that it causes worse spoke loosening and breaking problems.

Those 1991 Husqvarna WXE 350 White Power shocks really are pretty bad. It's amazing that back in 2014 I thought the suspension on my stock 1991 Husqvarna WXE 350 good. The 1991 Husqvarna WXE 350 White Power forks are actually quite a lot better than the 1991 Husqvarna WXE 350 White Power shock, so that was part of what was throwing my judgment off. The other thing was that what I had to directly compare the 1991 Husqvarna WXE 350 to was the atrocious 1992 Husqvarna Showa suspension. In thinking through the differences between the various suspension systems I have come up with an interesting concept; to rank suspension systems in order of overall competence and functionality.

Clearly the 1992 Husqvarna Showa shock is at the bottom. The reason that I consider the 1992 Showa shock the worst rear shock I have is that it does most things very poorly and there is very little that it does well. The 1992 Showa shock is even harsher than the 1991 WXE 350 White Power shock, and the 1992 Showa shock also bottoms extremely easily under all conditions. Add to that the fairly severe problem of packing up over successive widely spaced bumps and the 1992 Showa shock is the clear looser under nearly all conditions. The only thing the 1992 Showa shock does well is keep the bike low and well in control over jumps on a smoothly groomed motocross track. It is a beginner shock for motocross racing, and it doesn't do anything else at all well.

Next on the list would be the 1980's Husqvarna Ohlins shocks. The only reason that I rate the old Ohlins shocks so low is that they get harsh at high speeds, and I personally don't like that. Even with a weak little 125cc air cooled two valve Chinese motor the bike goes too fast for those old Ohlins shocks. That obviously is a very severe problem on big 59" wheelbase 60hp open class race bikes, because they tend to go pretty darn fast. The only thing that those old 1980's Husqvarna Ohlins shocks are good for is slow riding on very tight single track, and they are pretty good for that. The level of plushness, comfort and control down at low 10 to 20mph speeds in first and second gear is very impressive, and that can make single track riding very fun. In the end though the 1980's Husqvarna Ohlins shocks are again beginner shocks. When I first rode the 1986 Husqvarna WR400 I was so distracted by the abrupt power delivery, weak brakes and wet noodle flexi 40mm conventional forks that I hardly noticed the excess harshness at higher speeds. I say hardly, because it was actually on that very first ride that I noticed that pot holes in a dirt road connecting sections of single track were just a whole lot more of a problem than they had been on my 1991 Husqvarnas. At first I thought that the 1986 Husqvarna Ohlins shock was pretty good at all lower speed single track riding; only getting excessively harsh when opening it up on more open dirt roads. Later though I began to realize that even on an entirely single track ride I was still getting up to speeds where the Ohlins shock became noticeably harsh. It really is only way down at supper low speeds on the tightest sections of trail that the 1986 Husqvarna Ohlins shock works well. It's just too slow.

The 1987 Husqvarna Ohlins shock has a similar problem with getting harsh at high speeds, but it's not exactly the same as the 1986 Husqvarna Ohlins shock. The 1987 Husqvarna Ohlins shock has much more substantial damping overall, which means that it isn't quite as supper comfortable at low speeds on tight single track. The very substantial overall damping does however mean that the 1987 Husqvarna 430 WR is a lot faster. The 1987 Husqvarna conventional 40mm forks are also faster than the 1986 forks with much more substantial damping. The harshness at high speeds from the 1987 Husqvarna Ohlins shock does however still mean that the bike mostly is only good for low to medium speed trails. The 1987 Husqvarna Ohlins shock is perhaps marginally smoother at high speeds than the 1986 Ohlins shock, but it's not good enough. The 1987 Husqvarna 430 WR still beats the rider up too much when the trail opens up a bit.

The 1991 Husqvarna WXE 350 White Power shock comes in third worst, or is that fourth worst counting the 1986 and 1987 Ohlins shocks as separate entries. In any case the 1991 Husqvarna WXE 350 White Power shock is better because it does everything fairly well. It is hard to find much fault with the 1991 Husqvarna WXE 350 White Power shock unless there is a 1991 Husqvarna WMX 610 White Power shock available for direct comparison. For beginner riders the problem with the 1991 Husqvarna WXE 350 shock is just that it is somewhat harsh and uncomfortable at all elevated bike speeds. For going fast or racing the 1991 Husqvarna WXE 350 White Power shock also fails to provide competent damping. It blows through the middle part of the stroke, gets even harsher towards the bottom and on top of all of that the rebound damping doesn't work well enough for aggressive high speed riding over rough terrain.

Slightly better than the 1991 Husqvarna WXE 350 White Power shock is the race shop modified 1991 Husqvarna WMX 610 White Power shock I have. Under certain conditions that race shop modified WMX 610 shock is actually quite a lot better than the 1991 Husqvarna WXE 350 shock. At just the right speeds around 35mph it seems pretty much just as smooth and comfortable as the stock 1991 Husqvarna WMX 610 shocks, and that is actually quite a lot better than the 1991 Husqvarna WXE 350 shock. Also in impact absorption from landings it seems substantially competent and a lot better than the 1991 Husqvarna WXE 350 shock. The rebound damping on the race shop modified WMX 610 shock also works better than the 1991 Husqvarna WXE 350 shock, at least at moderate speeds. Going faster the race shop modified WMX 610 shock feels very bouncy and out of control, which is only partially due to inferior rebound damping.

As second best the 1999 Husqvarna TE 410 Sachs shock is pretty darn good. It is actually amazingly similar to the race shop modified 1991 Husqvarna WMX 610 White Power shock, at least in terms of working fairly well other than some excess harshness and a bouncy feeling in aggressive riding. The Sachs shock clearly gets ranked as better though because it works fairly well under all conditions. There is nothing that the 1999 Sachs shock does very poorly, and it does pretty much everything pretty darn well. The only reason that the 1999 Sachs shock is worse than the 1991 Husqvarna WMX 610 stock White Power shock is that it isn't quite as good. Compared to the stock WMX 610 shock the Sachs shock is a bit harsher at all speeds and under all conditions and there is a bit of an excess bouncy feeling also. The bouncy feeling is the combination of compression damping that isn't quite as compliant as it should be at higher bike speeds and also rebound damping that has a very narrow range of adjustment and doesn't quite work as well. The rebound damping on the 1999 Sachs shock actually is very good, but sometimes it seems a bit excessive even with the clicker turned all the way out and sometimes it seems a bit under damped even with the rebound clicker all the way in. The biggest problem appears to be the very narrow range of adjustment on the rebound damping, but the fact that I turn that Sachs rebound clicker more often than any other rebound clicker also indicates that the rebound damping is somehow a bit off from what it should be. That's what it is with the 1999 Sachs shock, it's just a little bit off from what it should be. But it's only the very slightest bit off, so overall it is actually a very good shock.

And that leaves just the 1991 Husqvarna WMX 610 White Power shock on the top of the heap. Perhaps the only reason that I think the 1991 Husqvarna WMX 610 White Power shock is good is that everything else I have to compare it to is quantifiably worse. There are some clues though that the 1991 Husqvarna WMX 610 White Power shock is in fact very good. One is that the 1991 Husqvarna WMX 610 always seems to be the fastest dirt bike around regardless of conditions. I am not always the fastest rider, but whenever anyone is faster than me I can always attribute it to specific riding technique advantages that other riders have. It's never the 1991 Husqvarna WMX 610 that is slower, and that is very impressive.

The most surprising aspect of the competence of the 1991 Husqvarna WMX 610 is that it is the fastest bike around on tight technical single track trail also. How is it that a bigger and heavier bike is still faster? Well, that's just because the other bikes haven't gotten light enough yet. I also haven't done any serious riding with anyone on the new generation 2016/2017/2018 KTM/Husqvarna 450 race bike. The rest of the modern dirt bikes are only very slightly lighter than a 1991 Husqvarna WMX 610, especially the off-road specific models. It is only the new 450 that was introduced as the 2015-1/2 KTM Factory Edition that is substantially lighter, but even that bike has some problems. The main problem is just the transmission. The racing suspension is only available with the excruciatingly narrow ratio five speed, and getting the wider ratio six speed saddles the bike with all of the street legal accoutrements including unknown (to me) casual oriented XPlode suspension.

That's part of why I haven't bought a new bike yet. I wouldn't know which one to buy without riding all of them. The new electric start, spring fork Honda 450 with finger follower driven intake valves seems to check all the right boxes, but then the magazine writers talk about suspension that needs work. The electric start Yamaha 450 off road race bike is said to have excellent suspension valving, but it is a heavy beast with a difficult to manage race gas hungry cam and bucket motor. For me the Husqvarna 450 dual sport bike certainly seems appealing, but that's a lot of money to risk on totally unknown suspension. The KTM or Husqvarna 450 off-road or motocross race bikes seem more likely to have good suspension valving, but then they are also illegal and probably require race gas. The only sure fire option seems to be a KTM or Husqvarna 350, 450 or 500 dual sport bike upgraded with about six grand worth of factory racing suspension. Now there are 250, 350, 450 and 510cc engine options for those street legal KTM/Husqvarna dual sport bikes, which is a pretty bewildering array of options. The 250 is the clear choice for anything other than going very fast, although a lot of riders seem to think they need at least the 350cc version to have enough torque available for easy hill climbing. The 350cc version is just the 250 engine punched out with a bigger bore and a slight increase in the stroke length, so it comes with zero weight penalty. The reality though is that the bikes are designed around the 450/500 motor. Being a SOHC motor it's more compact than the DOHC finger follower 250/350 motor, and it hardly ads any weight to the bike. Between the 450 and 510cc versions it is easy to chose the 450 for several different reasons. One of course is just that the 2.5" stroke length engines run a lot better than the 2.83" stroke length engines. That's already a long enough stroke length that any reduction in stroke length pays huge dividends in broad and easily tuned power. The other big reason to go with the 450 is specific to the new 2016/2017/2018 engine platform. The new engine uses a shorter connecting rod that is sized for the shorter stroke 450 where the old 2013/2014/2015 platform used a bigger and longer connecting rod sized for the 510cc version.

Finally there is just the displacement itself. The correct displacement for a big heavy racing type dirt bike appears to be about 300 to 400cc. As just about anyone can tell you the 450 is already too much displacement for a dirt bike, so going up to 510cc is really very ridiculous. Lots of displacement can be fun, but a 50+hp KTM 350 is already so much power that just about anyone would agree that more is not required. Really there are only two reasons that good running dirt bikes ever need more displacement than 350cc. One of course is out on the interstate highway when people drive at up to about 90mph. That's a lot of sustained speed for a motorcycle, so more displacement up to really very huge sizes tends to seem like a big advantage. The other would be in very high speed off road racing where large open stretches of flat uniform terrain allow the bikes to be opened all the way up for extended periods. It sucks to be passed and dropped for absolutely no reason other than that you are running less displacement than your competitors.

If the connecting rod is too short for the 2016/2017/2018 KTM/Husqvarna 510cc engines though then that extra displacement might not actually translate into more reliable sustained peak power output. There might not be any reason at all to go with the 500 over the 450 on this new 2016/2017/2018 KTM platform.

Engine Performance Rankings

While on the topic of performance rankings it is interesting to compare peak power output also. On the top of the heap is probably the 1987 Husqvarna 430 two stroke; that thing definitely makes excessive amounts of power. Part of the excessiveness of the power is the sub-par suspension valving and terrifying wet noodle conventional 40mm Husqvarna forks. With that suspension it wouldn't' take much power to be scary fast, but the 430 two stroke is in fact very much power. At least when it is actually running strong. The biggest functional problem I have had with the 430 two stroke has been a very bad tendency to fall off the power band anywhere from 5,500 to 7,500RPM, which is both very annoying and also makes the bike mostly useless. This is however just a gasoline problem. The broad tuned non-power valve cylinder port two stroke is certainly more finicky and less reliable than other two strokes, but on a consistent gasoline supply it can deliver some big power.

After riding the 1987 Husqvarna 430 WR a few times on longer trail rides last summer my cobbled together top end fell apart. Not the lightened stock Mahle piston, no it was my home made circlip that failed. As tight as I though those things were in there... Well they weren't in there all that tight after all, as one fell out while I was riding the bike. The engine severely lost power all of a sudden, and then wouldn't low idle at all. I turned it around and headed back down the hill too keep it from stalling, and once I figured out that it would still run at 4,000RPM with a wide throttle opening but wouldn't idle I was able to devise a strategy to ride it back. I did manage to keep it running, and I made it back the short few miles without it stalling. When I shut it off it wouldn't restart at all. Obviously the top end had failed, and right away I suspected those funky home made circlips.

When I tore the top end off I could clearly see what had happened. The circlip had fallen out, and then gotten sucked up into the combustion chamber where it was crushed between the piston and the cylinder head and broken into many small pieces. Those small pieces were repeatedly smashed against the piston crown around the perimeter of the combustion chamber. Some of the broken pieces landed lengthwise and were smashed so far into the crown of the piston that the ring broke apart into several pieces. Those broken pieces of ring then jammed against the cylinder walls and just totally obliterated the top end. Interestingly the lightened piston didn't break apart. Even running with the piston pin hanging out one side so that the entire load was taken by the pin now in "single shear", instead of the usual "double shear" neither the piston or the pin broke. I guess it is a good thing that the circlip happened to pop out when I was cruising up a hill at 5,000RPM instead of blasting to 8,000RPM between the gears.

I bought a used 430 top end of unknown condition for the rather low price of about $100. The used top end turned out to be perfectly serviceable. The cylinder was in better condition than the old one I had before, and the original Mahle piston was essentially like new. The only thing wrong with the top end was that the ring had been gapped too wide when first installed. A full 0.04" (1mm) end gap! Yes that is far too wide. Instead of buying a new ring for another $100 though I just put it together with the pieces I had. And again I cut the stock Mahle piston down to the same lightened 329g weight. The second time around it was even easier to cut the piston down, and that same amount of weight came off very quickly and easily. There is in fact a whole lot of extra weight that can easily be taken off of those very heavy old Mahle two stroke pistons.

The hardest part of the rebuild job this time was just trying to make sure there weren't any broken pieces of circlip in the crankcase. I turned the bike upside down and rinsed the crankcase out with water pressure while also turning the crankshaft by hand. I didn't see any little bits of broken circlip come out, and after all that high volume rinsing I was fairly certain that nothing was likely to remain.

When I first fired the rebuilt 430 up it took quite a few kicks, but it did fire up without having to do anything drastic like change the spark plug or pull it with a car. The cranking compression had seemed quite low when cold, but as soon as it warmed up just a bit I shut it off and checked the cranking compression again. With the piston warmed up the cranking compression was substantially high, a sure sign that the 0.040" end gap was in fact way too wide. The 430 restarted easily when warm, and then after it fully cooled off the cranking compression was again rather low.

Since this piston, ring and cylinder had all been a matched set the top end didn't need a lengthy break-in period. I did however notice that the craning compression when cold did continue to increase a bit as I rode the bike more on subsequent days. The 430 motor ran exactly as it had before, including the large tendency to fall off the power band at 5,500 to 7,500RPM when running weak watered down garbage gasoline. By this point I had grown to hate the harsh Ohlins shocks quite severely, so I didn't ride the 1987 Husqvarna 430 WR much. I just took it out every once in a while to see how it was running, and usually that meant pretty bad.

Then one day I got the idea that all I really needed was some normal gasoline and I could get some really big top end power out of the high revving 430 two stroke with my lightened piston. The idea I had was that although the gasoline I had been getting was very weak there must be some more normal gasoline around somewhere. The gasoline coming out of the pumps had usually been very weak and severely watered down and then gotten even worse sitting around in gas cans. It occurred to me that perhaps the gallon of pre-mix for the chainsaws that had been sitting in a sealed container all winter might still be more like the gasoline that had been normal for many decades. I put three quarts of that old pre-mix in the empty tank on the 1987 Husqvarna 430 WR and headed out for a little test ride.

When I fired the 430 two stroke up it would hardly run at all on the garbage still in the carburetor bowel. Once running on the chainsaw pre-mix though it was a whole different story. It was actually running about the same as it often had in the past, with some torque building nicely from 3,500 to 5,500RPM and then the power hitting strong around 5,500 to 6,000RPM. It wasn't falling off the power band at all and power seemed quite strong at 7,000 to 8,000RPM. The power seemed to keep increasing at least slightly up to about 7,500RPM. What was really surprising was that the power continued all the way up to 9,000RPM! Wow, that's a lot of twist for the big 2.91" stroke length 430 two stroke. The power was dropping off a bit out there at 9,000RPM, but it wasn't falling off the power band and it was revving out seemingly rather willingly. Out to at least 8,100RPM there wasn't any sign of the power dropping off, which was very impressive performance for the 430 two stroke. Interestingly the 430 two stroke was also seeming rather quiet on that more normal chainsaw pre-mix. Low idle was quiet, and blipping the throttle the engine sounded healthy and responsive but it was quieter than it often had been. Even wide open up through the power band the 430 two stroke was seeming more quiet than it usually had when it was running worse and falling off the power band. Quiet and powerful and a higher maximum engine speed. Hmm, sounds like more powerful gasoline.

I don't know if it was actually more power from the 430 two stroke than my hot rod 610 motor makes, but it was certainly in that league. The 430 two stroke would be expected to make more power than a 577cc four stroke with a longer stroke length, but the actual maximum power output levels seem to be rather similar.

The day that I put the remaining 65mpg gasoline out of the 1999 Husqvarna 410 into my 12.2:1 hot rod 610 motor I got a lot of power at 7,500 to 8,400RPM. Really very big power, like lifting the front end at 50mph while in a slightly forward seated riding position kind of power. The 430 two stroke does a lot of front end lifting at rather high speeds also, but mostly just because of the abrupt open class two stroke power hit that often seems to catch me a bit off balance or back a bit too far. I don't know which one has made the most power, but both tend to be highly variable on weak and variable gasoline. The hot rod 610 motor used to pull really hard like that all the time in 2015 and even sometimes in early 2016, but by spring of 2017 when I tried the old chainsaw pre-mix in the 430 two stroke the power output from all the engines had mostly been seeming rather low.

The two most powerful Husqvarna engines I have certainly are the 430 two stroke and my 12.2:1 hot rod 610 motor, but the bone stock 1986 Husqvarna 400 WR isn't far behind either. In fact it is substantially right in there. The difference is that the 400 two stroke is brutally narrow where it makes really big power. Pretty much only from 6,200RPM up to 6,800RPM. The power often stays strong to 7,000 or even 7,100RPM, but it is still an extremely narrow power band that can't quite cover the gaps between the gears without either clutch slipping or using a bit of the rapidly dropping overrev that is often available out to 7,400RPM. Because the 400 two stroke is so much more reliable and consistent it can sometimes seem a bit more powerful than the 430 two stroke, but the reality is that it takes running both engines on some really spectacularly weak watered down garbage gasoline to actually get less power from the 430 two stroke than from the 400 two stroke on the same gasoline.

Fourth position is the period correct big cam but otherwise stock 10.2:1 610 motor. With the big camshaft top end power around 7,500 to 9,000RPM is strong and reliable, but the heavy stock Mahle piston and the small valves do hold the engine back. It certainly makes more power than my other entirely stock 10.2:1 Husqvarna 610 motor. It is probably more power than the 11:1 SEM ignition 610 motor with the 333g lightened Mahle piston, although that is a bit hard to call. The stock camshaft and 26 degree BTDC spark timing makes so much more torque everywhere from 3,000 to about 6,000RPM that the top end power difference tends to get overshadowed. And even at that the lightened 333g piston is enough of an advantage that it is hard to even say for sure that the big cam but otherwise stock period correct 610 motor actually makes more power.

Where my old, stock 10.2:1 points ignition 610 motor falls is also very hard to judge, mostly because I don't have any suspension for it at the moment so it mostly just sits there. It still has the stock 180 main jet and the needle clip position is still in the second groove, and that means that severely weak watered down gasoline can make that seem like the most powerful engine when compared to all my other 610 motors that have the same approximately 176 size main jets in them. It is a powerful engine though. The 1992 exhaust system probably flows somewhat better than what is on any of my other 610 motors, and cutting the piston down to 368g along with a 10g lighter piston pin did go a long way to free up top end power compared to the heavy stock Mahle pistons. The crankshaft wiggle advance hitting at around 6,300RPM is also very good for delivering strong and reliable feeling power even on rather weak or somewhat slow flame front travel speed gasoline. That worn out old 610 motor is a hard puller, and really the only thing that stops it is very high pressure gasoline that requires more than 26 degrees BTDC on the static timing setting.

Clearly the all stock 10.2:1 Czech Republic CDI 610 motor is the weakest of the big open class Husqvarna motors I have, by a rather large margin really. The specific weakness of that all stock motor is however a bit tricky and deceptive. When it is running strong and the compression ratio isn't too low it does pull fairly well everywhere from way down low up to fairly high engine speeds. Power always seems to peak down rather low, often way down at about 6,500RPM or so, but it is only sometimes narrow. Even though it is only 26 degree BTDC spark timing plus the perhaps degree and a half of advance up to 5,000RPM for a total of less than 28 degrees BTDC the Czech Republic CDI does actually put out a fairly powerful spark. It might be only 27.5 degrees BTDC at 5,000RPM and above, but it seems like more spark advance than the stock SEM ignition backed off all the way to 26 degree BTDC spark timing. Amazingly the stock 10.2:1 Czech Republic CDI 610 motor does sometimes pull all the way out to 7,700 and even 8,500RPM.

With the stock camshaft and small 35mm/30mm valves the intake stack boost isn't very noticeable at 7,500RPM, and the power has often ended right at 7,700RPM. Despite the lack of much of a noticeable intake stack boost though the all stock 610 motor does often keep pulling right past 8,000RPM on fast flame front travel speed gasoline. The power usually seems pretty flat beyond 7,000RPM, but being able to pull all the way out to 8,000 or 8,500RPM before shifting keeps the power all the way up to it's modest maximum all the way through every gear. It might not be all that much power compared to some of the other modified 610 motors, but the stock 10.2:1 Czech Republic CDI ignition 610 motor does move the bike briskly. It's a lot better than an all stock 610 motor running the stock 33 degree BTDC spark timing, that's for sure.

Next down is of course the 1999 Husqvarna 410 motor, which often runs great but never quite matches the 610 motors for shear pulling power. The very broad effortless power of the short stroke length 410 motor is a big advantage though, particularly when nothing but extremely weak low temperature of combustion potential gasoline is available. The 410 motor gets weaker on weak gasoline, but it doesn't surge and the quality of power doesn't drop off anywhere near as much as on the longer stroke length engines. The 410 motor runs great, but it won't start and it also doesn't have quite as good suspension. I would love to get a set of 1991 Husqvarna WMX 610 forks for my 1999 Husqvarna WMX 410, but short of that the Vespa is relegated to the same corner cluttering duty as the two strokes and the 1992 Showa Husqvarna chassis.

The 1986 Husqvarna 250 two stroke motor was a very hard pulling 250, and it might have made more peak power than the 410 when running on reasonably powerful gasoline. The severe lack of any midrange though meant that the 1986 250 two stroke wasn't very usable, and most of the time it seemed severely slow. Slower even than the stock 1991 and 1992 Husqvarna 350 four stroke motors.

The stock 1992 Husqvarna TE 350 motor with the gigantic 1992 camshaft was quite powerful. It ran horribly at low engine speeds with the stock 33 degree BTDC spark timing, and the power came on very abruptly right at 6,000RPM, but it did pull pretty hard. Even down at 6,000 to 6,700RPM where it was most reliable the stock 1992 Husqvarna 350 motor was respectably powerful if very narrow on high pressure gasoline. When it was able to really get going and make power at 8,500 to 10,000RPM though it was quite a screamer. So much of a screamer that the rod bearing didn't last through the first really long ride I took it on. Of course I didn't know about the limitations of the oil reed valve lubrication system back then in early 2014 either, so I just kept that thing zinging all the time. As strong as the stock 1992 Husqvarna TE 350 motor ran though it was in fact severely saddled by the very heavy 270g stock Mahle piston and the huge 313g Husqvarna 610 rod. All that weight was in fact such a problem that power didn't actually seem to increase much past 8,500RPM, just like the stock 1991 Husqvarna WXE 350 motor. What surprised me the most about the two stock 350 motors was that the huge 1992 camshaft didn't actually make all that much difference in peak power output. The 1992 350 motor was more powerful up on the top end, but not by enough to overcome the severe problems bellow 6,000RPM. The stock 1991 Husqvarna WXE 350 motor, even when running ridiculous huge 30 and even 34 degree BTDC static timing settings seemed very nearly just as fast. Even with that huge spark timing the stock 1991 350 motor was much stronger at 4,500 to 6,000RPM than the stock 1992 350 motor had been.

The 386 stroker motor is actually a more powerful and much better running engine than the stock 1991 Husqvarna WXE 350 motor. The much lighter weight cut down 245g Wiseco XR400 piston and significantly lighter Pro-X brand XR400 rod mean that the 386 stroker motor spins much more easily than the stock 1991 and 1992 Husqvarna 350 motors. With the reciprocating assembly sucking off much less power the 386 stroker motor has quite a bit more power production potential when burning the same amount of gasoline with the same amount of intake air. I think I have gotten more power out of the 386 stroker motor than I ever got out of either the stock 1991 Husqvarna WXE 350 motor or the stock 1992 Husqvarna TE 350 motor, but it is a difficult call to make since the stock 1991 Husqvarna WXE 350 ran on so much more powerful high energy density and high temperature of combustion potential gasoline. The stock 1991 Husqvarna WXE 350 motor was severely weak with that very heavy reciprocating assembly and rather small valves, but boy was that gasoline that was coming out of the pumps in 2014 some powerful stuff.

Easy Starting and More Power from 4-3/8" Stroke Jeep

Also right in line with the general trend back towards more normal gasoline is occasional improved performance from the big 4-3/8" stroke length 1953 Willies engine in my Jeep. One day the detonation wasn't as bad from the long stroke length Jeep engine. I noticed that torque at all small throttle openings everywhere from 1,100 to 2,000RPM was considerably stronger, and the bad detonation was only occurring around 1,800 to 2,000RPM at wider throttle openings. At small throttle openings there was very smooth and very usable torque all the way through from 1,500 to 2,500RPM. Enough torque at small throttle openings that the bad detonation at 1,800 to 2,000RPM actually could be avoided with little difficulty. When I opened the throttle I found that the power at 3,000 to 4,000RPM was back with no cutting out. The power was actually increasing from 3,500 up to 4,000RPM.

Starting performance was also improved on that gasoline. The engine just fired right up from cold. A few days later the Jeep again fired right up from cold, and at first ran the same as it had with good torque at small throttle openings. Then after driving for a few miles I noticed that the torque around 1,200 to 1,600RPM had dropped off. I also noticed that the detonation at 1,600 to 2,000RPM was back and the engine just seemed to severely lack torque at all throttle openings. The gasoline in the gas tank obviously had disappeared, but the engine had fired up easily and ran strong on the more powerful gasoline that was still in the carburetor bowel.

Something interesting that I had noticed with that hotter burning more normal gasoline in the long stroke length Jeep engine was that it was at the ends of the engine speed range where there was significant improvement. Torque had increased quite dramatically from 1,1000 to about 2,200RPM and power had increased dramatically above about 3,200RPM. In the middle from 2,200 to about 3,000RPM though there wasn't much of an improvement, and the long 4-3/8" stroke length engine was actually noticeably harsher without more torque at some engine speeds right around 2,300 to 2,800RPM on the seemingly better running and more powerful gasoline. The big difference on the more powerful gasoline though was that letting off the throttle a bit eliminated the harshness everywhere from 2,200 to 2,600RPM without all that much of a decrease in torque output. The more powerful gasoline provided some usable amount of smooth torque at small throttle openings at all engine speeds from 1,100 up to 2,500RPM. On the dramatically weaker gasoline the bad detonation at 1,600 to 2,200RPM is unavoidable in the 4-3/8" stroke length engine, and torque remains very low at all throttle openings.

It's certainly not all peachy for the long 4-3/8" stroke length engine, even on seemingly much more powerful and better running gasoline. There still tends to be dead spots somewhere in the 2,000 to 3,000RPM range of engine speeds. The small 3-1/8" bore diameter is good for full flame front mode operation, and the 134 cubic inch 1953 Willies engine often makes usable amounts of very smooth torque all the way up to around 2,400RPM. That's great for avoiding the bad detonation at 1,800 to 2,200RPM, but then up around 2,500 and 2,800RPM the long 4-3/8" stroke length engine is tending to go very flat and lifeless in full flame front travel mode. Late compression ignition makes more power at those engine speeds, but even with the very long 4-3/8" stroke length there is some harshness. It's just too low of an engine speed for late compression ignition, even though the long 4-3/8" stroke length is requiring late compression ignition down to those low engine speeds.

With the long 4-3/8" stroke length the harshness at the latest possible time of late compression ignition diminishes very quickly as the engine speed is increased, but then that very long stroke length requires earlier times of late compression ignition to rev higher. The long stroke length severely reduces the range of engine speeds where the latest possible time of late compression ignition will deliver good torque, and the result is that earlier times of late compression ignition are required to make power. Those earlier times of late compression ignition really don't work well at low engine speeds though, so the age old problem of gasoline engines needing to rev out far to make power remains. Even with the very long 4-3/8" stroke length the power is stronger, smoother and more reliable up at around 3,500 and 4,500RPM.

Ultimately all that the longer 4-3/8" stroke length accomplishes is to push the engine speeds where late compression ignition seems to sort of work down lower to around 2,400 or even 2,100RPM. Usable torque at the difficult low end 2,600 to 3,200RPM engine speeds is actually easier to reliably attain with a much shorter 2.75 to 3.25" stroke length. It's not that a 2.75" stroke length engine runs well at 2,600 to 3,000RPM, it just that the 2.75" stroke length engine is likely to run much better than a 4-3/8" stroke length engine at those difficult 2,600 to 3,000RPM engine speeds. The 2.75" stroke length engine runs a whole lot better up at 3,200RPM than it does at 2,800RPM, but at 2,800RPM the 2.75" stroke length engine is likely to do a whole lot better than the 4-3/8" stroke length at the same 2,800RPM engine speed.

Where does a 4-3/8" stroke length engine do better than a 2.75" stroke length engine? Way down at 1,200 to 1,800RPM in full flame front travel mode. Long stroke length engines do better in full flame front travel mode than short stroke length engines. Long stroke length engines appear to do very well in full flame front travel mode because they do rather poorly in late compression ignition mode. Long stroke length engines can also actually run better in full flame front travel mode than short stroke length engines can. The long stroke length gets the mean piston speed up high enough to extract reasonably efficient torque without the engine speed getting so high that the flame front propagation can't keep up.

Back to Index to Technical Articles

©2017 Michael Traum