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The Husqvarna 250 Two Stroke

When American rider Fred Hoess won the inaugural vintage class at the 2016 ISDE in Navarra, Spain riding a 1986 Husqvarna 250 WR I figured it seemed like a good time to recount my own recent experience with a 1986 Husqvarna 250 WR. Fred's 250 looks to be a beautifully restored example with a mix of perfect stock parts and some hard to find aftermarket parts. Mine was also fairly pretty, and also had a few aftermarket parts on it. It was advertised as not running, but in excellent condition. For the low purchase price of $1,000 I drove all day on the Interstate to go buy it. I just wanted the nice shiny parts for my 1986 Husqvarna 400 WR, as nearly all of the parts other than the cylinder, piston, cylinder head and pipe are exactly the same on the 1986 Husqvarna 250 and 400 two strokes.

The Bike
The Motor
The Rebuild

The Bike

When I got to the guys shop his '86 250 WR was everything he said it was, although the glitz in the photographs was largely applied with buffing compound to well used old parts. The bike was however in excellent condition overall, with tons of obviously very usable stock 1986 parts. It was well worth the $1,000 and the long drive.

I had noticed in the photographs that the rear brake was not stock. The guy selling the bike said it was a rear brake off of an XC model, but when I looked at part numbers in the parts manuals it seemed that only the CR models in 1986 came with the different rear drum brake setup. The difference was that instead of the brake backing plate being clamped down to the swing arm it was free to rotate, and held somewhat in position with a link all the way forward to the frame.

The guy selling the bike said a few other things that weren't true either. He said the engine was not running because the ignition system didn't have a spark. That was not true, the Spanish built Motoplate Mini 6 CDI ignition system did give a sufficient spark, and the engine fired right up after I cleaned and gapped the spark plug. The engine had pretty reasonable cranking compression, and it fired up really rather easily, largely due to the smaller 250cc displacement being easier to kick over than the big 400cc and 430cc big blocks.

All I had to do to get the 250 ready to ride was install an air filter, clean and gap the spark plug, adjust the controls and put fresh fluid in the front brake. The bike had obviously been sitting unused for quite a while, but it had been fairly well taken care of and never stored outside in the rain for long periods of time. The front brake had sort of worked when I bought the bike, but just as I had expected the system was full of thick brown gunk. Once I drained the very old fluid out and replaced it with fresh DOT 4 fluid the hydraulic system seemed to be in perfect condition. The front brake was extremely weak at first, but improved as I rode the bike. There was probably just some oily residue on the disk from sitting around so long.

The CR rear brake setup was a disaster. The peddle height changed dramatically as the rear suspension compressed and extended, and the brake feel was horrible. As the rear suspension extended under heavy braking the peddle came up and the rear wheel locked up easily. When the rear suspension compressed the pedal dropped and the rear brake seemed to disappear.

After a few very short test rides I pulled the rear wheel off. I happened to have an extra '86 Husqvarna rear wheel and packing plate that I had bought to replace the worn out aluminum drum on my 1987 Husqvarna 430 WR. In several months I had not gotten around to installing the new wheel as my modified brake came spacers were doing a fine job of working with the worn out drum. The '86 hubs have a steel liner for the drum brake, so they generally don't wear out like the aluminum drum on the '87 through '89 hubs.

Even with my spare wheel on the '86 250 I was not entirely without another spare wheel for the '87. The '86 CR wheel that was on the '86 250 WR is very similar. The steel liner just sticks out a bit farther on the CR hub, but it could be converted to be used as a WR hub just by turning the liner down flush with the aluminum hub. As it would turn out there is isn't exactly a shortage of '86 rear wheels in general, but that is for a another story.

With the '86 wheel and backing plate installed the 1986 Husqvarna 250 WR was mostly back to its stock configuration. There were however a few small differences. One was that it had a 38mm Mikuni carburetor off of a 400 motor. The 1986 Husqvarna 250 WR did come stock with a 38mm Mikuni, but the jetting was different. According to the spec sheets the '86 250 WR came stock with a Mikuni 420 size main jet, where the '86 400 uses a 400 main jet. The needle and needle jet were also different, with the 250 WR spec sheet calling for a 6DH20 needle in an R-4 needle jet and the 400 WR using a 6DH3 needle in a Q6 needle jet. Not only was the main jet a Mikuni 400 and the needle a 6DH3, but the needle jet appeared to be the same diameter as the Q6 needle jet on the 400. In every regard the carburetor that was on the 250 WR when I got it was exactly the same as the 38mm Mikuni on the '86 400 WR. This seemed good to me, as I had always thought that the Mikuni 400 main jet was way plenty fat for that carburetor. Initially I set the needle clip position on the 250 at the same 3rd groove where I had been running the '86 400 WR.

Most of the glittering beauty of my new bike was in the freshly painted frame, chromed sub-frame, polished swing arm and buffed out plastic. It also had a stiffer than stock shock spring and a tall and firm seat foam. Both nice features for more aggressive riding. The stiffer shock spring felt good all around, but unfortunately stiffer fork springs had not been installed to match. The pre-load had been increased on the forks, and this made them feel stiffer near the top, but diving under heavy braking was still a severe problem. Overall the modified suspension was much faster than the stock '86 400 WR suspension setup, but not quite as comfortable or confidence inspiring. It is always the conventional Husqvarna forks that are the problem with these late 1980's bikes. The rest of the chassis seems so good, but the forks fall short on performance. When it comes down to it though the drum brakes are not very good either, and along with the abrupt two stroke power the bikes end up being rather slow.

The muffler and spark arrestor on my new '86 250 WR were aftermarket parts, also polished up nicely. It was a larger diameter muffler more similar to the 1987 mufflers, and it had a Cobra Sparky USFS approved spark arrestor on the back of it. The larger muffler was not however quieter like the '87 muffler. The 250 was louder than the stock '86 400. Even just idling along the 250 with the aftermarket pipe was ear splitting loud. Up on the power band though the large muffler did a good job of keeping the screaming sound to a reasonable level. And it turned out that the 1986 Husqvarna 250 did do some serious screaming.

The Motor

The power band was up very high on the 1986 Husqvarna 250 WR. I never put a tachometer on it, but the start of the power band certainly seemed to be up above 7,000RPM. And did make some serious power, more power than any of the more recent Japanese or European 250 two strokes. Instead of a hard hitting narrow power band from 6,300 to 7,000RPM followed by flat overrev to 8,000RPM as is typical of 1990's Japanese 250 two strokes the '86 Husky 250 just had an extremely top end oriented power band. The power hit abruptly at about 7,500RPM or so and then built to a screaming crescendo around 9,000RPM with little to no overrev. The obvious difference is in the stroke length. All modern water cooled 250 two strokes have a 72mm stroke length, where the 1984 through 1988 water cooled Husqvarna 250 two stroke has a much shorter 64.5mm stroke length.

I don't have much experience with 250 two strokes in general, but when I got the '86 250 zinging I certainly could tell it made some serious power. Considerably more I am quite sure than any of the later long stroke 250 two strokes were capable of in stock form on the same pump gas. The long stroke 250 two strokes were sometimes modified to run on race gas with as much as 70hp output at 9,000RPM, but that is far from the tame 41 to 45hp that stock bikes did on pump gas. The all stock 1986 Husqvarna 250 WR is more powerful with it's shorter stroke length and very high power band. I didn't like the '86 Husqvarna 250 two stroke at all though. The problem was that it was weak as a kitten everywhere bellow the power band. And when I say weak, I mean extremely weak. Like so weak that it would hardly pull out in first gear. I dare say that the bottom end power on the '86 Husqvarna 250 two stroke is weaker than many 125 two strokes. And the weakness was not just way down low either, it extended all the way up to the power band with essentially no midrange torque at all.

A big Part of the problem was the extremely late 9 degree BTDC spark timing and the extremely high 15:1 compression ratio. That is a combination that helped to deliver big power and a fairly wide power band to very high engine speeds, but it was disaster for midrange torque. The engine just refused to run in late compression ignition mode anywhere bellow the power band. It was supper weak full flame front travel mode combustion up to way higher engine speeds than 9 degree BTDC spark timing could well support, and then the power band would hit suddenly with all sorts of very early times of late compression ignition up to very high engine speeds. It made power to be sure, and the power band was actually wide enough to get between the gears, but there was nothing but that very high and powerful powerband. For someone who was used to riding weaker 250 two strokes I am sure that this would have been lots of exiting fun. Coming off of the 610 four stroke and the 400 two stroke though the 250 two stroke just felt slow. It is not that it did not make enough power, it certainly made plenty of power. It was just that it was tons of very high engine speed screaming for considerably slower maximum acceleration than the 610 four stroke or the 400 two stroke. Essentially what it comes down to is that I ride much of the time at the 25 to 35hp that the 610 four stroke does at 3,000 to 4,000RPM, and I expect that power to be efficient, reliable and easy to get without clutch slipping. The much higher 60hp or more that the open class bikes make up in their power bands is more than a dirt bike needs for most purposes, but it is fun shred tires and throw rocks sometimes. And then there is racing. Any time the trail opens up a bit more power provides opportunities to pass and stretch out a lead.

Even the 400 two stroke nearly always has some little bit of midrange torque starting down around 5,000RPM that allows the engine speed to hover near the bottom of the power band. My 610 four strokes always pull hard down to around 3,100 or 3,300RPM, and even the 5,000 to 6,000RPM midrange of the 400 two stroke is much more usable for normal riding than the absolutely top end only powerband of the '86 Husqvarna 250 two stroke. I hated the power delivery of the '86 250 two stroke, but it was impressive just to see that 250cc of two stroke displacement could make such a large amount of peak power output with a wide enough power band to get through the gears.

When I ran all three of my two strokes on the same gasoline by swapping it from tank to tank, one bike after another, I was amazed that they were all indeed tuned to be able run on the same gasoline. The day I did the three two stroke gasoline test it was some rather weak and also higher pressure gasoline that was somewhat resistant to popping off in all of the bikes. Interestingly it was the lowest compression ratio 12.1:1 '86 400 WR that was the most reliable, delivering usable torque from 6,200 to 6,800RPM once well warmed up. On the same gasoline the 12.6:1 '87 430 WR was making torque from around 4,500 to 6,500RPM, but it was unreliable, and was falling off the power band all over the place. Mostly it was above 5,500 and 6,000RPM that the 430 was falling off the power band that day, but even down to 5,000RPM the torque was not reliable. On that same gasoline the 15:1 '86 250 WR made power in the power band, but it was difficult to get it going and the power was unreliable unless the engine was well heated up from some big pulls. The '87 430 WR ran the worst on this gasoline, and it seemed like low energy density watered down gasoline. The '87 430 WR is leaner at throttle openings bellow 3/4 throttle on the 6HD20 needle with the clip in the stock 3rd possition even though it is the same 38mm Mikuni with the same Mikuni 400 main jet. Both of the '86 bikes had the clip on the 6DH3 needle at the same 3rd groove position, and this provides a substantially richer mixture across the medium throttle openings.

Then the 250 seized up on me. I had opened it up in sixth gear at high speed on a dirt road on a hot summer day, and it just stopped. I felt the power drop, I closed the throttle and downshifted and the motor just stopped. It was seized solid, and would not turn over at all with the kick starter. Stranded, hot, and very disappointed I sat there for a few minutes wondering how I was going to get back. When the engine cooled off a bit I checked the coolant, and it was sill all the way up. Once the engine cooled off some I was able to turn it over with the kick starter, but there was a bunch of drag and the cranking compression was gone. With the extra drag it would not kick over freely, and it would not start. Amazingly though I was able to restart the engine by rolling it down a hill. Once restarted the engine ran horribly, with no power and it got hot again very quickly. If I ran it along very slowly at low engine speed it did not boil over, but the bike would hardly move. I could still rev it out to the power band, but as soon as it started to make power in the power band it got very hot and would start to seize up again.

I put more two stroke oil in the gasoline and I rode it around for a while shutting it off periodically at the top of a hill to let it cool all the way off. The cranking compression came back up a small bit, but it still would not kick start. After riding it around for a while like this with about a 20:1 mix ratio the engine freed up and started to work as it had before. Seemingly all of a sudden the extra drag went away, and the power came back up in the power band. It seemed like I had managed to repair the seized up engine just by riding it. The problem though was that the cranking compression did not come back and it absolutely would not start from cold with the kick starter. Sometimes if I shut it off hot it would start right back up with the kick start lever, but if it sat for more than a few seconds it would not restart.

The Rebuild

Not having any interest at all in a bike that would not kick start I tore the motor apart. I thought perhaps I could get it going again with just a new ring. When I saw the piston though my fantasies of just a re-ring job melted. The piston had smeared all over the cylinder walls on both sides of the exhaust port and there was tons of wear to both the piston and cylinder on the intake side also.

It was an aftermarket piston in the motor. I thought it looked like a Wiseco piston, and it was 0.5mm over bore at 70mm. The piston was marked 020, indicating 0.5mm oversize. The rest of the top end looked new. The piston pin was new and showed essentially no wear, and the areas of the cylinder that had not been damaged by the overheating and seizing were also like new. I thought the problem had probably been that the aftermarket piston had been installed with insufficient skirt clearance. When I measured the undamaged parts of the cylinder it was seeming like it had been only about 0.002" of skirt clearance. A very skimpy amount for such a high revving high output two stroke.

While I had the motor apart I also had a chance to look over the porting. The guy I had bought the bike off of said that the cylinder had been ported by a local race shop, and it was signed with an engraving tool near the part number. The reality though was that it was that the porting was bone stock. The only modification had been to slightly increase the chamfer size at the top of the exhaust port, a difference of less than 1/32" in the port height. An essentially totally insignificant difference.

To verify that the cylinder was stock I looked at photographs of used '86 Husqvarna 250 cylinders advertised for sale on eBay, and I also bought one of them for a modest $40. They all appeared to have the exact same port shapes and port locations. When I got the other used '86 Husqvarna 250 cylinder and measured it the ports were indeed exactly the same. The signed, and supposedly ported, cylinder just had that chamfer at the top of the exhaust port opened up to about a 1mm 45 degree chamfer.

Interestingly the port heights on the 1986 Husqvarna 250 cylinders were only very slightly higher (as a percentage of stroke length) than on the '87 Husqvarna 430 cylinder. I had expected a much bigger difference in porting what with the 430 hitting hard way down at around 4,500 or 4,800RPM and the 250 having nothing bellow 7,500RPM. The difference really is largely in the pipe.

The used cylinder I bought was rusty, but in fairly good condition. There was some wear around the exhaust port, but not so much that it would not run. The cylinder was obviously usable, but it was bored oversize to 69.75mm. The 0.25mm overbore had been a standard size on the 1986 Husqvarna 250 two stroke, but new 69.75mm pistons were no longer available. The only sizes available were the stock 69.5mm bore, 0.5mm over at 70mm, 1mm over at 70.5mm, 2mm over at 71.5mm and 3mm over at 72.5mm. I did find an unused 69.75mm stock Mahle piston advertised for sale in England, but the price was high and the shipping charge was also very high. It was just way too much to spend putting a piston in a cylinder that was only marginally serviceable and far from new. Instead I ordered a 70.5mm Wiseco piston. The Wiseco pistons had been discontinued, but I found one in stock for just $75 delivered. Much lower than the approximately $250 for the 69.75mm Mahle piston. When I got the 70.5mm Wiseco piston it was in fact nearly identical to the 70mm Wiseco piston that came out of the motor.

To use the 70.5mm piston I reamed out the 70mm cylinder with my Size 0 Japanese hand reamer. It was pretty easy to cut the cylinder out 0.5mm, but the job did not turn out all that well. The ports in the '86 Husqvarna 250 cylinder are extensive. The cylinder is just all holes and very little cylinder, which made the reaming turn out crooked. I ended up with much more tapper than I would have liked, and I had to keep reaming out to 0.008" of skirt clearance at the top of the cylinder in order to get the 0.005" of skirt clearance that I needed at the bottom of the cylinder. After having the piston seize up like that I wanted to make sure there was plenty of skirt clearance.

Once bored out to the 70.5mm size the chamfer at the top of the exhaust port was considerably down in size, and the porting looked essentially identical to the other used cylinder I had. It was not really a ported cylinder to begin with, and it was even less different after having been bored another 0.5mm oversize. On the intake side some of the small chamfers had disappeared entirely, so I worked in the edges a bit to make sure that the ring would slide easily and not catch anywhere.

Something that I did not like about the Wiseco pistons was that they had two compression rings. Cylinder port two stroke dirt bikes nearly always have just one compression ring, and this is as it should be. The second ring does not do much good for improving sealing, and it wears the edges of the exhaust port out twice as quickly. I decided to just leave one of the compression rings out, but I had a bit of a hard time deciding which one to leave out.

Since it was the top ring that had gotten so stuck in the seized piston I decided to install only the lower compression ring. That turned out to be a mistake though. The engine would not kick start when cold. It started easily when I rolled it down a hill, and it also restarted easily with the kick starter once it was warmed up, but it would not kick start cold. The engine did start much more easily than it had with the seized up piston, and it ran somewhat differently also.

At first the cranking compression was actually alarmingly low, and I had to tow the bike to get it started. After just a few minutes of run time though the new ring seated in on the much less than perfect reamed out cylinder and the cranking compression came way up. Very quickly the cranking compression was higher than it had been on the last top end. The cold the cranking compression seemed sort of weak, but once the engine was warmed up the cranking compression was huge. It started easily when warm, even after sitting for 15 minutes or so, but it would not kick start from cold. Even after sitting for a half hour or so it just would not kick start and had to be rolled to get it started.

Interestingly the low end power was even weaker with the top ring left out, but the midrange was considerably improved. The difference with the midrange was however due to a difference in the gasoline and possibly a slight increase in the compression ratio with the larger bore diameter. I had used the same base gasket and head gasket, so the compression ratio should have come out very close to the same. The 70.5mm piston even had the required very slightly lower compression height to compensate for the increased bore diameter. With such a high compression ratio though I was still not entirely convinced that some small compression ratio change might have taken place.

The difference really was in the gasoline, as there had been a few times on the last top end when I had gotten some increased midrange also. What caused the midrange torque to increase was much lower pressure gasoline that resulted in crisper operation. The low end was always extremely weak, but sometimes there was a bit of midrange just below the power band. Usually the midrange was just as non-existent as always when the engine was at normal operating temperature, but if I got it well heated up on some big pulls a little bit of midrange torque would start to creep in bellow the power band. What I always noticed was that when the gasoline was allowing that little bit of midrange torque to creep in the engine was then extremely crisp up in the power band. Excessively crisp really, and this no doubt has a large amount to do with why the engine seized up. On the day that it seized up there was a little bit of midrange, but mostly only once the engine was well heated up from heavy acceleration.

Something else I had noticed was that the maximum power output up at the top of the power band was low on the lower pressure gasoline that was allowing that little bit of midrange to creep in. The engine had made it's best power when it was running just crisply enough to reliably get through the gears up in the power band under sustained heavy acceleration.

With the rebuilt top end there was even more midrange, and there was even more excessive crispness also. Top end power was not seeming like it was all that low, but there was also a piston weight difference. I had cut 22g off of the new Wiseco piston to bring it down to a 212g weight. In addition to that modest weight savings I had switched from the stock Husqvarna pin to a Wiseco S508 pin and B1022 wrist pin bearing for another 24g weight reduction. The total weight reduction was 47g, a massive 15% reduction in total piston and pin weight. Way up at 9,000RPM that weight savings translates into quite a bit more power on the same gasoline. The engine was also noticeably smoother with the lighter piston, but the 250 had been a fairly smooth running engine to begin with. It had been under balanced like most 1980's and 1990's Husqvarna motors, but the small displacement and short stroke had resulted in fairly smooth operation none the less.

The increased midrange power was interesting, but I still did not like the '86 250 two stroke. It was only a little bit of midrange torque that showed up, and the bike still needed to be kept zinging in the power band all the time to scoot along. On the trails the midrange was not really enough, and the engine still needed to be kept in the power band all the time to get up the steep hills. And then it seized up on me again.

I had been climbing up a series of moderate hills gaining elevation up into the mountains, and at one point the engine just slowed way down. I kicked down to first gear and turned the bike back down the mountain. By ridding along slowly for a few minutes the engine freed up again rather easily. I stopped to check the coolant, and this time it was low. I added water and limped back with no difficulty. The motor seemed normal once it had cooled down. It was spinning over freely, and the power was still just as big up in the power band.

I decided to disassemble the engine again. Both because I wanted to see what damage I had done by that little bit of piston seizing, and also because I wanted the engine to kick start when cold. When I got the top end off the ring was slightly stuck in the lower groove, and the piston and cylinder were a bit damaged from the overheating and seizing. The damage was however very slight compared to previous massive smearing of aluminum on the cylinder wall and large amount of wear. This time I the other new ring in the top groove and cleaned up the piston and cylinder with 400 grit silicon carbide paper. The top end still seemed just about as good as it had when I put the new piston in. And it worked well also.

Again the cranking compression was low when I first put the top end back together, but after an even shorter run in period the new ring was sealing well. The cranking compression was noticeably higher than it had been with the ring in the lower groove, and the engine once again kick started easily as it had when I first got the bike. I was generally very pleased with the rebuilt engine, and I was even thinking that I might actually ride the 250 more. I was kind of liking the lower maximum power output just because the rear tire was lasting longer. The 400 two stroke just absolutely annihilates rear tires, but the somewhat lower power of the 250 two stroke is a whole lot easier on the tires. The 250 two stroke was slow, but I was getting into the challenge of keeping it in the power band. In many ways it was a lot like the '87 Honda CR125 I had ridden back in the mid 1990's.

The top end only power band was however not something that continued to work. I had some more problems with the engine loosing coolant. At first I thought it might be a poorly sealing head gasket, but when I rode the bike slowly for 30 miles using the power band for only very for short periods of time the coolant did not go down at all. Then it seized up on me again, this time catastrophically. I was climbing a rather long series of moderately steep trails up into the mountains, and when I closed the throttle at the top of the last climb up at 5,000 feet of elevation the motor seized up hard. This time it was out of water and very overheated. Once again I got the motor re-started by rolling it down the hill, but it would not free up and continued to run poorly.

It seems that increasing the skirt clearance had allowed the engine to get even hotter when seizing up, so the coolant boiled over. With the skirt clearance set very tight at 0.002" the piston had seized up without the engine ever heating up to the point where the coolant boiled over.

This time I did disassemble the bike for parts, and now I have a 1986 Husqvarna 400 Enduro with an aftermarket muffler and lots of shiny new looking parts. It turned out that my '86 400 had aftermarket Boyesen reeds in it, so I put the reed block with the stock Husqvarna reeds off of the 250 motor on the 400 motor, and I also used the plastic reed block insert that only came with the 250 motor. I had thought that using the 250 reed block insert on the '87 430 probably blustered midrange torque, and putting one on the '86 400 motor seemed to support this idea. It really seems like the '86 400 makes broader midrange torque than it did before, but this may just be the difference between the Boyesen reeds and the Husqvarna reeds. The Husqvarna reeds look like a much better flowing design, where the Boyesen reeds are stiff and look excessively resistant to opening at small flow rates.

What also seems to have come over from the defunct 250 motor to the 400 motor is the lower pressure gasoline and higher levels of crispness. When I took the newly shinned up '86 400 WR on a long ride up to 6,500 feet of elevation I had power all the way up, something the '86 400 WR never did for me before. In the past the power used to go away above about 4,000 feet of elevation, and sometimes it would hardly get into the power band at lower elevations either.

The 400 has sometimes been seeming extremely excessively crisp, but it does not seize up. With a broader and more powerful midrange I can run the engine at whatever engine speed happens to be working well for the current gasoline. When it is extremely low pressure gasoline I can hide from the crispness by keeping the engine speed a bit lower at low elevations. I have also changed the needle clip position from the 3rd groove to the 2nd groove, which helps a lot with running over a wider range of altitudes. The 3rd groove needle clip position was one leaner than the stock 4th groove needle clip position, but even with the needle clip in the 3rd groove the mixture was way on the rich side at all medium throttle openings. With the clip in the 2nd groove on the 6DH3 needle the mixture control appears much better, with a more even mixture across all throttle openings.

The big aftermarket muffler is still loud on the 400 motor, possibly slightly louder even than the very loud small stock 1986 muffler. When I saw that big aftermarket muffler I was hoping for a quiet exhaust note like the 1987 430 WR, but that has not been the case on either the 250 or 400 motors. It is just a loud muffler.

I still hardly ever seem to ride the two strokes, but they do continue to be an interesting novelty. One thing that I have noticed is that the '87 430 WR seems unstoppable even on very weak gasoline. The gasoline has often been seeming very weak and watered down, resulting in poor performance and lower power output from all of the bikes. The '87 430 WR though just keeps charging hard, with terrifying amounts of torque building dramatically from 4,800 to 6,000RPM and not letting up much until 7,700 or 8,000RPM. Scary fast to be sure, with the emphasis on scary thanks to the flexy conventional Husqvarna forks.

What had really convinced me to give up on the 250 two stroke was the poor gas mileage. Just like my old '87 Honda CR125 the '86 Husqvarna 250 two stroke got unbelievably poor gas mileage. The '86 250 WR actually used more gasoline on the same loop than the '86 400 WR. I checked the gas mileage several times by riding a dirt road loop of known mileage, and the best I could get out of the '86 400 WR was 27mpg. When I ran the same loop on the '86 400 WR I got 31mpg! Compared to 45 to 50mpg from the 610 four stroke on the same loop both of the two strokes did very poorly, but 31mpg from the 400 is much better than 27mpg from the 250.

The fact that I had such bad experiences with the '86 Husqvarna 250 two stroke does not mean that it can't be made to work though. One obvious solution would be to lower the compression ratio and increase the spark advance correspondingly. The same Motoplate Mini 6 CDI ignition system works very well with the 17 and 18 degree BTDC spark timing that the '86 400 WR and '87 430 WR run. In fact spark timing all the way up to about 20 degrees BTDC could probably be used with this basic CDI ignition system. Anything more than about 20 degrees BTDC though and kicking back while starting can easily be a huge problem. There is in fact a world of difference between 9 degrees BTDC and 17 degrees BTDC. Going up to around 15 to 19 degree BTDC spark timing with a lower compression ratio not only delivers much stronger operation in full flame front travel mode, but crispness at the bottom of the power band and across the midrange engine speeds in late compression ignition also increases with more spark advance. In late compression ignition mode more spark advance is like slower flame front travel speed gasoline or a larger bore diameter. This sounds backwards, but it really is so. The limits for how much spark advance can be used are twofold. First and most importantly for the old Husqvarna two strokes is that the Motoplate Mini 6 ignition system has just a fixed spark timing value, so spark timing earlier than about 20 degrees BTDC is not normally possible. Kick starting can be impossible with spark timing earlier than about 20 degrees BTDC. The second severe limitation for spark advance is that spark timing earlier than about 23 or 25 degrees BTDC does not allow the latest possible time of late compression ignition, which means excess harshness down at less than about 5,000 or 6,000RPM depending on the fuel used and the stroke length of the engine.

Clearly the '86 Husqvarna 250 two stroke could use considerably earlier spark timing than the stock 9 degree BTDC spark timing. The trade off is that earlier fixed spark timing tends to result in a narrower power band with less snap up at the highest engine speeds. Just how much of a problem this might be depends on the fuel used. The little 250cc engine does not have all that big of a bore, so an inability to rev out and make power at the top of the power band is not as likely as on a larger bore engine. Add to that the fact that two strokes heat up readily under heavy loads, and this might not seem like much of a real problem. Still though, there certainly are slower flame front travel speed gasoline types that could easily result in disappointing top end yank even on the 70mm bore diameter with fixed spark timing.

Another solution would of course be a race gas and slow flame front travel speed regular gasoline mixture. The proportions of the mixture can be varied to tune the power band to the specific requirements of the rider. More slow flame front travel speed regular and less race gas results in a more difficult to tune engine, but absolutely more midrange grunt at the bottom of the power band. More race gas and less slow flame front travel speed regular results in more bottom end power way down bellow 4,000RPM, cleaner and more efficient operation, more power at the top of the power band and of course somewhat more overrev.

The worst gasoline for the '86 Husqvarna 250 two stroke is what kept showing up for me. Extremely weak watered down gasoline with lots of ethanol for a rather fast flame front travel speed, but a low energy density and a very low temperature of combustion potential. Normal premium gasoline would be somewhere in the middle, and would not necessarily be all that great for the '86 Husqvarna 250 two stroke either. From what I have seen the '86 Husqvarna 250 two stroke certainly will run on normal premium gasoline, but some mixture of race gas and slow flame front travel speed regular would of course deliver much better performance and a lot more power from the small bore two stroke. On high pressure race gas the '86 250 two stroke could also tolerate 15 or 18 degree BTDC spark timing with the stock 15:1 compression ratio, and that is the setup that would probably be preferred by most race teams. The 15:1 compression ratio on the cylinder port two stroke is after all probably equivalent to about a 12.5:1 or 13:1 compression ratio in a four stroke engine. It is a very high compression ratio to be sure, but there certainly are high pressure types of race gas that do great with high compression ratios like that. On most types of normal pump gas it is an astronomically high compression ratio that is difficult to impossible to get to work. It is easy to see why the dirt bike industry rapidly transitioned to power valve type two strokes and more sophisticated advancing type CDI ignition systems in the mid to late 1980's.

Perhaps the last word on the '86 Husqvarna 250 two stroke though is that it would seem possible for a differently shaped pipe to deliver considerably more midrange torque down to around 5,000 and 6,000RPM at any spark timing value. The port timing is really extremely similar to the '87 430 WR, which hits very hard down a whole lot lower. The 15% longer stroke of the 430 does not alone come anywhere near explaining the 4,500RPM versus 7,500RPM difference in onset of power. The 15% longer stroke of the 430 might mean that the 250 would not be able to make torque bellow about 5,200RPM, although even that theoretical limit is not hard and fast. A 2.5 inch stroke length gasoline engine can in fact make torque down to somewhere around 3,500RPM at the latest possible time of late compression ignition, well below where a cylinder port two stroke does best.

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