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The Swedish Two Stroke Husqvarna

I had always had a fanciful interest in the old Swedish two stroke Husqvarna race bikes, and it is often said that the two stroke Husqvarnas were the greatest dirt bikes ever built. When we were able to trade my father's little used 2002 Yamaha TTR225 for a 1986 Husqvarna 400 Enduro two stroke in fairly good condition we jumped on the opportunity.

Swedish History
One Bike Many Classes
Compression Ratios
The Last of the Breed
Which Fork is really Upside Down
Two Stroke Starting
Staying on the Pipe

Swedish History

The Husqvarna company has been around for quite a long time, and the company has a long history of being heavily mixed up with the Swedish government. Going back hundreds of years Husqvarna is said to have gotten it's start as an arms contractor for the Swedish government. Husqvarna motorcycle production began in 1903 with rather mundane single cylinder overhead valve dirt bikes similar to what other manufacturers turned out in the following decade. Husqvarna's best known early racing accomplishments were in the 1920's and 1930's with a 500cc v-twin air cooled side valve road race bike that was produced with few changes for many years.

The Husqvarna company again became embroiled with federal government surrounding the partial nationalization of heavy industry in Sweden following the Second World War. Husqvarna as well as other Swedish motorcycle manufacturers dabbled in air cooled four stroke motorcross race bikes through the 1950's, but it was the rise of the two stroke that brought Husqvarna to the forefront of motorcross racing. Throughout the 1960's Husqvarna two strokes were the world standard in off road motorcycle performance. With continuous improvements in design and manufacture Husqvarna two strokes continued to be a major force in motorcross racing well into the 1970's.

By the mid 1970's the Husqvarna motorcycle division was a very small operation, just barely hanging onto some racing success, but the innovation and development continued at blistering pace. Around 1980 Husqvarna was an early innovator of the mono shock rear suspension and water cooling. The 1981 Husqvarna model year two stroke race bikes from Husqvarna came with a single Ohlins rear shock with linkage and water cooled engines.

The really big innovation was the four stroke race bike introduced late in 1983. This first modern four stroke racing dirt bike was air cooled and shared the transmission and clutch with the water cooled two strokes of the time. With a few upgrades over the years this original 1983 four stroke is substantially the same engine that continued to be produced as the Italian 610 and later 570 which were produced until about 2004.

As significant as the introduction of the four stroke Husqvarna ultimately proved to be it was just a freak show at the time. The focus was still firmly planted in two stroke race bikes both at Husqvarna and in the dirt bike industry in general.

In the 1980's Husqvarna had some success in Enduro racing with open class two strokes, and the bikes were wildly popular with dirt bike riders in general because of their overall high level of functionality and refinement. Extremely high purchase prices compared to the more narrowly focused Japanese motorcross bikes did however keep sales numbers rather low.

One Bike Many Classes

Arguably the most striking thing about the 1980's Husqvarna dirt bikes is that they are all the same. That is the 125cc, 250cc, 400cc and 500cc two strokes all used the same frame, the same engine cases, the same transmission and the same 134mm long connecting rod. This made for a very small and light open class race bike, but it also made for a rather huge and heavy 125 class bike. The one difference was that more primary reduction was used on the smaller displacement two strokes. By spinning the transmission more slowly light load efficiency with the smaller displacement engines was improved. The same transmission that was used with the 500cc engine was however still rather heavy and bulky compared to some of the other 125 class transmissions on the market at the time. The amount of additional primary reduction chosen also was not quite what it should have been. The 500cc two strokes, 400cc two strokes and the 510 four stroke used 2.3:1 primary gears, where the 250cc two stroke had only just a bit more primary reduction with 2.63:1 primary gears. The 125cc two stroke had much more substantial 3.89:1 primary gears, and it would seem that the 250cc two stroke also should have used this much more substantial level of primary reduction.

Other shared parts, such as the same 38mm Mikuni round slide carburetor, being used across the entire model range made the 125cc bikes look quite ridiculous. What did bolster the performance of the 125cc two strokes though was a very high compression ratio. The compression ratio of the 125cc engines varied from year to year, but was usually up at 15:1. The 250cc Husqvarna two strokes typically used about a 12:1 compression ratio while the big 400cc and 500cc open class bikes had compression ratios that ranged from 9.0:1 up to 12.6:1 depending on year and model.

Compression Ratios

The compression ratio of the 125cc Husqvarna two strokes varied from year to year, but was usually up at 15:1. The 250cc two strokes typically used about a 12:1 compression ratio while the big 400cc and 500cc open class bikes had compression ratios that ranged from 9.0:1 up to 12.6:1 depending on year and model.

The first important point about two stroke compression ratios is that they tend not to be exactly equivalent to four stroke compression ratios. Cylinder port two strokes typically do not attain quite as high peak compression pressures as four strokes with the same compression ratio. Right at the narrow torque peak of a two stroke the cylinder filling can be pretty close to as high as is typically attained on a four stroke, but there are some substantial differences. The large exhaust port quite high on the cylinder wall always lets some of the intake charge get pushed out as the piston rises. Within the "power band" of the engine this escape of the intake charge is smaller, but it still means that cylinder filling will not be quite as high as for a four stroke.

In general it might be said that two stroke dirt bikes need about a 15% higher compression ratio than a four stroke dirt bike running on the same fuel. This would mean that the 15:1 Husqvarna 125cc two strokes were roughly equivalent to 12.5:1 or 13:1 four stroke dirt bikes. When the Husqvarna 510 four stroke was introduced in 1983 it was a 9.5:1 engine, which might be considered roughly equivalent to the 11:1 or 11.5:1 compression ratios on the 1981 through 1983 open class two stroke Husqvarnas.

There has long been a myth that smaller displacement per cylinder can use a higher compression ratio, and no doubt the higher compression ratios on the 125cc Husqvarnas was a result of this rather widespread misconception. The reality though is that smaller engines can rev higher, and maximum compression ratios tend to be extremely universally applicable to all gasoline engines. All of the two stroke Husqvarnas probably should have had roughly the same compression ratio. A 9:1 compression ratio on a 500cc two stroke and a 15:1 compression ratio on a 125cc two stroke just means that the big engine is not going to run well on the same fuel. Eventually the Husqvarna open class two strokes got much higher 12:1 and 12.6:1 compression ratios and were able to make a whole lot more power while also using less fuel.

The Last of the Breed

Apparently the Husqvarna Motorcycle Division was sold to Cagiva sometime around or shortly before 1987. The 1986 models were the last fully Swedish Husqvarnas even though the 1987, 1988 and 1989 Husqvarnas were nearly identical. It was not until the 1991 model year that saw the introduction of the 610 that the Italians made substantial changes to Husqvarna dirt bikes. The 1991 WMX 610 is a very well thought out bike, so it is likely that the changes had been in the works since shortly after the 1987 acquisition of the brand.

The main change for 1991 was of course the big bore kit which raised the displacement of the four stroke to 577cc on the same 3.01" stroke bottom end and transmission. The other big change was the White Power rear shock which replaced the Ohlins rear shock used before. Along with the new shock and shock linkage the bike got a rear disk brake, a new smaller gas tank, a new seat, a new rear sub frame and all new plastic. The frame stayed pretty much the same aside from new mounting locations for the new sub frame and new shock linkage. These changes were clearly the work of Cagiva in Italy. What is not so easy to sort out are the changes that were implemented for the 1987 model year.

The big change for 1987 was of course water cooling for the 510 four stroke. That is a really big upgrade because high performance air cooled four strokes have such a propensity for melting pistons. It seems likely that the new water cooled cylinder and cylinder head castings were developed under Swedish ownership, but this is not entirely clear. Aside from the new cylinder and cylinder head castings the 510 motor remained essentially identical to the earlier air cooled 510 motors. The only other changes that came with the water cooled upgrade were larger 35mm intake valves to replace the old 33mm intake valves and a bump in the compression ratio from 9.5:1 to 10.0:1. The other big upgrade for 1987 was the use of the White Power 40mm upside down forks on the four stroke and some two stroke motorcross models. The switch to an already commercially available fork might have been done at the last minute under Italian ownership, but I don't have any specific information about just who made the decision to upgrade to the White Power forks.

Which Fork is really Upside Down

The first thing that is noticed when ridding the 1986 Husqvarna 400 Enduro is that the front end is extremely imprecise with a vague steering feel. The 40mm conventional forks with 11.2 inches of travel just twist and flex all over the place. Even a pee wee 50 rider would think the steering and handling left something to be desired. The Husqvarna forks do work very well, the valving is extremely competent giving both a comfortable and plush ride while also allowing good control at high speeds over rough terrain. Those old Husqvarna forks were widely lauded as the best conventional forks ever built, and they are in fact extremely competent. Clamping the smaller diameter steel inner legs in the triple clamps would however tend to require a fork brace. Fork braces were abandoned many decades previously simply because low fenders and fork braces were a huge disadvantage in mud races. In muddy conditions a high front fender was absolutely necessary, so the fork brace was left by the wayside. It took a few decades but eventually the forks got flipped over and mounted correctly in the triple clamps. The reason that this is such a big deal is that the outer fork legs are just a whole lot stiffer. Not only are they made of much stiffer aluminum, but they are a much larger diameter than the inner legs. The net result is that the 40mm "upside-down" White Power forks are leaps and bounds more rigid and precise than the old 40mm "conventional" Husqvarna forks.

It is fairly easy to get used to the flexy feel of the old "wrong side up" Husqvarna forks, it just requires going slower and being willing to let the bike go where it wants to go. Sort of like the difference between dragging a dog by its leash or following the dog as it takes you on a walk. If you have to get a dog to follow a particular path then the leash is necessary. If on the other hand it does not matter which direction the walk takes you then simply following the dog is much more enjoyable. The same is true with the Husqvarnas. Letting the 1986 Husqvarna 400 Enduro bounce where it will along wide open trails is loads of fun. If however there are trees or other obstacles that have to be avoided on a rough descent then the flexy conventional forks are downright scary and somewhat dangerous.

Two Stroke Starting

The 1986 Husqvarna 400 Enduro actually starts extremely easily for two separate reasons. One is that the 400cc two stroke is very easy to kick over compared to a big 577cc four stroke, it just does not require nearly as big a kick to get it going. The other thing about the 1986 Husqvarna 400 Enduro is that the Spanish built Motoplat Mini 6 CDI ignition system provides a very strong spark that prevents spark plug fouling problems. Even after the needle clip fell off and the engine flooded horribly restarting was no problem without even taking the spark plug out. I just pulled the top off the carburetor, put the needle and needle clip back in position and the bike fired up. It just required a few kicks with 1/4 throttle to clear the combustion chamber and the engine fired up without even having to remove the spark plug. As long as there is gasoline in the tank the 1986 Husqvarna 400 Enduro fires up extremely easily on the first kick every time, probably the easiest starting two stroke I have ever ridden.

Staying on the Pipe

The big challenge with the 400 Enduro these days is getting gasoline that will work in it. As good as the Motoplat ignition system is for easy starting it offers limited adjustability for dealing with changing fuel. The 1986 Husqvarna 400 Enduro has a rather high 12.1:1 stock compression ratio, which I would estimate to be about equivalent to a 10.5:1 four stroke. This would seem to be high enough, but the spark timing is fixed at somewhere close to 20 degrees BTDC. The Motoplat ignition system looks like it is easily adjustable, the hold down screws for the stator are accessible without even removing the flywheel. When the stator hold down screws are loosened and the stator is turned though the spark timing does not appear to actually change much if any. Just how the stator can be turned without actually changing the spark timing is somewhat of a mystery, but that is what is observed.

Two stroke dirt bikes always tend to have a rather narrow power band, and with no power valve the 1986 Husqvarna 400 Enduro is at an even greater handicap compared to modern two strokes. With fuel that does not have quite a fast enough flame front travel speed or fuel that is requiring too high of a temperature and pressure to light off on late compression ignition the power band on the 1986 Husqvarna 400 Enduro simply disappears. The big two stroke starts easily, idles as well as just about any two stroke and delivers nice usable smooth cruising power from 2,500 to 4,500RPM without loading up or stalling. Idling along in full flame front travel mode though the big two stroke just does not make much power. In order to make power the engine has to be revved up to where the port timing and expansion chamber length work together to deliver high cylinder filling. Within the narrow power band of approximately 5,500 to 7,000RPM the 396cc two stroke does make some substantial power, probably in the neighborhood of 60hp. Actually getting the engine to make that power is however not always the easiest thing to accomplish. When the engine is running well on sufficiently fast flame front travel speed fuel that is lighting off easily enough on late compression ignition the power is available simply by going through the gears. With slightly slower flame front travel speed gasoline though the big two stroke can be extremely reluctant to get going and make power. Slower flame front travel speed fuel also causes the power to end sooner, making for an even narrower power band and reducing maximum power output.

As with most two strokes slipping the clutch helps for several reasons. The big thing about slipping the clutch on a two stroke is that the engine speed can be maintained up near the top of the power band as the bike accelerates. Clutch slipping is the standard solution for dealing with extremely narrow power bands on two strokes, as long as the engine will make big power at some engine speed clutch slipping can keep the engine right at that engine speed where it makes power.

Then there is also the somewhat mysterious tendency for clutch slipping to get the engine to light off on late compression ignition when the fuel otherwise seems to be of too slow a flame front travel speed or is requiring too high of pressure. It is sometimes the case that a two stroke will appear to be severely lacking in power at all engine speeds, but slipping the clutch will get it to pop off on late compression ignition and make power. What is going on is that for some reason the engine will only get into late compression ignition mode under medium loads, but then once in late compression ignition mode more fuel can be fed in and more power can be produced.

This phenomenon of an engine only entering late compression ignition under lighter loads is sometimes also seen on four stroke dirt bikes. The easiest explanation would be a load dependent timing mechanism. Later model dirt bikes from after about the year 2003 often came with throttle position sensor equipped ignition systems capable of delivering load dependant timing adjustments. Points ignition systems where the points ride on top of the end of the crankshaft would also have a small load dependent timing effect.

Explaining how a two stroke with a traditional CDI ignition system could exhibit this phenomenon of lighting off on late compression ignition more easily under medium loads is a bit more difficult. One thing going on is the air/fuel mixture delivered by the carburetor. Carburetors on two strokes are usually rather rich on the pilot jet within the first 1/5th of the throttle opening. This rich mixture allows the engine to light off on late compression ignition more easily, but once the throttle is opened a bit more on the needle the mixture leans out a bit. When the throttle is opened further towards half and three quarters of the throttle opening the mixture again richens up on the main jet for maximum power output up at maximum engine speed. The lean spot in the middle delivers better fuel economy under normal ridding, and also helps with smooth power delivery over a range of engine speeds and engine loads. If however the fuel has too slow of a flame front travel speed or is requiring too high of a pressure to light off on late compression ignition then the lean spot in the middle becomes a dead spot and the engine appears to severely lack power.

The way that clutch slipping helps deal with this dead spot on the wrong fuel is that the engine can be brought up into the power band on the rich mixture of the pilot jet for a second before the throttle is opened further to make more power. Sometimes two stokes that are running fairly well will stay in full flame front travel mode until the clutch is momentarily slipped and then the engine will take off and make power over a reasonably wide range of engine speeds in late compression ignition. When this is working well it only takes just the barest of initial clutch slipping to get the engine running in late compression ignition mode, and then no further clutch slipping is required for the duration of acceleration even when going up through the gears. Only after the engine has cooled down again from coasting or slow cruising is another blip of the throttle required to get the engine going again in late compression ignition mode.

This has long been described as "keeping it on the pipe", because the size and shape of the expansion chamber on a cylinder port two stroke has so much to do with how well the engine is capable of running. Only over a certain range of engine speeds does the tapered expansion chamber provide powerful exhaust gas scavenging to fill the combustion chamber with air and fuel. At lower or higher engine speeds the exhaust gas scavenging works less powerfully, or does not work at all, and the cylinder does not fully fill with the intake charge. the placement of the intake and exhaust ports as well as the diameter and length of the intake tract also have a large deal of influence over what engine speeds will work on a cylinder port two stroke, and the reality is that what was traditionally described as "on the pipe" is more accurately on the pipe, on the ports and on the intake stack.

In any case a two stroke dirt bike only runs over some rather narrow range of engine speeds, and if the combustion properties of the fuel, the compression ratio of the engine and spark timing are not mutually compatible then the engine will either be extremely loud, harsh and inefficient or it won't make power at all. Of course the fastest flame front travel speed fuel available does wonders for any two stroke as it stretches the width of the powerband as wide as it possibly can be. If the flame front travel speed of the fuel is fast enough then the compression ratio can be a bit on the low side so that it takes a deliberate yank of the clutch to get the engine going in late compression ignition mode. For a big open class two stroke it can be very useful to have a broad lower engine speed range in full flame front travel mode for best possible power modulation and smooth delivery for slippery or challenging trail conditions. At the same time though the engine has to light off on late compression ignition easily enough that big power is always available with a touch of the clutch.

The commonly mentioned two stroke disadvantages of high fuel consumption and short clutch life certainly are of concern, but it should also be kept in mind that two strokes also tend to be harder on rear tires and harder on trails than four strokes. Yanking the clutch to hit the power band also tends to break the rear tire loose, if this clutch and launch power delivery is used out of every single turn rear tires get chewed off fast and ruts develop quickly in trail surfaces.

Fuel and Oil

Fuel consumption on the 1986 Husqvarna 400 Enduro is actually quite low by two stroke standards. Idling along on trails with only sparing use of big power in the power band the bike burned just under a gallon an hour, about the same as for many 250cc two strokes. This is not exactly good fuel consumption, but it is not quite as thirsty as two strokes often are seen to be either. Considering that the 610 four stroke is well capable of 0.55GPH on a similar ride while ridding faster with much more heavy acceleration the big two stroke looks pretty inefficient. Compared to a particularly inefficient 125cc or 250cc two stroke that would use just as much fuel though the 400 Enduro does pretty good.

When ridding fast using lots of power up in the powerband the 3.2 gallons of fuel that the 1986 Husqvarna 400 Enduro carries does a disappearing act that would make Houdini's head spin. The 400 Enduro makes lots of power, and using lots of power does suck down some fuel. That is however not the whole story. The 610 usually appears to go farther on a gallon of gas the harder and faster it is ridden (on dirt), big power up at 5,000 to 7,000RPM is delivered quite efficiently by the four stroke. The two stroke on the other hand is not efficient under full power for a couple of reasons On is that it blows quite a lot of unburned fuel out the exhaust. Lots of clutch slipping is also inefficient, wasting power to heat up the gear oil. The other difference of course is that the 610 four stroke makes several times more power at 2,500 to 4,000RPM than does the 400 two stroke. With the same transmission and clutch the transmission losses are much lower for the engine that is making more power at the same engine speed.

I have always mixed two stroke oil at about 40:1, and that is where I have been running the 1986 Husqvarna 400 Enduro. On my 125 two stroke I usually used about 40:1 for trail ridding, but out in the sand dunes I went up to 30:1. Since the big 400cc engine does not need to rev as high or stay at high engine speed for as long 40:1 will probably always be sufficient. The Husqvarna recommendation back in 1986 was to run a 25:1 mix, but that was with somewhat different two stroke oil that what has been available since the 1990's. My favorite two stroke oil has always been Yamalube R since it is rather cheap and does not stink as bad as other two stroke oils. What the Yamalube R does however do is drip lots of black crud out of the exhaust if the engine is not running crisply enough.

What I have also done sometimes over the years is put 40:1 premix in four stroke engines just to see what the smoke would be like. When I ran 40:1 Yamalube R in the 11.3:1 Husqvarna 610 motor it did not smoke as long as I was using substantial power and keeping the engine in late compression ignition mode. Even at idle there wasn’t much in the way of smoke or smell. The 40:1 Yamalube R mix smokes more in the 400 Enduro, but the amount of smoke is moderate for a two stroke. Once "on the pipe" the smoke goes away (or at least is blown away by so much airflow that it is not noticeable). Other supposedly "premium" two stroke oils smoke less at idle but smell rather strange when the engine is running in late compression ignition mode. Most commonly available two stroke oils for chainsaws and string trimmers also sometimes smoke a bit at idle and at low power output, but do not smell strongly when the engine is running in late compression ignition mode.

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