There were a lot of things to like about the 1999 Husqvarna TE 410, so it was disappointing to not be able to ride it. At first I thought I could just take it out now and again for some casual rides on smooth trails despite the horribly harsh 1999 suspension, but this wasn't working out at all. Just going down a short one mile stretch of rough and rocky dirt road was a brutal beating that I soon grew to despise. The fact that the Ducati Energia CDI ignition equipped motor often had difficulties with idling and kick starting also made taking the 1999 Husqvarna TE 410 out annoyingly difficult. When I saw a set of 1991 Husqvarna WMX 250 forks advertised for $200 delivered I saw an opportunity to fix up the 1999 Husqvarna TE 410 and get it going as a usable dirt bike.
1991 WMX 250 Suspension Speculating
1991 Fork Installation
Riding the 1999 Husqvarna WMX 410
1991 Husqvarna WMX 610 Comparison
Race Gas After Regular, Sort of
The really big problem with the 1999 Husqvarna TE 410 was the suspension valving. It just didn't work worth a darn. I could have dealt with it being less competent over big bumps and at high speed than the 1991 Husqvarna WMX 610 suspension I am used to, but the brutal harshness of the 1999 suspension was a total deal breaker. Not only was the 1999 suspension bouncy and slow, but it was totally irrationally harsh and uncomfortable also. Perhaps even more annoying than the actual harsh and uncomfortable ride was the simple fact that it was just worse in all regards. How could the 1999 suspension be both more uncomfortable and also much slower under all conditions? It just didn't make any sense at all
It is essentially the same bike. The frame geometry is identical, the total weight is nearly exactly the same and the suspension systems appear to be of exactly the same type. Upside down forks and linkage type rear monoshocks on a steel single back bone frame with ridged mounted engines. All exactly the same, but the 1999 suspension valving just didn't work for anything.
The other severe problem with the 1999 Husqvarna TE 410 was that it often wouldn't kick start and also sometimes had difficulty idling without stalling. The 410 motor did run, and it usually was able to kick start from cold. Once warmed up though it would only sometimes kick start, and both the low idle performance and the hot starting performance were extremely variable from day to day.
The hard starting problem was obviously related to the weak spark Ducati Energia CDI ignition system, but there also seemed to be some sort of carburetor problem. The fact that the 410 motor always did best with the idle mixture screw on the 36mm DellOrto bottomed out at zero turns out was very suspicious. The idle mixture screw looked like it should work similarly to the idle mixture screws on the 34mm and 40mm DellOrto carburetors. I even went so far as to mark the idle mixture screw on the 410 with black permanent marker to see what the seating pattern looked like. I seated the idle mixture screw down in the carburetor and then took it out again. Sure enough there was an even contact ring all around the needle where it was seating down on the carburetor body. The idle mixture screw was turning off completely, but there appeared to be gasoline leaking in through other circuits at low idle.
The thing is though that the 1999 Husqvarna 410 was still idling rather lean with the idle mixture screw bottomed out at zero turns out. It was so lean that there was usually quite a bit of popping out the exhaust on deceleration, and also so lean that it wouldn't low idle when first fired up cold even on a scorching 105 degree summer afternoon. The idle mixture appeared to be plenty lean, arguably too lean even, but not lean enough to kick start hot with the weak spark Ducati Energia CDI ignition system.
Getting the 410 motor to start and idle seemed possible. If nothing else I could have just put a points ignition system on it. The problem though was that I just couldn't seem to care much about the starting and idling performance when the suspension was so bad that the bike wasn't usable. The 1999 Husqvarna TE 410 was usually able to fire up in a few kicks from cold, and then by the time I got back from just a short ride I was so disgusted by the crappy 1999 suspension that it just went back in the corner of the garage and I didn't look at it for weeks or months. The bad suspension was in a very real way preventing the hard starting problem from being fixed. If the bike wasn't usable anyway then what did it matter if it was hard to start and often stalled at low idle.
When I ordered the forks off of a 1991 Cagiva/Husqvarna WMX 250 two stroke that was being parted out I wasn't sure just what they would be like. The advertised description didn't even say "WMX", it was just listed as parts off of a 1991 Husqvarna 250 two stroke motocross bike.
When the package arrived the first thing I checked was the stamped model indication. The 1991 Husqvarna WMX 610 White Power forks and shocks are stamped "WMX" and the 1991 Husqvarna WXE 350 forks and shocks are stamped "WXE". The 1991 Husqvarna WMX 250 forks were also stamped "WMX", so this seemed very encouraging.
The next thing I noticed though was that the 1991 Husqvarna WMX 250 White Power forks were short like the 1991 Husqvarna WXE 350 White Power forks. This was a bit disconcerting, as it is the length of the damper cartridges on the 1991 Husqvarna four strokes that clearly indicates whether it is a WMX race bike or a WXE trail bike. My new set of 1991 forks were stamped "WMX" but they were shorter and this seemed somewhat suspicious. I decided to install the new forks on the 1999 Husqvarna 410 anyway, hoping that the "WMX" stampings meant that it was usable suspension valving.
The main challenge in installing the 1991 Husqvarna WMX 250 forks on the 1999 Husqvarna TE 410 chassis was in sorting out the triple clamps. The upper triple clamps are interchangeable between all of the 1990 through at least 2001 Husqvarna four strokes, and the 1991 Husqvarna two strokes with the 40mm USD White Power forks obviously also go in the same top clamps. The lower clamps are however different. As expected the 1992 through 2001 45mm USD forks take a larger diameter lower clamp than the 1987 through 1991 40mm USD forks, but somewhat confusingly the 1992 45mm forks also take a slightly different lower clamp diameter than the 1999 45mm forks. Luckily all of the 1980's and 1990's Husqvarna four strokes take the same steering head bearings so that any of the triple clamps will go on any of the bikes. The problem I ran into though was that the 1991 Husqvarna two stroke is actually on a Cagiva frame that takes different size steering head bearings. The 1991 Cagiva/Husqvarna WMX 250 triple clamps look exactly like the 1990/1991 Husqvarna triple clamps, but the steering tube is a slightly different diameter to go in the very slightly smaller steering head bearings. The 1991 Cagiva/Husqvarna WMX 250 triple clamps wouldn't work on the 1999 Husqvarna TE 410 chassis.
Luckily I happened to have an extra set of 1991 Husqvarna triple clamps that had come with the 1991 Husqvarna WMX 610 suspension that I put on my 1991 Husqvarna WXE 350 chassis. As expected the 1991 Husqvarna WMX 610 triple clamps bolted right up to the 1999 Husqvarna TE 410 chassis. I decided to re-use the 1999 upper clamp though. By using the 1999 upper clamp the taller 1999 bar mounts just stayed mounted in exactly the same position relative to the frame as on the stock 1999 Husqvarna TE 410. It seemed like a good idea to start with exactly the same bar to frame positioning. The only downside is that it looks a bit strange to have 6mm Allen bolts on the lower clamp and 8mm flange bolts on the upper clamp.
There is actually another problem with the triple clamps also. The 1980's and 1990/1991 Husqvarnas all use adjustable steering stop bolts attached to the sides of the frame. The 1992 through 2001 Husqvarnas on the other hand use a tab sticking out the front of the head tube between two stops on the lower triple clamp. The 1992 and later system is annoying because it is not adjustable, or at least it is not two way adjustable. The stops can be cut down to allow the front wheel to turn farther, but then that setting is a permanent modification. With the 1991 lower clamps on the 1999 chassis there just wasn't any positive steering stop at all. The radiators then became the steering stops, which is not good for radiator survivability in a crash.
Visually there doesn't appear to be much of anything wrong with the 1991 fork installation on the 1999 Husqvarna. In fact it looks really great. The darker anodized color of the 1991 White Power upper fork tubes actually goes with the yellow and black color scheme of the 1999 Husqvarna much better than the flashy light gold color of the 1999 45mm upper tubes.
All of the 1990, 1991, 1992 and 1993 Husqvarna front wheels are interchangeable, but the 1999 front wheel uses a larger diameter axle. Luckily I happened to have a few extra 1991 and 1992 front wheels sitting around. One I bought with the 1991 Husqvarna WMX 610 suspension that I put on my 1991 Husqvarna WXE 350 chassis, and another one I bought off of a 1991 Husqvarna WXE 350 that was being parted out. I also had the front caliper off of that 1991 Husqvarna WMX 610 that the wheel and the clamps had come off of, and it turned out to be a good caliper in perfect working condition.
The only difficulty I had in the front brake installation was that the 1999 front brake line ended up being too short. Initially I just put the 1991 caliper on the 1999 brake line with the 1999 Brembo front brake master cylinder. The way that the brake line is routed forward from the caliper to the front of the fork leg on the 40mm USD White Power forks it ends up needing a longer line. Initially I just crammed the shorter 1999 line in, but it was a tight fit.
The oil that was in the 1991 Husqvarna WMX 250 forks when I got them looked thin like 5W, not thicker like 10W or 15W. I decided to give them a run just as they were, but I did top the oil up with some 5W since the levels in both legs were down much lower than I usually run.
With the forks and front wheel installed I headed out for a test ride. Right away I was very impressed that the 1991 Husqvarna WMX 250 40mm USD White Power forks were a whole lot better than the 1999 45mm USD forks. The plushness and comfort was drastically improved, and the bad manners of the front end squishing out to the side on jump landings was totally gone. At first I was very impressed with the 1991 Husqvarna WMX 250 folks, but I did notice right away that the bike didn't feel exactly the same as the 1991 Husqvarna WMX 610. The 1999 Sachs shock seemed much better with the 1991 Husqvarna WMX 250 40mm USD White Power forks than it had with the crappy 1999 45mm USD forks. As good as the 1999 Sachs shock was seeming it certainly wasn't the same as the 1991 Husqvarna WMX 610 White Power shock, that much was very clear right away.
The new 1991 Husqvarna WMX 250 forks seemed to be working a whole lot better than the 1999 45mm forks, but right away they needed service. The seals were leaking and also sticking. Pushing down on the bars with the bike stationary there was a very noticeable sticking feeling. I pulled the forks off again, replaced the seals with ARI-035 fork seals and refilled them with fresh 5W fork oil.
When I headed out for another test ride I was even more impressed with the 1991 Husqvarna WMX 250 forks. Plushness over small sharp bumps was even better, and the action over bigger bumps and jumps also seemed improved with the fresh oil. It had indeed been 5W oil that was in the forks when I got them, but the fresh 5W oil seemed to work even better. For clicker settings I was using just two clicks in on the rebound clickers for the stock #3 position and just one click in on the compression dampers for the #2 setting. This seemed to work very well, and I was very impressed with the very large improvement over the 1999 Husqvarna TE 410 45mm USD forks. It really seemed like I had managed to fix the 1999 Husqvarna TE 410. There was however the nagging issue of the different feeling of the 1999 Sachs shock versus the 1991 Husqvarna WMX 610 White Power shocks.
I filled the tank on the 1999 Husqvarna 410 and I headed out for a longer ride through the hills. Right away I thought there was something wrong with the 1999 Sachs shock. Just going down the same dirt road that I had been test riding the upgraded 1999 Husqvarna WMX 410 on the rear end felt wrong. It felt harsher and bouncier and it also felt more busy. I stopped and checked the clicker settings and I also turned the rebound clickers in. On the Sachs shock the low speed compression setting was still at two clicks in where I had been running it and the high speed compression clicker was still all the way out where I had been running it. I went in from 10 clicks in on the rebound damping to 15 clicks in on the rebound damping on the Sachs shock and I also went in from the #3 rebound position on the White Power forks to the #4 rebound position. This increase in rebound damping removed the busy feeling at the rear end, and it also quieted down the bouncy feeling some. The extra harshness though still seemed like a problem. The 1999 Sachs shock really did seem a small bit harsher than it had on previous days.
When I got to the first trail I learned a lot more about my new 1999 Husqvarna WMX 410. It worked pretty good. Darn good really. Better than just about any dirt bike I have ever ridden. But not as good as the 1991 Husqvarna WMX 610 suspension, and that was a big letdown.
The 1999 Sachs shock did do a pretty good job on the trails. The control over medium to large rolling bumps felt pretty good, and the impact absorption from landings felt good enough. The most significant difference was that the bike felt bouncy and harsh. Even though I was able to go fairly fast on the upgraded 1999 Husqvarna WMX 410 it was seeming like a more demanding bike than the 1991 Husqvarna WMX 610. The extra harshness was more tiring, and the bouncy feeling meant that timing and body position was more critical for keeping the bike in control at higher speeds.
Despite the obvious problems I did like the 1999 Husqvarna WMX 410. It seemed fun to ride. The challenge of dealing with the bouncy suspension became a fun little game, and the bike did go fast enough to be quite thrilling. The 1999 Husqvarna WMX 410 was reminding me of late 1990's and early 21st century Japanese 250 two stroke motocross bikes I had ridden; fairly fast and fairly comfortable and just generally fun and easy to ride at a moderate pace. The 410 motor was also running pretty well, and that was helping a lot with getting the 1999 Husqvarna WMX 410 to go fast and deliver thrilling entertainment. What I was really impressed with was the broad and usable torque on the steep trails. I found that I was trying to climb hills up a gear in third gear like I often do on the 610 bikes, and amazingly this was sort of working. The torque from the 410 motor was sizeable and very usable. To get up the steepest climbs though I did find that I had to shift down like on the 350 motor to find stronger torque and more power. The torque from the 410 motor was amazingly strong way down low around 3,000 to 4,500RPM, but the torque did increase considerably up at 5,500 and 6,500RPM and this meant that more speed in third gear kept the bike accelerating up the steep hills. The 410 motor was also revving out willingly with a sizeable amount of top end power, and this meant that second gear was also very effective up to rather high speeds on the trails. With the 13/48 gearing 10,000RPM in second gear is a whopping 50mph!
As I climbed higher the 410 motor became reluctant to get going at the low end and lower midrange engine speeds. By the time I got to 6,000 feet the barking strong torque across the entire lower operable engine speeds was gone. Interestingly though the 410 motor was still able to get going and make power up above 6,000 feet of elevation, the power just was only available up at higher engine speeds. Also of interest was that torque at very small throttle openings remained very good up at 6,000 feet of elevation. The throttle response at small throttle openings seemed good, and the smooth and quiet torque was perfectly instant and very usable for casual cruising over an amazingly wide range of lower engine speeds. Twisting farther just didn't do much of anything until the engine speed came up quite substantially. Then at about 6,000RPM or so the 410 motor came alive with a loud thundering bark and made power over a seemingly rather wide range of upper engine speeds even up to 6,500 feet of elevation.
When I dropped back down to 4,000 feet of elevation the midrange torque came back strong, and continued to get stronger down to lower elevations. Top end power was seeming really quite strong for the 400cc motor, and it was pulling over what seemed like a fairly wide range of upper engine speeds without difficulty.
When I got back from that 72 mile loop ride though the mountains I drained the remaining gasoline from the 1999 Husqvarna 410. I got 5 quarts out, which seemed like a lot. That meant I got 65mpg on that mixed ride, which is the best I have ever gotten from any of my 1990's Husqvarna four strokes. Better than I ever got out of the smaller displacement 350 motors, which is interesting. It would be expected that the shorter 2.39" stroke length would be even more of a problem than the 2.48" stroke length of the 350 motor when put on the same bottom end and transmission as the 610 motor. As it turns out it is the extra 50cc of displacement that is such a big advantage on the 410 motor. Keeping the stroke length short is very good for overall operating efficiency on a dirt bike, but the extra 50cc of displacement works towards mitigating the oversized bottom end. I was also running 10-40 oil in the 410 instead of the 15-50 I usually have run in the 350 motors and the 386 stroker motor, and that might help a small bit with fuel mileage.
I was interested to learn more about this gasoline that had delivered such amazingly high 65mpg performance, so I put a gallon of it into the empty tank on my 1991 Husqvarna WMX 610 with the 12.2:1 hot rod 610 motor. The big thing I noticed in running this gasoline in the hot rod 610 motor was that it had a rather low energy density. The hot rod 610 motor was running and pulling well without cutting out, stumbling or surging but it was popping out the exhaust on deceleration much more than it had been the last time I rode it and the idle quality was also very poor. On this lower energy density gasoline the points ignition 610 motor was actually stumbling and missing at low idle, and the idle speed was very unstable. Kick starting hot the hot rod 610 motor was also acting up, popping but not starting on the first kick. Sometimes it was giving a pop and not starting on the second kick also. It did still always fire up fairly easily, but it was often taking a few kicks where it just popped and didn't start before it finally fired up.
With the big 175 size main jet though the 12.2:1 hot rod 610 motor was able to make some power. Torque at 3,000 to 5,500RPM was smooth, quiet and fairly strong, but noticeably not quite as strong as the hot rod 610 motor had been doing the last few times I had ridden it. The smooth torque was building nicely from 4,000 to 6,000RPM without the slightest bit of surging. Then the torque increased quite noticeably right at 6,000RPM and continued strong up to 7,500RPM where the power hit hard. Power from 7,500 to 8,400RPM was seeming quite strong and also very reliable without any cutting out. When I got back I checked the static timing setting, which I found to be at 23 degrees BTDC hot. Very close to the 22 degrees BTDC it seemed to be at the last time I rode the hot rod 610 motor.
Over the past few months I have been having pretty good luck with not having bad surging from the 12.2:1 hot rod 610 motor, although slow flame front travel speed gasoline has again been coming out of the pumps as 91(RON+MON)/2 octane rating premium gasoline. Running 22 degrees BTDC the slow flame front travel speed gasoline has been almost working in the 610 motor, but not quite. With that rather large 22 degree BTDC spark timing on the 12.2:1 hot rod 610 motor operation right at 2,900 to 3,100RPM had been overly crisp to the point of torque output suffering, but then by 3,200RPM it was only somewhat overly crisp and actually working well. Backing off to 18 degrees BTDC torque at 2,800 to 3,100RPM improved quite a lot, but then the engine wouldn't easily rev up past about 5,000RPM. At a 22 degree BTDC static timing setting though I was getting strong and reliable torque everywhere from 3,200 to 6,500RPM. The power was even taking off noticeably at 6,000RPM, but by 6,800RPM the slow flame front travel speed gasoline was cutting out some. Once I got it up to 7,000RPM it pulled smoothly again, but only to 7,300RPM where it again was cutting out so bad that power output suffered dramatically. In lower gears I was able to accelerate right past the cutting out around 7,000RPM, and there was some fairly big smooth power from 7,500 to 8,100RPM. Very close to working, but not quite. The problem seemed to be that this particular slow flame front travel speed gasoline had a lower energy density than the slow flame front travel speed gasoline I had gotten in 2015 and 2016. There wasn't any noticeable blackening of the exhaust, which is unusual for slow flame front travel speed gasoline in the hot rod 610 motor. The flame front travel speed also seemed boosted a slight bit, so it was probably a mixture of slow flame front travel speed regular and some considerable amount of ethanol. Probably about 20 or 30% ethanol or some other low energy density specialty fuel. If it had been just 10% ethanol mixed with 90% slow flame front travel speed gasoline the energy density would not have been that low. As low as the energy density of this slow flame front travel speed gasoline seemed the idling and starting performance of the hot rod 610 motor was still pretty good. The 22 degree BTDD static timing setting and 18 degree BTDC static timing setting was all on the same tank of 91 (RON+MON)/2 octane rating premium straight from the gas station. It was sold as 91 (RON+MON)/2 octane rating premium gasoline, but it turned out to be slow flame front travel speed regular gasoline. And watered down at that.
The fast flame front travel speed gasoline that delivered 65mpg in the 410 motor had an even lower energy density, causing much worse popping out the exhaust and an unusual unstable low idle in the hot rod 610 motor. That gasoline did however deliver some considerable broad top end power in both the 410 motor and in the 610 motor. Obviously a fast flame front travel speed gasoline means quite a lot for both performance and efficiency when run in mechanically controlled engines operated over wide ranges of engine speeds and engine loads.
That was why I had swapped the gasoline into the 1991 Husqvarna WMX 610 immediately after a long ride on the 1999 Husqvarna 410; to see what was going on with the gasoline. What I also noticed was the difference in suspension performance. As soon as I rode off on the 1991 Husqvarna WMX 610 I was struck by how tall it felt compared to the 1999 Husqvarna WMX 410. Even with the tall 1999 bar clamps and the 40mm White Power forks all the way down in the clamps the 1999 Husqvarna WMX 410 was a lot shorter at the front. The 1999 Husqvarna TE 410 chassis is also lower at the rear. It is just lower overall. As soon as I rode off on the 1991 Husqvarna WMX 610 I noticed that it just felt a whole lot taller at both ends. The bars felt a lot taller, but the rear end felt higher also.
The next thing I noticed was that the 1991 Husqvarna WMX 610 suspension was a lot smoother. Even just cruising off down a small dirt road at around 20 to 30mph I was very impressed with how much smoother the 1991 Husqvarna WMX 610 suspension felt over all types of bumps. Then when I opened the hot rod 610 motor up and went faster I was totally blown away by the suspension difference. The 1991 Husqvarna WMX 610 was just a whole lot smoother and more comfortable at all elevated speeds, and the faster I went the larger the difference seemed. The level of control at high speed was also in a whole different realm with the 1991 Husqvarna WMX 610. Instead of the bouncy and difficult to control suspension on the 1999 Husqvarna WMX 410 the 1991 Husqvarna WMX 610 was just totally at ease over all types of bumps up to very high speeds.
What I realized in switching between the two bikes one after the other was that the 1991 Cagiva/Husqvarna WMX 250 White Power 40mm USD forks probably have exactly the same valving as the 1991 Husqvarna WXE 350 White Power 40mm USD forks. That slight bouncy feeling and lack of good rebound control at high speed was exactly the same. As was the increased harshness over all types of bumps at all speeds above about 15 or 20mph. What had thrown me off was that the 1999 Sachs shock is actually a lot better than the crappy 1991 Husqvarna WXE 350 shocks. I actually have two of those 1991 Husqvarna WXE 350 shocks now, as I for some crazy reason decided to buy the shock off of one of the 1991 Husqvarna WXE 350 bikes that was being parted out. I guess I just didn't trust that the stock 1991 Husqvarna WXE 350 shock really could be as bad as I had experienced on my 1991 Husqvarna WXE 350 back in 2014. I bought that second 1991 Husqvarna WXE 350 shock to try on my period correct 1991 Husqvarna WMX 610 with the crappy race shop modified suspension, but it didn't work. The 1991 Husqvarna WXE 350 shock was actually worse than the race shop modified 1991 Husqvarna WMX 610 shock. It turns out that the shock on that period correct 1991 Husqvarna WMX 610 is only slightly modified. After trying the crappy stock 1991 Husqvarna WXE 350 shock I put that stock spring on the race shop modified 1991 Husqvarna WMX 610 shock. Just as I had expected the very stiff 68 spring was causing a lot of the problems. The bike was much better with the stock 58 spring. With the same spring rates as my other 1991 Husqvarna WMX 610 bikes I was able to get a good idea of what the valving differences were. What I found was that the race shop modified 1991 Husqvarna WMX 610 suspension was just as good as the stock 1991 Husqvarna WMX 610 suspension over a narrow range of speeds around 30 to 45mph. At lower speeds the race shop modified 1991 Husqvarna WMX 610 suspension was harsher and had a harder time dealing with ruts. At higher speeds the race shop modified 1991 Husqvarna WMX 610 suspension again got harsher than the stock 1991 Husqvarna WMX 610 suspension, but it also got bouncy and out of control with poor rebound control. As bad as that sounds the race shop modified 1991 Husqvarna WMX 610 shock was still noticeably a lot better than the stock 1991 Husqvarna WXE 350 shock, which is interesting. The shock is the worst part of the 1991 Husqvarna WXE 350 suspension package. The 1991 Husqvarna WXE forks are inferior to the 1991 Husqvarna WMX 610 folks, but the difference isn't quite as large as the difference between the 1991 WXE and 1991 WMX shocks.
So that was what was going on with the upgraded 1999 Husqvarna WMX 410. The combination of the drastic improvement over the 45mm 1999 forks and the fairly competent 1999 Sachs shock made the 1999 Husqvarna WMX 410 seem to work fairly well. At least in isolation. In direct comparison to a stock 1991 Husqvarna WMX 610 though the slight problems became obvious. The really tricky thing is that the 1999 Sachs shock is quite good, but noticeably slightly inferior to the 1991 Husqvarna WMX 610 White Power shock. The 1999 Sachs shock is much better than the 1991 Husqvarna WXE 350 White Power shock, and the 1999 Sachs shock is also quite noticeably better than the race shop modified 1991 Husqvarna WMX 610 White Power shock. The 1999 Sachs shock is darn good, and that actually made figuring out just exactly what was going on with the upgraded 1999 Husqvarna WMX 410 rather difficult. Riding the bikes back to back on the same day the 1999 Husqvarna WMX 410 ended up being a disappointment, and back into the deep dark corner of the garage it went.
Just as happened in the summer of 2015 slow flame front travel speed gasoline coming out of the pumps as 91 (RON+MON)/2 octane rating premium was followed by fast flame front travel gasoline requiring a high compression ratio. This time though it was considerably watered down and rather weak. One month it was watered down slow flame front travel speed regular gasoline being sold as 91 (RON+MON)/2 octane rating premium, and then the next month it was very fast flame front travel speed gasoline with a high temperature of combustion potential and also had a higher temperature and pressure requirement for late compression ignition. Here is what happened:
On a tank of 91 (RON+MON)/2 octane rating premium gasoline straight from the gas station the 12.2:1 hot rod 610 motor was hesitating and reluctant to get going with a static timing setting of 22.5 degrees BTDC. It was pulling at all engine speeds, but not willingly. Once fully warmed up on some big pulls I was able to get power at 7,500 to 8,100RPM without any cutting out, but it was reluctant to make power up at those higher engine speeds and torque down at 4,000 to 6,000RPM was also unreliable. Once fully warmed up on some big pulls the torque at 4,000 to 6,000RPM increased to a respectable level, but when cooled off from coasting and cruising it was flat and dead feeling across the entire midrange. There wasn't any surging at any engine speed, and there wasn't really any cutting out either. The engine just seemed very reluctant to get going and torque output was low.
I stopped and bumped the static timing setting up to 24.5 degrees BTDC, and this got the engine going more easily and more reliably. Power from 7,500 to 8,400RPM was available reliably each time I went after it, and the torque at 4,000 to 6,000RPM was reliable, more instant and a bit stronger. Again there wasn't any surging. Not the slightest hint of any surging at any engine speeds. Bumping the static timing setting up just that small 2 degrees of crankshaft rotation had gotten the power and torque coming on considerably more instantly and reliably, but the change in the amount of power output was not all that large. Just that small two degrees of crankshaft rotation also made a big difference in how far the engine was willing to rev out. At 22.5 degrees BTDC on the static timing setting (about 26 degrees BTDC above 7,000RPM with the crankshaft wiggle advance) the power was done at 8,100RPM, and sometimes it was ending at 7,900RPM when the engine was somewhat cooled off from coasting. Then with 24.5 degrees BTDC on the static timing setting (about 28 degrees BTDC above 7,000RPM with the crankshaft wiggle advance) there didn't seem to be any hard rev ceiling. The engine was able to keep revving and kept making about the same amount of power up to higher engine speeds when I stayed on it.
Even with the static timing setting bumped up to 24.5 degrees BTDC there also wasn't any excess crispness anywhere down low at 3,000 to 4,000RPM, although even with just 22.5 degrees BTDC on the static timing setting the engine had seemed sufficiently crisp at around 3,000 to 3,500RPM. A rather large change in crispness and willingness to rev out above 8,000RPM but not all that much change in the level of crispness down at 3,000 to 4,000RPM clearly indicates a fast flame front travel speed gasoline. This certainly was very fast flame front travel speed gasoline. The fact that there was no surging even after the static timing setting was bumped up by two more degrees of crankshaft rotation indicates a rather high temperature of combustion potential. Also the simple fact that the three inch stroke length engine was able to continue to rather easily make power well beyond 8,000RPM indicates a rather high temperature of combustion potential.
Requiring that large amount of spark advance in the 12.2:1 hot rod 610 motor also indicates a rather high temperature and pressure requirement for late compression ignition. It was high pressure gasoline, but it was also low energy density gasoline that seemed to need richer jetting than the 175 size main jet I was running. The strange thing though was that even with the needle clip in the first (leanest) groove the engine was running and pulling at all throttle openings from 1/5 to 1/3. The gasoline seemed weak and unable to make really big torque, but it was actually able to make some lower midrange torque at small throttle openings on a rather lean mixture ratio.
This clearly was some sort of high pressure specialty gasoline for high compression ratio engines, but it was watered down with a low energy density specialty fuel. That low energy density specialty fuel wasn't just low pressure premium gasoline either. No, it was something with a rather high temperature and pressure requirement for late compression ignition. Something expensive probably. This low energy density but higher pressure specialty blend might actually be more expensive to produce than race gas. It probably is about half race gas, but what it is watered down with remains a total mystery. Weak, but not all that lean. Hot burning, but not all that powerful. Some very strange combustion fuel to be sure.
The next day this fresh 91 (RON+MON)/2 octane rating gasoline had changed slightly while sitting in the tank on the bike. The static timing setting had also mysteriously been adjusted back to 22 degrees BTDC when measured hot. With 22 degrees BTDC on the static timing setting the 12.2:1 hot rod 610 motor was now running fairly well, without nearly as much reluctance to get going as it had been exhibiting the day before with very close to the same 22 or 22.5 degree BTDC static timing setting. Instead of a reluctance to get going and even some hesitation at a 22 degree BTDC static timing setting like the day before the 12.2:1 hot rod 610 motor was now only exhibiting some lag and a slight bit of delay before the onset of torque. The torque was however reliable nearly 100% of the time, even after the engine was cooled off a bit from cruising and coasting. The biggest single difference was that the energy density of the gasoline no longer seemed to extremely unusually low. Most of the popping out the exhaust on deceleration was gone, and the idle quality was more solid and more consistent. The 12.2:1 hot rod 610 motor was also starting more easily, firing on from cold the second very small kick without the choke the 78 degree Fahrenheit ambient afternoon heat.
With this new mystery gasoline that had showed up overnight there were some very interesting performance capabilities. One was fairly strong power to 8,900RPM without cutting out or surging or much of any problems of any kind. There very interesting thing was amazingly strong torque BELLOW 3,000RPM. It was sort of unbelievable how low the torque continued fairly strong. I tired at 2,800RPM, and it pulled somewhat harshly but it did make usable torque. Then I tried accelerating from 2,600RPM, and again it actually pulled. Considerably less torque than 3,000RPM, but still usable. Going a bit lower it was only sort of working and it was getting very harsh, but even all the way down to 2,400RPM opening the throttle actually resulted in acceleration. That was pretty amazing, especially combined with fairly good power all the way out to 8,900RPM.
The top end power was however not all that strong. In fact torque and power everywhere were only sort of reasonably good. The torque seemed about like normal at 3,000 to 3,500RPM, and there wasn't much excess harshness to be found even all the way down to 3,000RPM. It certainly was popping off on late compression ignition from 2,500 to 3,500RPM though. Opening the throttle a small crack at those engine speeds resulted in extremely smooth and extremely quiet operation with some small amount of usable torque. Then opening the throttle a bit farther the torque increased quite a bit with a noticeable little bark from the exhaust. Then from 3,500 to 4,500RPM the engine was seeming flat. It was pulling at those engine speeds, and torque was even increasing from 3,500 to 4,000RPM and also increasing from 4,000 to 4,500RPM. Just not as much as is typical. The three inch stroke length 610 motor normally makes considerably more torque at 4,200RPM than at 3,700RPM, and that wasn't really happening. Instead it was just getting extremely quiet from 3,700 to 4,500RPM with only very slightly increasing torque. It wasn't until nearly 5,000RPM that the torque started to increase significantly with some bark to it, but even then it wasn't much of a hit. It wasn't abrupt at all at any engine speed, but by 6,000RPM it was pulling pretty good. It does sometimes pull even harder, but it certainly was pulling.
Then the performance at 6,500 to 7,500RPM was somewhat strange. In all lower gears it just accelerated right up and the power increased noticeably at 7,500RPM and continued to pull above 8,000RPM. The power was seeming to peak early around 8,000RPM, but that power was continuing all the way up to 8,900RPM before dropping off noticeably. In sixth gear with the 14/48 gearing though it was a different story. There actually was some slight cutting out, but at a somewhat strange engine speed. On slow flame front travel speed gasoline the 610 motor tends to cut out at about 6,800 to 7,000RPM. On this faster flame front travel speed gasoline though it just continued to pull right up to 7,000RPM and 7,200RPM without the slightest bit of cutting out. Then it got stuck at 7,300RPM. Wide open at 7,300 and 7,400RPM in sixth gear the engine was cutting out in a very rhythmic way that was reminiscent of rhythmic surging. The cutting out was however fairly sharp, so it wasn't actually rhythmic surging where the time of late compression ignition is falling over to very early before TDC times of late compression ignition. No, this was more like cutting out, but it was doing it regularly every second or so. This little bit of cutting out was causing the bike to get stuck right there at 95mph, but then after just a few seconds it was able to pull up to 7,500RPM where the intake stack boost hit and it accelerated farther. The acceleration from 7,500RPM up to 7,900RPM at 103mph seemed quite a bit slower than usual, but it did pull consistently without any cutting out from 7,500 to 7,900RPM. Power output was low. It would only do 103mph at 7,900RPM, where 8,500RPM is the more typical top end engine speed for the hot rod 610 motor. That 8,500RPM would be 111mph with the 14/48 gearing, although I haven't hit 8,500RPM in sixth gear yet with the 14/48 gearing. I was very easily pulling to 8,500RPM in sixth gear with 15/50 gearing two years ago though.
The difference between 103mph and 115mph might not seem like all that much, but the reality is that it takes about 1/3 more power to go 115mph than to go 103mph. That is in fact a substantial difference.
So the power output was low with this mystery gasoline, but the range of engine speeds that it was pulling over was astonishingly wide. There was nowhere from 2,700 to 8,900RPM where the 12.2:1 hot rod 610 motor wasn't making pretty good torque, and there weren't any horrible problems like surging, hesitation, excess harshness or cutting out. Just that slight bit of getting stuck at 7,300RPM in sixth gear, but in fourth and fifth gears it pulled right through from 6,500 to 7,500RPM with seemingly no difficulty at all.
Was this then the best performance I have ever seen from the hot rod 610 motor? No, not by a long shot. On normal types of gasoline the torque is stronger and the three inch stroke length 610 motor is more abrupt everywhere from 3,500 to 6,500RPM. The combination of usable torque down to 2,500RPM and usable power to 8,900RPM is impressive, but the reality is that the 610 motor runs most of the time between 3,200 and about 6,500RPM, so it is the performance within that range of engine speeds that is most important.
It is important for the three inch stroke length engine to pull well down to at least 3,500RPM, but performance bellow 3,000RPM can very easily seem irrelevant. Likewise it is important for the 610 motor to pull up to 7,500RPM without cutting out, but whether it pulls to a maximum of 8,200RPM or a maximum of 8,900RPM doesn't usually seem all that significant. If it were actually raced at speeds above 100mph then stretching the shifts to 9,000RPM to keep the engine speed constantly above 7,500RPM would be required. In lower gears though it just doesn't matter much if the engine speed drops down to 6,700RPM or all the way down to 6,000RPM after shifting. It pulls hard everywhere from 5,500RPM to 8,500RPM.
Absolute peak torque might actually come up at 7,500RPM where the intake stack boost first hits, but torque is also very strong from 5,500 to 7,000RPM. If it were not for the intake stack boost at 7,500RPM peak torque on the hot rod 610 motor would be at about 6,000RPM. Torque doesn't actually peak bellow 6,000RPM, but the torque does typically remain very strong down to somewhat lower engine speeds. Torque nearly always seems to remain very strong down to around 5,500RPM on the three inch stroke length 610 motor, but then bellow 5,500RPM can be a bit sketchy. The shape of the torque curve from 3,500 to 6,000RPM on the 3" stroke length engine tends to jump around some with changes in the combustion properties of the gasoline.
On powerful hot burning gasoline it is just the latest possible time of late compression ignition around 15 or 20 degrees ATDC everywhere from 3,000 to 5,500RPM; getting harsh and loosing torque bellow about 3,300 or 3,400RPM but providing very powerful and also very smooth torque everywhere from 3,500 to 6,500RPM.
On slightly weaker gasoline the earlier and easier to hit time of late compression ignition is required down to lower engine speeds bellow 6,500RPM. The earlier and easier to hit 5 degree ATDC time of late compression ignition can still work fairly well down to 5,500RPM and even 5,000RPM, and the minimum engine speed then drops down to 3,200RPM or even 3,000RPM. On dramatically weaker lower temperature of combustion potential gasoline torque bellow 5,000RPM plummets. On very weak gasoline the earlier and easier to hit time of late compression ignition is required down to 4,500RPM and even 4,000RPM, and this is where spark timing up around 24 degrees BTDC and 26 degrees BTDC becomes such a huge advantage. The earlier spark timing allows the time of late compression ignition to very easily fall over to 5 degrees ATDC to get some of the lost torque back at 4,500 to 5,500RPM. The problem though is that the earlier times of late compression ignition simply don't work well at lower engine speeds. The engine becomes finicky and difficult to tune and efficiency and torque production tend to fall off somewhat.
As the earlier and easier to hit 5 degree ATDC time of late compression ignition is pushed down to 4,500 and 4,000RPM the engine still won't run well much bellow 3,000RPM so the range of engine speeds at the latest possible time of late compression ignition gets narrower. If the earlier and easier to hit 5 degree ATDC time of late compression ignition gets pushed down to 3,500RPM then the range of engine speeds for efficient torque generation collapses to nothing and the engine can hardly be made to work at all. Gasoline engines do run best at the earlier and easier to hit 5 degree ATDC time of late compression ignition, but the engine speeds where this earlier and easier to hit 5 degree ATDC time of late compression ignition works well appears to be in the 4,500 to 9,000RPM range of engine speeds. Best possible performance is likely right in the middle somewhere around 7,000RPM, and that probably requires a 2" stroke length not a 3" or 4" stroke length.
If excessive crispness causes the earlier and easier to hit 5 degree ATDC time of late compression ignition to occur down to slightly lower engine speeds than required for 2" and 3" stroke length engines this usually only causes some excess harshness without much reduction in torque production. This is why shorter 2.5" and 2" stroke length engines tend to hold onto their torque down to 6,000RPM even when running slightly too much spark advance. For a shorter 2.5" stroke length even running powerful hot burning race gas 6,000RPM is only slightly too low of an engine speed for the earlier and easier to hit 5 degree ATDC time of late compression ignition. Said another way 6,000RPM can just about always tolerate the earlier and easier to hit 5 degree ATDC time of late compression ignition even in rather short stroke length engines. A short stroke length engine might get somewhat harsh at 6,000RPM at the earlier and easier to hit 5 degree ATDC time of late compression ignition, but it doesn't lose much torque at that engine speed. Down lower around 4,500 to 5,500RPM the earlier and easier to hit 5 degree ATDC time of late compression ignition doesn't work was well, so timing becomes more critical on longer 3" and 3.5" stroke length engines. This is why good running 3.25" and 3.5" stroke length engines tend to get so harsh around 4,000 to 4,500RPM. Just a small bit of excess crispness that pushes the earlier and easier to hit 5 degree ATDC time of late compression ignition down to say 4,000RPM when 4,300RPM is all that is required tends to cause quite a lot of extra harshness at 4,000RPM. The long 3.25" or 3.5" stroke length engine is already harsh at 4,300RPM, but pushing the earlier and easier to hit 5 degree ATDC time of late compression ignition down to 4,000RPM adds a lot of unnecessary extra harshness that does nothing but reduce efficiency, increase nasty high pressure pollutants and increase engine wear.
With even longer stroke lengths the earlier and easier to hit 5 degree ATDC time of late compression ignition is required down to lower engine speeds than it really works at. When the earlier and easier to hit 5 degree ATDC time of late compression ignition is already being required down to inappropriately low engine speeds then any excess crispness that pushes the earlier and easier to hit 5 degree ATDC time of late compression ignition down to even slightly lower engine speeds than required causes disproportionately large problems.
How low the 610 motor will pull is very significant, but from a different perspective than the actual RPM. It is very important that the long three inch stroke length engine will run down as far as it is will run. What this really means is that it is important that the spark timing isn't so early as to prevent the engine from running as low as it otherwise would be capable of. Just what engine speed is required depends on what gasoline is being run. On normal powerful gasoline 3,300 or even 3,500RPM is low enough, and excess spark advance totally ruins the torque everywhere from 3,000 to about 5,000RPM. On weaker gasoline it is important to be able to torque down to 3,000RPM or even 2,800RPM simply because that weaker gasoline needs the big three inch stroke length engine to generally stay at lower engine speeds to work at all well. Even on very weak low temperature of combustion potential gasoline the big torque is always up above about 4,500RPM, but on that weaker gasoline the three inch stroke length engine tends to need drop down much much lower for reasons like efficiency and quality of power delivery. If it is necessary to drop down to 3,200PRM to get the big three inch stroke length engine to run well on weak gasoline, then it tends to be necessary to go even a bit lower to get some usable range of bottom end engine speeds for delivering smaller amounts of power.
The big danger with excessively long stroke lengths in gasoline engines is that the power becomes "on and off" and there is then nowhere that the engine will run along well over a range of loads. This is why automotive engines have so commonly ended up with very small camshafts that deliver high cylinder filling way down to 2,000RPM. If the stroke length is so long that the engine won't make good torque at 3,200 to 4,000RPM then obviously sub 3,000RPM engine speeds are going to be used a lot. Perfect tuning can get some usable and reasonably efficient torque down a bit bellow 3,000RPM, but not all the way down to 2,000RPM. That is just impossible. Still though the automotive camshafts have traditionally been sized for high cylinder filling down to 2,000RPM because the transmissions have nearly 2:1 jumps between the gears. If the engine gets kitten weak or balky with "on off" power delivery above 3,500RPM then clearly it would be necessary to run down to 2,000RPM to get between the gears. Unfortunately sub 3,000RPM engine speeds just don't work in gasoline engines, and the automotive camshafts that deliver high cylinder filling down to 2,000RPM are a total mismatch.
So how is it that the Husqvarna 610 motor often actually makes more torque per cubic inch at 2,500 to 4,000RPM than traditional automotive gasoline engines? The camshaft is part of it. Getting reasonably smooth torque to continue down as low as possible in a gasoline engine requires less combustion way down at the bottom, and a big camshaft is a more efficient means of reducing cylinder filling than closing the throttle. Closing the throttle reduces cylinder filling, but also dramatically increases pumping losses. A big camshaft reduces cylinder filling with less of an increase in pumping losses.
Mixture control is part of it also. Many automotive engines have run extremely rich at small throttle openings. A rich mixture actually hinders 2,000 to 3,000RPM torque production in 3.25 and 3.5 inch stroke length engines. The rich mixture can reduce torque production down low for two different reasons. One of these reasons is related to the cylinder filling issue. A functionally lean mixture means less combustion, and that is good for supporting engine speeds way down at the bottom of where late compression ignition can work. The other reason that a rich mixture hurts low end torque production has to do with full flame front travel mode operation. Mostly it's going to be all full flame front travel mode operation bellow about 2,800RPM even with a long 3.5" stroke length. An overly rich mixture depresses the actual flame front travel speed, and that means less torque in full flame front travel mode up at the 2,000 to 3,000RPM engine speeds.
The spark timing is an even bigger part of the explanation of big torque from the Husqvarna 610 motor. Not only does the spark timing need to be less than about 24 degrees BTDC for there to be any chance of strong bottom end torque in late compression ignition mode, but the advance shoulder also needs to be bellow the bottom of the big torque. Traditional automotive engines typically used 3,300 or 3,500RPM advance curve shoulders, and that usually totally precluded good torque production bellow about 3,300RPM. The lack of any advance curve shoulder anywhere between 2,500 and 5,000RPM on the Husqvarna 610 motor is much more likely to actually stretch the usable torque down bellow 3,300RPM.
And finally the Husqvarna 610 motor is allowed to run harshly. The roller rod bearing can take a lot of pounding punishment at low engine speeds, and accelerated wear on pistons, piston rings and cylinders is generally not considered much of a problem on dirt bikes. Emissions compliance is also part of this. Harsh low speed operation is a red flag for oxide of nitrogen production, traditionally enemy number one for photochemical smog issues. Nobody cares about oxide of nitrogen production from dirt bikes, and that is for two different reasons. One of course is just that there aren't many dirt bikes running around, and even the most extensively used dirt bikes just don't get all that many miles on them. The other part of this is that although dirt bikes benefit greatly from being able to torque down as low as possible they don't actually end up being run at those low engine speeds much of the time. Mostly engine speeds on dirt bikes stay up somewhat higher where more torque is available and where a wider range of engine loads can be accommodated.