The stock Swedish built SEM ignition system had worked out fairly well on my newly rebuilt 1997 610 motor with the 11.3:1 compression ratio, but it had been getting hard to start. Eventually I could not get the engine to fire up at all with the stock SEM ignition, so I ripped it off and built another points ignition system for the engine. For more on the 11.3:1 610 motor build project see
Resurecting the 1991 Husqvarna WMX 610.
Another New Points Ignition
Late Timing and Big Torque
Slow or Cold?
From Slow to Weak
Back to Race Gas
After closing the spark plug gap up to 0.009" and turning the SEM stator all the way forward for the latest possible spark timing (about 25 degrees BTDC) the 11.3:1 engine had been working fairly well and starting easily. The easy starting did not however last. At first it was starting easily on the first kick much of the time, and only sometimes requiring two three or four kicks to get going. After a few weeks though the bike was requiring a bunch of big kicks to get it going hot or cold. It was still reliably starting with the kick starter, but a half dozen big kicks to get it going each time was getting quite annoying.
The first really big trouble I had with the SEM ignition was on a long ride I took where the bike just would not start with the kick starter, and I had to roll start it every time. Then the bike died on me while I was ridding. I eventually got it going again by switching to another plug also gapped down to 0.009", but then it died again a few miles up the trail. I tried different plug gaps, but I could not get any pop out of it with more than a 0.009" spark plug gap. Reducing the spark plug gap even smaller than 0.009" did not get it to fire up either. When I kicked the bike over with the spark plug out I could see just a very faint pale blue spark that seemed woefully inadequate. I had an enormous amount of difficulty in getting the engine started again, finally carefully cleaning the spark plug with my pocket knife and carved sticks got the engine to start and I made it home.
Then the next time I tried to start the bike it gave a few pops when I rolled it down a hill, but it would not fire up. When I towed the bike behind a car spinning the engine up to 4,000RPM it fired up but ran poorly. I was able to ride the bike for a few miles, but it was hardly running. It would not idle and would only run with the throttle cracked open just the right amount at about 4,000RPM.
The next day when I tried to start the bike it again would not fire up, even with a brand new spark plug. Again I tried different spark plug gaps, but none seemed to work. When I rolled the bike down a hill in third gear I got some pops and then engine was almost running. It would not however stay running, and after several big hills I had to give up and walk back to get a car. Again we towed the bike behind a car, but this time I still could not get the engine to start and run. It gave a lot of little pops, but it just would not rev up or stay running under it's own power. When I kicked the bike over with the plug out I could hardly see any spark, just the smallest and faintest little white spark with the plug gapped all the way down to 0.009".
I checked all the wiring connections, and I replaced the spark plug wire but the engine still would not fire up and only the faintest little white spark could be seen. I even tried removing the 5 kilo ohm resistor spark plug cap, but again the bike would not fire and I could only get the faintest little white spark. It seemed that the 1992 stock SEM ignition system also had finally completely failed.
I was reluctant to give up on the stock ignition since I really had been enjoying not having to charge batteries up to go ridding, but once again it was time for a points ignition.
The new points ignition system I built was basically the same as the one I had built back in February for the 1991 WMX 610 motor in the 1992 TE chassis. I started with a piece of 3/16" aluminum plate to mount the points on, and I mounted the points and condenser directly to the plate without using any other parts from an automotive distributor.
Again I cut the ignition cam to have only about 0.015" of points lift to reduce wear on the follower, which had worked out very well for prolonging the intervals between points adjustments. Again the opening ramp has a rather shallow angle attained by offset grinding the ignition cam. This shallow angle is what gives the three or four degrees of crankshaft wiggle advance up at high engine speed. And again I cut the ignition cam to have a dwell angle of approximately 200 degrees.
I weighed all the parts of both of the ignition systems to see which one was lighter, and I was somewhat surprised to find that the points ignition system is actually a whole lot lighter than the stock SEM ignition system. The big difference really is in the flywheel, that stock flywheel is really quite heavy at four pounds. The rest of the SEM ignition system, including all the wiring, is only two pounds. The big automotive coil for the points ignition system seems quite heavy, but it is in fact only a pound and a half. The rest of the points ignition system is quite light, with the points mounting plate, mounting hardware, points, and condenser coming to less than a half a pound. Just how much lighter the points ignition system is than the stock ignition system depends on what battery is used. With a four pound 5Ahr 12V AGM lead acid battery like I used to use 10 and 15 years ago the total weight of the points ignition system comes right up to the same as the stock ignition system, the difference is that the bulk of the weight is in the battery instead of in the flywheel. Adding a second 5Ahr lead acid battery as a backup then makes the bike four pounds heavier than with the stock ignition system. Using lithium-ion batteries of course saves quite a bit of weight. A small 2.2Ahr 7.4V Li-Po model airplane battery weighs a third of a pound and will go for nearly a full tank of gasoline, although engine performance does drop off a bit towards the end of the discharge period when the voltage drops bellow 7V. Even a big 10Ahr 13V LiFeP04 battery that is good for at least two full tanks of gasoline and will also power incandescent lighting weighs only three pounds.
When I put the bike back together it fired up on the first kick and ran well. It took a while for the new ignition cam and follower to break in, and on the first few short test rides the spark timing fell off to 13 degrees BTDC as the follower wore in. Amazingly the engine ran quite well with a spark timing of 13 degrees BTDC with no stumbling or cutting out. It would even rev up to about 4,500RPM, but there was no late compression ignition and little in the way of power generation above about 3,000RPM. At first I also had a bit of trouble with the cam follower not being centered on the ignition cam as the ignition cam had pushed farther onto the tapered crankshaft than I had at first expected. This problem was easily solved by removing the points mounting plate and cutting down the mounting spacers to move the points mounting plate closer to the engine cases. Once the follower was riding on the smoothest and flattest center part of the ignition cam it wore in evenly and the timing began to stay where I set it.
The engine was running amazingly well with a static timing setting of around 21 to 24 degrees BTDC and I did not miss the stock ignition system at all. The engine was of course harsher down at less than 2,000RPM than it had been with the stock ignition, and torque generation way down to 1200RPM was not as good as it had been with the stock ignition system that backed off on the spark timing below 2,000RPM. Above 2,000RPM though the points ignition system was working much better than the stock ignition system had. Torque was bigger over a wide range of engine speeds, and the crankshaft wiggle advance allowed the engine to rev all the way out with ease even when the flame front travel speed of the fuel was only moderately fast. The feel of the clutch at the normal 2,000 to 3,000RPM engagement speed also felt much better with the flywheel removed, and all in all I was pleased to have my new 11.3:1 610 motor running every bit as well as my old 10.2:1 1991 WMX 610 had in past years.
As soon as I took off for a longer ride though a problem cropped up. The engine would run great at first, but then it would cut out and die after about 10 minutes of run time. At first I thought it was a carburetor problem, so I blew through all the passages. The bike fired right back up, but again after a few minutes it cut out and died. Quickly I realized that something was heating up and causing the engine to die, but figuring out what that something was turned out to be rather difficult.
I had heard stories of carburetors overheating and experiencing "vapor lock", but the carburetor still felt cool to the touch after the engine cut out.
I thought the problem might be with the condenser as I had experienced several condenser failures in the past year. When I switched condensers back and forth between my three bikes though they all appeared to work on the running bikes and the new 11.3:1 motor continued to cut out after 10 minutes on all of the condensers.
I was stuck on it being a condenser problem though, and I wasted many hours swapping condensers back and forth. The reason I was so stuck on condenser problems was that I had had three condensers fail recently. I had always used the same 1968 Dodge condenser on the points ignition system on my old 10.2:1 WMX 610 motor and it never gave any trouble. When I moved that same ignition system over to the 1991 WXE 350 in 2014 again the same condenser worked flawlessly for several months. Then something when wrong with the ignition system, so I replaced the condenser. As it turned out it was only the condenser mount that had failed, but a new condenser and mount got the engine going again. Then the replacement condenser really did fail three months later. When I put the old condenser back on the bike ran perfectly.
When I built the new points ignition system for the 1991 WMX 610 motor in the 1992 TE chassis I at first had trouble with the new condensers failing. Two of them failed after just one hour of operation, and each time the engine again ran perfectly when I put my old condenser on. Eventually I did get a new condenser that continued to work, but those three failures had me worried that the new condensers just are not sufficiently durable.
I should have known earlier that it was not a condenser problem this time by the way that the engine was cutting out. When a condenser fails the spark becomes very weak and the engine will not make power with wider throttle openings. This very weak spark caused by the lack of a functional ignition condenser can also make the engine a bit hard to start, but the most noticeable difference is just that the engine will not take any throttle opening without missing and losing power.
The way that the rebuilt 1997 motor was cutting out on me after ten minutes was quite different. The engine ran well up to the point where it cut out, and then it would start missing and stumbling horribly before dying completely a few seconds later. After sitting for 20 minutes or so the engine would again fire up and run normally for a short period of time.
Once I was satisfied that all the condensers I had were working perfectly my attention shifted to the ignition coil. I tried two different stock Bosch coils off of Porsche 914 cars and the cutting out problem was exactly the same with both. I also bought a new Chinese made replacement coil for a Porsche 914 like I have been running on my other two Husqvarnas, and the cutting out problem still existed.
Then one day I took off ridding and the engine cut out and would not restart. Even after waiting a considerable length of time it just would not fire up. I was not sure if it was the higher ambient temperature (as the early June weather had suddenly turned much hotter) or if it was the new Chinese made ignition coil. In either case I was quite stuck. Eventually my neighbor came by on his way back from town and we lifted the bike into the back of his pickup.
Back home I still could not get the bike to fire up. This time I was worried that something more severe was wrong with the engine. I checked the valve lash, and it was still very close to where I had set it when I first assembled the engine. After 30 hours of operation the intake valves had opened up by about 0.002", so I set them back down a bit tighter and looked elsewhere for the problem.
With the centrifical de-compressor on the 1994 camshaft it was a bit difficult to feel what the cranking compression of the engine was, but eventually I realized that if I turned the engine over backwards on the power stroke I could feel the full cranking compression of the engine with the piston going up and the valves fully closed. The engine still had perfect compression, much better than the old worn out 10.2:1 stock 610 motor that still starts up on the first kick every time.
Once I got the bike all back together it fired right up with the kick starter. This time I was wary of being stranded so I just rode back and forth up the hill from the house until the engine cut out. Again it was about 10 minutes of run time, and the engine would not restart for a long time.
With the ignition cam on the crankshaft instead of in a distributor spinning at camshaft speed there are some peculiar problems that can crop up because of all the extraneous firings of the spark plug. Not only does the spark plug fire at the end of the compression stroke when the points open, but it also fires into the exhaust when the points open at the end of the exhaust stroke. This firing into the exhaust does not cause any problems, but the fact that the spark plug also fires each time the points open is sometimes quite problematic.
When I built the points ignition system for the 1991 WMX 610 back in 2002 I at first tried to set the dwell angle down at about 130 degrees to save battery power. This did not work though as the spark plug was firing into the intake and causing the engine to miss and stumble horribly. With the points closing just after bottom dead center on the compression stroke the intake charge was sometimes being ignited before it was even compressed, and this was causing the engine to stumble and die. Increasing the dwell angle up to 180 degrees got the bottom firing to occur before bottom dead center when the intake charge was still being drawn into the cylinder, and this prevented the bottom firing of the spark plug from causing problems. With the bottom firing occurring no later than 20 degrees before bottom dead center the intake charge was not ignited and the engine ran well.
Again when I built the new points ignition system for the 1991 WMX 610 motor in the 1992 TE chassis I had the same problem. I had tried to skimp on dwell angle to save battery power and the engine did not run right until I increased the dwell angle up to over 180 degrees of crankshaft rotation.
The dwell angle on the new points ignition system on the 1997 motor was right at 180 degrees, which I thought should work fine. Having no idea what was actually wrong with the engine I began to go on a dwell angle witch hunt. I increased the dwell angle a bit to perhaps 200 degrees and took a test ride, the engine still cut out after 10 minutes just as it had before. I then tried decreasing the dwell angle to a bit to perhaps 160 degrees, again the engine ran well at first but cut out after 10 minutes.
I then tried radically increasing the dwell angle to well over 220 degrees. Again the engine ran well for a while, but cut out after it got hot. With the dwell angle radically increased the points gap was however really extremely small. I began to think that there might be some problem with the points gap being too small. With the ignition cam cut the way it was I could not increase the points gap without reducing the dwell angle.
To remedy this potential problem I ground some off of the ignition cam where the points close. I did this just by eye with a hand grinder, but it turned out reasonably smooth. The result was a more rapidly closing cam profile to increase the dwell angle while maintaining the 0.020" point gap.
All my effort was for naught though, the engine still ran well when first fired up but cut out after 10 minutes.
I was quite confused by this mysterious failure, and it took me a few days to figure out what was going on. Eventually I noticed that the points seemed sticky when the engine was hot. I took the points off, and as they cooled down in my hand the stickiness disappeared. To be sure of what was going on I heated the points up a bit with a propane torch, and sure enough they jammed and became difficult to open.
This then was the problem, the points pivot was jamming when it heated up. The points I had been using were the old set of 1968 Dodge points that I had bought new shortly after I first got the 1991 WMX 610 up and running on the points ignition back in 2002. I had run that same set of points for many years, and when the follower was finally worn all the way down I had riveted a new follower on and used them some more. Because I had not trusted my re-rivet job I had not wanted to run those repaired points, but I had kept them around as a spare set. When I built this latest points ignition system for the 11.3:1 1997 motor I had just used the old spare set of points.
The problem with the jamming pivot probably had something to do with my new aluminum points mounting plates with two points hold down screws. The stock 1968 Dodge advance plate just has a pin that comes up through the points pivot, and the points are held down with a single hold down screw through the adjustment slot. By adding the second hold down screw through the pivot I was putting an additional load on the pivot that had not been part of the original mounting arrangement. The two hold down screws had however worked just fine on the points ignition system I had built for the 1991 WMX 610 motor in the 1992 TE chassis, so it was probably just a matter of the pivot screw having been torqued down much too tight on my new points ignition system. As soon as I installed a new set of 1968 Dodge points the engine fired up and continued to run.
On the first few short test rides with the new points the timing again slipped down quite a bit as the new follower wore in. After just a few minutes of engine run time though I set the timing to 21 degrees BTDC and headed off to town. The bike was running quite well, with lots of smooth torque generation over a wide range of engine speeds. The flame front travel speed of the gasoline was not as high as what I had been getting in the past, but the good match between the pressure capabilities of the fuel and the 11.3:1 compression ratio meant that the engine ran really quite well. Under a very light load cruising along at 35-50mph in sixth gear (3,000 to 4,000RPM) the engine ran quietly in full flame front travel mode, but just a very small additional twist of the throttle got the engine into late compression ignition mode and quite a bit of torque was available.
In town I put a gallon and a quarter of 91 (RON+MON)/2 octane rating premium gasoline in the tank, and the engine seemed to run about the same. The engine ran great, but at higher engine speeds it was seeming to require an unusually large throttle opening to stay in late compression ignition mode. Still though there was quite large torque available over a wide range of engine speeds from 3,000RPM up to 6,000RPM and the bike was very usable with smooth and controllable power delivery in low gears.
When I got back from the hour and a half test ride I checked the static timing setting and found that it had slipped all the way down to 18 or 19 degrees BTDC. This was the latest spark timing that I had ever seen any of my four stroke Husqvarna engines run at without hesitation.
After another hour long ride the static timing setting was still right at 18 or 19 degrees BTDC, not the slightest bit later than when I started the ride. If anything it was seeming like the engine was lighting off on late compression ignition more easily down at 3,000 to 4,000RPM than it had been on the last ride. There was however still somewhat of a problem of the engine being reluctant to rev all the way out.
It might be expected that this inability to rev past 7,000RPM might have to do with a lower temperature of combustion potential of the fuel, but this did not seem to be the case. If the maximum temperature of combustion potential of the fuel was in fact lower, then the engine would run more smoothly down to lower engine speeds. In general I have been amazed with how smoothly and powerfully the new 11.3:1 engine runs all the way down to 3,500 and even 3,000RPM. When it is running smoothly and powerfully down at 3,000RPM though it also typically revs all the way out and makes huge power up at 7,500 to 8,500RPM.
With the gasoline that has been running well at a static timing setting of 19 degrees BTDC but not revving out all the way the low engine speed performance is still about the same. The engine is smooth enough to be usable down to 3,000RPM but it still continues to get smoother and smoother as it is revved out past 4,000RPM. If the maximum temperature of combustion potential of the fuel were actually lower, then the engine would be able to operate at somewhat lower engine speeds than 3,000RPM without extreme levels of harshness. As it is the engine becomes extremely harsh as soon as it enters late compression ignition mode at any engine speed less than 3,000RPM, and it really needs to be up at 3,500RPM or more to be fully usable.
It is not that the flame front travel speed of this 91 (RON+MON)/2 octane rating premium gasoline I have been getting is extremely slow, it is still fast enough that the big 3.86 inch bore engine runs reliably without any stumbling with a spark timing of just 18 or 19 degrees BTDC. Even with that rather late spark timing the engine will still rev all the way up to about 4,500RPM in full flame front travel mode under very light loads. It is just that the flame front travel speed is a bit lower than what 91 (RON+MON)/2 octane rating premium gasoline usually has been, and this slightly lower flame front travel speed is capping the maximum engine speed at about 7,000RPM. The crankshaft wiggle advance of the points ignition certainly helps with high engine speed performance on this slightly slower flame front travel speed fuel, but it is only of very limited utility.
With slightly low flame front travel speed gasoline the three or four degrees of crankshaft rotation of crankshaft wiggle advance that the points ignition system gets really is only a very small amount of additional advance. The slower flame front travel speed gasoline really would require a more substantial advance curve matched to the exact properties of the fuel being used.
On another day I brought two gallons of fresh 91 (RON+MON)/2 octane rating premium gasoline back from town, put it in the bike and immediately took off on a ride. Before heading out I checked the static timing setting, and it looked like it was at about 17 or 18 degrees BTDC. This new gasoline was seeming more like 91 (RON+MON)/2 octane rating premium gasoline that I had gotten in the past in that more power was being generated at 4,500 to 5,500RPM with smaller throttle openings. There still seemed to be a severe rev ceiling around 7,000RPM, but the engine was running better and making more power over a wider range of engine speeds. There was some considerable lag over a wide range of engine speeds, but the engine was always willing to pop off on late compression ignition with a bit of a twist of the throttle.
Up at over 4,000 feet of elevation though the engine was hesitating too much for trail ridding, so I stopped to check the static timing setting. The timing was looking more like 16 or 17 degrees BTDC, which was substantially less than the 18 or 19 degrees BTDC I had been running on previous rides so I bumped it back up to 19 degrees BTDC.
With the timing up at 19 degrees BTDC the engine ran much better up at over 4,000 feet of elevation with instant big torque available over a wide range of engine speeds. Even down at lower elevation the 19 degree BTDC static timing setting was seeming better, with much less lag and instant power over a wider range of engine speeds. The rev ceiling at around 7,000RPM was however still present and somewhat annoying.
It is not that the big 610 usually needs to rev much over 7,000RPM, 577cc of displacement makes plenty of power for a dirt bike down at 4,000 to 6,000RPM. It is just that losing any measure of performance is troubling.
The troubling thing about premium gasoline suddenly having a lower flame front travel speed is that there is nothing that can be done to get back lost performance. Even if a digital advance curve can be reprogrammed to remove the "rev limiter" imposed by the slower flame front travel speed fuel the engine still is not going to run well over as wide a range of engine speeds and engine loads as it did with faster flame front travel speed fuel.
On a mechanically controlled engine or a CDI ignition engine with a fixed advance curve a switch to a slower flame front travel speed fuel is particularly problematic in that nothing can be done to get the engine to rev out as far as it did on faster flame front travel speed fuel. On slower flame front travel speed fuel changing the static timing setting has a large influence on how the engine runs at lower engine speeds, but has little or no influence on how the engine runs up at the highest engine speeds. Advancing the spark timing three degrees with slow flame front travel speed fuel will make an engine much crisper down at 2,000 to 3,000RPM, but that three degrees is essentially nothing for slow flame front travel speed fuel up at 8,000RPM.
Over the next several weeks I endeavored to get fuel for the 11.3:1 engine directly from gas stations. Several times I brought a two gallon can of 91 (RON+MON)/2 octane rating premium gasoline back from town and immediately went for a ride on the fresh gasoline after draining whatever was in the tank out. I also went for a long ride on some highways and byways where I stopped several times during the day and took on a gallon and a half at a time of 91 (RON+MON)/2 octane rating premium gasoline.
Gradually the gasoline began to seem like it had a bit higher flame front travel speed again, and the engine was able to rev up to 5,500 and 6,000RPM more easily with smaller throttle openings. Operation over a range of elevations also improved with the faster flame front travel speed fuel. Up at 5,000 and 6,000 feet of elevation with a static timing setting of about 18 or 19 degrees BTDC I was still getting quite a bit of hesitation, but the faster flame front travel speed of the fuel allowed the engine to make more power in full flame front travel mode at 3,000 to 4,000RPM so that the hesitation was not so extremely debilitating to performance.
I even tried backing all the way off to 13 degrees BTDC on the static timing setting, and the engine ran fine at this spark timing with no stumbling or cutting out of any kind. Down at 13 degrees BTDC the engine also did not make much power at all as there was for the most part no late compression ignition. When well warmed up after a few pulls down at 1500 feet of elevation I could get a bit of late compression ignition at around 3,000RPM with the rather late 13 degree BTDC spark timing, but the late compression ignition did not stick around above 3,500RPM so that power output was still quite low. Going up to just 16 degrees BTDC on the static timing setting yielded big torque with only small amounts of hesitation from 3,000 to 4,500RPM and the engine would make power in late compression ignition mode all the way up to 5,500RPM once well warmed up in higher gears.
The problem of a low rev ceiling however continued to persist. I tried bumping the static timing setting all the way up to 22 degrees BTDC, and this did help a bit with reducing hesitation up at 5,500 to 6,500RPM. The earlier spark timing also made 3,000RPM operation worse though, and the engine pretty much had to always be kept above 3,500RPM to make any substantial amount of torque.
It was really feeling like the main jet was too small. The engine would pull well up from 3,500RPM to 5,500RPM with about a quarter throttle opening but then it would hit a wall and even opening the throttle a bunch more did not get it to rev much higher. Wide open throttle on the main jet got just a bit more power and a bit higher engine speeds up to about 6,500 or 7,000RPM, but it was feeling like there was just not much fuel getting into the engine at the higher engine speeds.
Since the 175 size main jet I had been using was somewhat smaller than the 180 main jet that is stock on the 1991 610 motor I began to think that there might be some reason to go bigger. It was seeming like the energy density of the gasoline was just low. Back in May the new 11.3:1 610 motor had been pulling really hard all the way up to 8,500RPM with ease on the 175 main jet, and even with that smaller main jet there was quite a bit of black soot residue around the exhaust pipe. Then when the low rev ceiling problem showed up the engine seemed to be running lean on the 175 main jet, with less noticeable new build up of black soot around the exhaust outlet.
Somewhat out of exasperation and desperation about a total loss of the 7,000 to 8,500RPM range of engine speeds I decided to try going up to a fatter main jet. I just drilled the 145 main jet that I had previously drilled out to the 175 size with a #50 drill out to the 185 size with a #49 drill. This is a fairly sizeable increase in jet size. The 12% increase in cross sectional area would tend to flow at least 12% more fuel. Because the cross sectional area increases with the square of the diameter but the wall area increases proportional to the diameter this increase in jet size from 175 to 185 in practice yields even more than a 12% increase in the maximum fuel flow rate. The main jet mostly sets the mixture up at 3/4 throttle to wide open, but all the way down to about half throttle going with a radically larger main jet does provide a slightly richer mixture. At throttle openings less than half throttle the richer main jet makes essentially no difference, and the mixture is still quite lean with the needle clip in the top (#1) position.
Dumping more fuel in certainly did help to reduce hesitation at high engine speeds, the difference though was disappointingly small. The engine was more willing to rev out to 7,000RPM and there was more power up at high engine speeds, but it still just would not rev like it had before. Still very disappointed about the total loss of higher engine speed operation I bumped the static timing setting up to 24 degrees BTDC. Again this helped a small amount with reducing hesitation and increasing power at high engine speed, but the difference was disappointingly small. The engine just did not want to rev all the way out.
At this point it was obviously a problem of a mismatch between the fuel being used and the advance curve of the ignition system. The engine would rev to 7,000RPM, but it was hesitating up at that high engine speed and kept falling off of late compression ignition. I could feel the engine pull hard for a second in late compression ignition mode, and then it would cut out and run in full flame front travel mode for a few seconds with much less power output before once again lighting off on late compression ignition and pulling hard for a short time.
The flame front travel speed of the gasoline may still be a bit lower than what 91 (RON+MON)/2 octane rating premium gasoline has been in past years, but there also seems to be some other difference. The gasoline just does not have as much power in it. This could be a slightly lower maximum temperature of combustion potential, but it could also be a difference in the amount of heat released by the gasoline. A lower density fuel that requires fatter jetting would tend to not be able to make quite as much power on a carbureted engine since more fuel evaporating in the intake tract would displace some of the intake air, resulting in less oxygen in the combustion chamber.
And of course there is also the possibility that the 91 (RON+MON)/2 octane rating premium gasoline from the past 20 years had some specialty compounds in it that were just capable of releasing more heat. This could either be in the form of a high explosive that releases heat without the need for oxygen or some specialty combustion fuel that just happens to make better use of the oxygen. It normally tends to be the case that all combustion fuels release the same amount of heat for a given amount of oxygen, but it may be possible for the fuel refiners to attain a noticeable difference in oxygen utilization by selecting certain specialty compounds at the extreme ends of the normally very small range of relative levels of oxygen utilization.
I have some reason to believe that the standard premium pump gas from the past several decades did not have much if any high explosive in it. I have several times tried radically richening the main jet on engines without getting any substantial increase in power output. When running nitro methane it is possible to get much more power just by dumping a bunch more fuel into the engine. On a combustion fuel dumping a bunch more fuel into the engine certainly can help with getting the engine into late compression ignition mode more easily, but once it is running in late compression ignition richening the mixture beyond a certain point yields only very small increases in power output. Twice on the smaller 350 Husqvarna engines when I was having trouble with the premium gasoline being for much higher compression ratio engines and requiring a whole lot of spark advance to light off on late compression ignition in the 10:1 engine I tried richening the main jet.
Back in July of 2014 the stock 10.2:1 1991 WXE 350 (with the points ignition) was making big power up at 7,500 to 9,500RPM, but it was requiring a huge amount of spark advance to light off on late compression ignition at all. With a static timing setting of 28 degrees BTDC the engine did run in late compression igition mode, but it was hesitating over a wide range of engine speeds. With the static timing set at 28 degrees BTDC I temporarily replaced the stock 145 main jet in the 34mm DellOrto with a 180 main jet out of a 40mmm DellOrto. This yielded absolutely no better performance, all it did was blow black smoke if I twisted the throttle much past half open. Going back to the stock 145 main jet and bumping the static timing setting up to 30 dgrees BTDC got the 10.2:1 engine to run crisply without hesitation. On another occasion the fuel in my new 9.7:1 386 stroker motor was seeming to be of unusually low energy density, twisting the throttle just did not do any good and it felt like the main jet was way too lean. I replaced the stock 145 main jet with a 160 main jet, and this did make some small difference. The jetting no longer felt too lean, but the engine still was not running all that well on the low energy density fuel and I had to bump the static timing setting all the way up to 30 degrees BTDC to get it to run at higher engine speed without hesitation.. The radically richer jetting did get the engine to light off on late compression ignition a bit more easily, but it also covered the exhaust outlet with black soot very quickly.
If it is not a high explosive that was yielding more heat from the 91 (RON+MON)/2 octane rating premium gasoline in the past 20 years then it is just as likely that this new 91 (RON+MON)/2 octane rating premium gasoline is made up of specialty compounds selected specifically for their noticeably worse oxygen utilization.
Again this goes to the point that it does not so much matter what the combustion properties of standard premium gasoline actually are, what is important is that it is the best compromise between refining efficiency and engine performance and that it does not change much over a long period of time. The worst thing for engine performance is when the combustion properties of the gasoline change dramatically and frequently.
At least my dirt bike is starting extremely easily with the new points ignition system. Even the rather worn out old stock 10.2:1 1991 WMX 610 motor in my 1992 TE chassis always starts easily on the first kick, but it does require a rather brisk kick to get going. The new 11.3:1 engine fires extremely easily with the points ignition, it still sometimes requires the choke and two kicks to get going when cold but once warmed up it always fires on the first kick. Just how easily the engine fires with the points ignition also does depend somewhat on the exact properties of the fuel being used. Often after the engine is warmed up it will fire on just a small causal kick, hardly more than half stab at the leaver. Sometimes though it takes a bigger more deliberate kick to get it going even when it is warmed up. Advancing the spark timing to 24 degrees BTDC also seems to require a slightly bigger kick to get the engine going than is required with the spark timing down at 18 degrees BTDC.
This slightly bigger kick required up at 24 degrees BTDC may also explain why the old 1991 WMX 610 engine has been requiring a full kick to start. With the lower 10.2:1 compression ratio I have usually been having to run the static timing setting up in the 26 to 29 degree BTDC range.
Then the gasoline changed again in early August. The lower pressure fuel had only stuck around for a short period of time, and the gas stations were quickly back to dispensing race gas sold as 91 (RON+MON)/2 octane rating premium gasoline.
One day I brought a two gallon can of 91 (RON+MON)/2 octane rating premium gasoline back from town, poured it into the 11.3:1 Husky and headed out for a ride. With the static timing setting at 23 degrees BTDC there was quite a bit of lag but the engine did light off on late compression ignition and make big torque. Up at over 4,000 feet of elevation there was a bunch of hesitation, and the engine would make torque over only a narrow range of engine speeds. I bumped the static timing setting up to 25 degrees BTDC, and this helped considerably with instant power at higher elevations. Even down at 1,000 feet of elevation the 25 degree BTDC static timing setting worked better as there was less lag and the engine seemed to use less fuel. The engine was however noticeably louder and harsher than it had been previously on the slower flame front travel speed fuel that was running with a static timing setting of 16 to 19 degrees BTDC.
Another day I brought two gallons of 91 (RON+MON)/2 octane rating premium gasoline back from town and immediately went for a ride. This time the 11.3:1 engine was extremely hesitant to light off on late compression ignition with a static timing setting of 24 degrees BTDC. The engine was making an amazingly large amount of power in full flame front travel mode, but there was hardly any late compression ignition at all.
I stopped and bumped the static timing setting up to 27 degrees BTDC and this got big power out of the engine. There was however a bunch of hesitation, wow that's some really high pressure fuel. The engine also seemed quite loud and harsh over a wide range of engine speeds. I stopped again and bumped the static timing setting up to 30 degrees BTDC, and this got the hesitation down to a manageable level. The engine was then crisp with near instant power, but it was also even louder and harsher. Just really unbelievably high pressure fuel. On this very high pressure race gas the engine was so harsh as to be practically unusable down at 3,000RPM, and continued to be rather harsh as it was revved out past 6,000RPM. Torque generation was also way down from what it had been on lower pressure fuel.
The most surprising thing about the extremely high pressure and also high flame front travel speed fuel was the way the engine behaved at slightly higher elevation. When I climbed up from 1,000 feet to 1,800 feet of elevation a bunch of hesitation showed up at the lower engine speeds. All of a sudden the engine was reluctant to light off on late compression ignition at any engine speeds bellow about 4,500RPM, but would still pop off and make power from 4,500RPM up to about 6,000RPM once well warmed up.
The engine was so loud and harsh on this new extremely high pressure race gas that I just took it out of the tank after one short ride. When I put the old gas back in I set the static timing setting back down at 24 degrees BTDC and the engine ran a whole lot better. Wow, much much more torque from 3,500 to 5,000RPM and it actually revved out higher as well, all the way to 8,000RPM and if felt like it had more to go as well.