I didn't modify the ignition box yet, as I started to suspect that the issue might also be related to so called "squish velocity" inside the engines combustion chamber and how the worn out crank bearings might cause too high squish velocity.
Basically it means that when the piston comes up and squeezes the mixture on the very small squish gap, the mixture will be pushed out with x velocity. The smaller the squish gap is and the larger the squish area is, the higher the velocity. Then I speculated that if the velocity is too high, it might have undesirable effects on combustion. Maybe it would somehow "blow out" the spark. As my crank bearings were worn out, it would cause the squish gap to get smaller at high rpm, because the piston would pull on the bearings and reach higher. There is evidence of this, marks on the piston crown hitting the stock head back when it still had it. With the new head there was no new marks.
So I decided to change the bearings. I again went with low budget and bought some chinese bearings from hardware store and just ripped the seals out of them. Might be a mistake to use general chinese bearings here ha ha..
At least the new bearings felt good. There is a lot of play on the old ones and new ones can feel absolutely no play.
When I assembled the engine and tried to start it, I immediately noticed that the engine now turns a lot smoother and with less noise. I always wondered why the engine is so noisy and seems to have this high pitched rattle when running. I thought that it is coming from the pull start, but it was actually the bearings...
It also seemed to start and run better. I had high hopes but unfortunately the dyno said different.
The red curve is with new bearings. If anything, there is about 0.2hp loss on peak power and no change on the problem.
On green curve I got the most power with cold engine so far, 4hp. That was after I tried to clean the spark plug.
Speaking of the spark plug, that was the next idea to try. Maybe it would be that simple? The spark plug was years old and not very clean. First I tried to just clean the old plug by heating it with a gas torch burning old carbon away. There was a noticeable improvement at first(the 4hp curve), but then it seemed to reduce back to old level. Then I bought a new spark plug to confirm it. That didn't make a difference either.
Maybe too high compression ratio, or still too high squish velocity.
I took the head off and slightly enlarged the combustion chamber, making squish area smaller.
No change.
Then I started to try various adjustments. It feels like it's still a bit rich with the 76 main jet, but I'm missing a 74 jet, 72 was too small. I leaned out the needle and mixture screw if that could make it slightly leaner. Well, it only lost power as a result. Then I adjusted ignition timing again. It was 25 degrees constant curve, first I tried to increase it to 30. That didn't make much difference, there was maybe slightly more low end torque but more misfiring at high. Then I tried 22 and 20 degrees and with those, it started to act even stranger. Cold engine power slightly reduced and with hot engine, a huge power loss (about 0.5hp) at all rpms... it just didn't make much sense. I also tried a larger 78 main jet with the 22 degree timing, because it felt like the timing changed it so that it went lean when hot, but that was also a wrong assumption, it got only worse.
Then I sighed oh well, and thought that maybe I just continue with the original tuning plan improving the parts and see how it changes. Sometimes these things are so random that it's impossible to predict what change makes it better or worse so I might as well change something that shouldn't relate to the problem. So the next thing to improve was the squish clearance. It was around 1mm, so I took the cylinder off and mounted it in my lathe and machined 0.5mm off the top.
Turns out that I had measured the 1mm squish clearance in a hurry. It was actually less and now the piston was hitting the head lol.
To fix it, I slightly machined the head, but that was not bad since the squish band angle was already wrong, I could fix that at same time.
After that I measured that the squish clearance is now about 0.32-0.35mm. That might be too little. A rule of thumb for minimum squish clearance is 1.5% of stroke, and since the stroke is 32mm, this is now around 1%. But it was getting late so I left it. Might be a bad mistake, the piston may hit the head at high revs, I have to check for that.
Then, testing the new squish clearance.
Obviously the compression is now higher. Bike sounds much more snappy at around 2000-5000rpm. But again a bummer, after I dynoed it, it became clear that it is again a backwards step..
sorry about the bad image, had to take it with a gopro
The highest curve is the best overall hot engine curve from last week(3.7hp).
Pink curve is with cold engine with the 0.32mm squish gap. Barely 3hp. If I remember right, it still had 3.6hp just before the change.
Then when hot it just gets worse. This thing is annoying. It seems that I could easily spend all my spare time tuning it if I want to get great results.
Although these curves are still with the 22 degree ignition timing. I'm pretty sure that the worst drop in performance came from changing it from 25 to 22. That's when hot engine power dropped under 3hp.
There is a slight improvement in the high rpm region though. Now it continued to rev over 11000 even after couple of pulls although still with some misfiring. But it might just be that it didn't get so hot, because it was making less power overall.
I have to put the timing back to 25, and also test settings between 25 and 30. Although I'm a bit afraid that there might be something else going on. The cylinder is also pretty worn, I think that may also contribute to power loss when hot and makes it harder to tune.
Here's a couple of pics of the cylinder. The exhaust port is raised(cannot remember how much in mm), I have ground a boost port and slightly filled the transfer corners with JB weld, but that was a poor job and it really needs much more filler. Planning to redo it at some point. I also plan to do some proper porting on the transfers, but I would like to get the engine running well and fix the misfiring problem first, to really see the effects of porting.
I have to say I had much higher expectations at first about a cag engine power capability with tuning. Seems not easy at all. I'm still keen to improve it and solve the issues, but it seems to steal so much time that I may need to take a break..
