Jaguar and Butre and a couple others, you are worth reading. Thanks for being there...
I did a lot of work on cylinder head chamber shapes and volumes, and rebuilding and modifying 2 stokes on snowmobiles, bikes and most recently Blaster quads, since the 1970s. I've stuffed, timed, cut, filed, ported and swapped. My experience is with 125-250cc engines. Jaguar, I really respect your website and CDI box. Excellent work. I only have limited experience with my pre-used HT motor. I checked and set the squish while getting it reliable with an overall bolt check, but I feel I have to wade in on the head issue.
First of all, chamber shape is most critical to combustion characteristics, comparatively more than compression ratio. A wide squish band and tight flat pocket combustion chamber support low rpm torque better. The best shape for broad torque is a torroidal chamber that will support a donut shaped tornado of twirling combustion. The squish has to be set (on a 125 anyway) to 0.5 to 1mm max to get this effect. The present HT head supports high rpm power and the squish was off on mine at 1.5mm.
Jaguar is right about cranking pressure, but head volume, compression ratio and port timing all go hand in hand. Set the port timing highter and you can raise the CR without putting the cranking pressure too high. One balances the other. Compression ratio mainly affects the low rpm and tends to drive heat into the head and piston if too high. Jaguar is right about needing more timing retard at higher rpm. critical. In fact, the more power you make, the better the combustion shape, the less timing advance a 2-stroke needs.
About plugs, READ THEM. The condition of the the deposits on the insulator near the electrode tip tells you about the plug heat range. First however, you have to have the carb mixture right. That is read on a new plug insulator down next to the shell after a long full throttle run. We sometimes even cut the threads off the plug to see this area to read it. Just looking for a brown soot ring at the base of the insulator.
Even before that, check for vacuum leaks. 2 strokes are very vulnerable to vacuum leaks. Plug the intake and exhaust and pump up the cylinder and crankcase to 5-7psi with a handpump and see how long it holds it. 3-5 minutes would be a minimum. This is an important test for seals and gaskets.
Butre, I am interested in this bronze rod bearing idea. We used to use them in our universal joints when drag racing for the same reason. Many old outboard engines used them too, but used lots of oil and low rpm. Modern roller bearings have proven able to stand up, so what is the failure mode? I am guessing a bronze bushing would only be needed to overcome:
1) poor rod bearing surface hardening
2) poor pin hardening
3) poor roller quality
4) detonation problems
Thoughts?
Steve