RingWinger,

Thanks for the links. The two articles gave me a much better understanding of why you could get different performance from engines which appear to be very similar. It would appear that for engines running fixed ignition timing such as one running on glow/gas, the corrected compression ration would be a good indicator of what kind of fuel mix would be required. Higher CCRs would require higher fuel octane number. Aeronut's post about needing to go to 91 octane gas on one of his engines would seem to give this some credence. I'm convinced that ignition timing is the root cause of the vibration problem since the engine does not vibrate on EI syncrospark. Since an engine running on glow is essentially operating on "fixed" timing for a given fuel, it would seem to follow that modifying the fuel mix is about the only variable which could be changed.

I'm going to determine the CCR for the Stihl and an Echo 38cc which runs very well on the standard glow/87 octane gas. I am also going to run the Stihl on an EI unit without the syncrospark unit to see at what point advancing the timing starts to cause vibration.