This has always been the problem with the "old non Surpass" engine TOO HIGH A COMPRESSION RATIO (CR). Compound this with the fact that when it was in production the recommended four cycle fuels had about half the oil content (12%) of the fuels recommended today (18% TO 20%). Drop down to a fuel that uses about 12% oil by volume. This will help effectively lower the CR as oil is not a comprisable fluid unlike fuel vapor (in the gaseous state). The true solution that I found to work very well was the replacement of the original mild cam to a Surpass cam. The increased dwell angle (overlap) effectively lowers the compression ratio at low RPM ( high loads) yet allow for the superior breathing at high RPM levels. This is a win win situation as far as performance goes for this engine you get both better low end and high end performance. This cam is a direct replacement.

This is not just my fix but one I first read about in Clarence Lee's engine clinic in the now defunked magazine RC Modeler.

Friends don't let friends fly nickel,

Konrad