Hi!
It is also vital that you run these small COX engines on good fuel! Use at least 10% nitro....preferable 20%-25% and a good prop like a 6x3 or 5x3 ...7x4 is too large if it is a COX Babe-Bee or Black-widow.
Also see to that you use the high compression head on these engines ..originally made for the TEE-DEE 0.049 and 0.051 series of engines.

Regards!
Jan K
Sweden