wolf - I have run almost every 50-cc class engine out there but two... and ALL of them have a natural "resonance" point in their rpm range. I think it has to do with the counterweight/con rod design, piston mass, etc. Just a click below, or a couple clicks above, and it seems to go away. Mine all seem to be around 3000-3100 rpm or thereabouts. When I realized that it was just 'the way it is', I quit being concerned.

Glow engines also have a similar resonance.

I don't know enough to explain the "why" very well, but they do...