+1 on the Bowman rings. I've installed them on several engines. Since the engine make and model are unknown I would have to ship at least the cylinder head and piston to him as he would not know what size ring to provide. I'll first try a smaller loading prop and if that doesn't improve top end I may just send it to him for a ring.

As a side note, I quite enjoy tinkering with engines, carbs, and such. It is satisfying to fix something when it doesn't perform as it should. I also still enjoy building models from kits rather than assembling ARF's. I'm old school. I prefer gas or glow over electric. However, I will admit it is pretty much less costly now days to go with an ARF rather than the cost of building. Covering is getting darned expensive.

Mike