If the engine is hot and not run lean then I would say the prop is too small and the engine revs too fast. But I usually don't know what I'm talking about...Had a OS FP-40 that would run hot and dead stick with a 10X6, went to a 10X5 and solved the problem. If you bought the engine from Mecoa or Tower, ask them, couldn't hurt. I just realized that the FP-40 problem is opposite from your problem, but I've heard that too small a prop wpould cause overheating just as a prop that taxes the engine would.