One thing I can suggest is what size prop are you running and perhaps too much load? 16*8 on petrol with spark ignition is max. 18*8 on glow and methanol. That is with a well run-in engine.

The other question if the engine wasn't new, perhaps it was already worn before it was converted?

I guess you ran in a couple minutes at a time then cooling and not too chugging away at full throttle for an hour!