Is the engine inverted? Where is your fuel tank in relation to your carb? I have TH 75 in a bashed Four-Star clone biplane. It just has cheeks, no cowl. This engine is a power house for me, and has never given me trouble. It is mounted upright, fuel tank centered with the carb. I always leave the ground a tad rich, and let it lean out in the air. You may be leaving the ground too lean, and it leans out even more in flight.