Does it rev high when cold or does the engine have to be hot? Compare the temperature of this engine to a proper running engine, this engine is probably way hotter. If not an air leak it could be that the jet is too small, I've had a kit carb that was not a true NT that needed opening up. The bofeng carburetor that several of us have tried shows a lot of promise, maybe try a different carb and see if that helps.