I'm not sure how new you are to the RC world, so you may already know these things. When I started a few years back, I didn't put the prop washer on before the nut, as in spinner backplate, then prop, then washer, then nut, then spinner cone. I tightened that nut down so many times, but it still came loose until someone asked me where the washer was, which fixed that. If you use a spinner nut, all you need is the prop, then the spinner nut.

Is the engine mounted inverted?

Do you disconnect the muffler pressure line when you refuel? If not, that will flood your engine sometimes.