It's hard to say without seeing the plane, but it sounds to me like the tank is too high compared to the carb. Nominally, you would like the center of the tank in line with the jet in the carb. If the plane is designed for an upright engine, for example, and you invert the engine, you will end up with the tank high so fuel siphons into the engine at idle or when shut down. This could be why you are getting a wet plug.