Could be the clunk, or it could also be that you have a very tiny air leak that increases during negative G maneuvers. Being a side mounted engine I don't buy the fuel being pulled out of the carb by excess G's. Might have a pinhole tear somewhere, could even be in the tubing in the tank. Normally, the pinhole stays sealed becasue it is so small. Under negative g's, the tubing flexes the right way for the pinhole to expand and bingo, air in the fuel line. Seen it before. Just replace all your fuel line (including lines inside the tank) and try it again. If it doesn't solve your problem, at least you know all your lines are good.