I think you are not flooding the engine, but that gas simply drops out of the carb instead of going into the crankcase. The ëxplosion with firing back into the carb is a sign of lean mixture, so very little gas had entered the engine. This idea is further supported by the dry plug.
Is your choke closing properly?
Is the carb mounted such, that fuel that drips out of the jets immediately drips out of the carb?

Try flipping the engine with the carb sloping down towards the engine and see what happens.