Are you suggesting that without the flapper in the manifold there will be no exhaust flowing through the intake? My experience says otherwise. Certainly less flow, but with today's gasoline, how much exhaust under the carb would it take to cause a problem? Given the OP's issue, it is not something that should be ignored and in fact, would be the first thing I would make sure is done. Today's gas is far more volatile than at the time that system was designed. And ANY extra heat to that fuel is a bad thing.