First off - if you shut the fuel off when you shut the engine off, the carb will not overflow into the air box.
That being said, the culprit is usually a defective float, float needle, float needle seat or the o-ring that seals the needle seat to the carb body. The o-ring is damaged by alcohol, shrinks and loses it's seal. With out working on the carb, shutting off the fuel will end the problem unless the petcock is damaged and will not shut off. In that case, the petcock needs replaced before doing any other work.
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