Clean or rebuild the carb. Most likely problem is something is either stuck in the needle/seat or one or the other has wear or damage. You could conceivably have a "sunk" float also, but that's pretty uncommon. If the needle can't close the passage, fuel will just continue to flow until the tank is empty and/or the crankcase is full.i have a john deere 11 with a 12hp briggs motor and it floods the motor with gas when i shut it off so i put a shut off valve in it but by the time i get to it it has dumped gas into the crankcase. is this a problem with these carbs or do i nreed to rebuild the carb.and will that help?