Haven't been down here in a while, but if the carbs are overflowing it's the float valves, not the petcock. Shouldn't matter what position the petcock is in, even if you leave it open the float valves should stop the flow of fuel to the carb when the float is full. The vacuum valve on the petcock is there as a backup incase the float sticks so you don't dump gas all over the place.
But, if the Petcock needs to be left in the prime position, there is a vacuum leak to the Petcock.
So I would say you have 2 problems, the carb float valves and the petcock.
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