Fuel: I'm told (thanks fearlessfred) that the diaphram in the petcock can go bad/crack/pieces broke off and that allows fuel to go down the vac hose and into the motor.
Ah ha!! So this is why it will flood the case!!
I have been trying to sort out why you say a bad petcock will flood an engine when all carbs have floats that shut off the fuel supply, so a faulty float will flood an engine regardless of petcock or not. Or am I missing something else?