Originally Posted by wingam00
I not 100% sure but think I remember reading where someone had their stock petcock leaked through the vacuum line from the stock petcock which filled the cylinder with fuel. The leaking petcock's fuel did go through the carb but through the vacuum line to the cylinder.
So an in-line shut off would not help in this example. I believe that this is rare buts has happen before.
That's pretty unlikely, as there are typically two layers to the diaphragm in a vacuum operated petcock, with the outer edges sandwiched between a spacer. The spacer typically has a drain hole in it, so that if the layer nearest the fuel gets a hole in it, the fuel will exit there. If the layer nearest the intake port gets a hole in it, there will be a vacuum leak, which will cause the engine to either idle high or die (depending on engine temp and the severity of the leak) until the engine dies from fuel starvation as the bowl empties (if the leak in this layer of the diaphragm is severe enough, the petcock will close/not open). The only way for fuel to fill the engine from a diaphragm failure is if both layers fail simultaneously and the drain hole in the spacer is plugged, or if both layers fail simultaneously and so spectacularly that the volume of fuel that leaks past the diaphragms is far greater than can exit the drain hole.