I had the same problem. The fuel was coming from the two (vent?) hoses that go horizontally back from the carb under the seat. (Top tip is to make sure that when you position those hoses NEVER point them in a direction that would be a major problem if gas spurts out, ie into the air filter)
The problem on mine was a stuck valve in the carb. The floats in the carb bowl were not rising to shut off the float needle. (excuse terminology if wrong) Its happened twice now, both times after the bike had sat for a while.
I stripped the carb to fix it, but if it happened again I would disconnect the gas from the carb, drain the bowl. Then I'd squirt some gas from a squeezy bottle into the drain hole at max force and see if I could knock the floats free
Re petcock - PRI lets gas drain via gravity into the carb, until the floats rise to stop the flow.
Other two positions rely on suction, so if motor isn't turning then gas supply is shut off at the petcock.
Leaving the petcock on PRI should not flood your bike, unless the floats aren't working.