First, remove your fuel supply line from the petcock. Then unplug the vacuum line at the back of the fuel valve. When the bike is not running, the fuel valve should only supply fuel in the prime (PRI) position. When in the on or reserve positions no fuel should be supplied.  If fuel is flowing freely, your petcock has been either altered or has internally failed.

Check the vacuum line port on the petcock. You should not have fuel coming out here, in any position, ever. Not even a drip. If fuel is leaking through the vacuum port this is very dangerous. Do not attempt to run this bike again until you have determined the cause of the fuel entering the engine oil. You can start a fire or gas wash your engine bearings, ruining the engine.