There is no "OFF" on the petcock, only "RES", "ON" & "PRI". That was the first thing I thought of doing, but to no avail, it didn't work. I opened the drain for the bowl on one of the carbs and waited for a while to see if it emptied, the flow was steady. So then tried to clamp the furl line to that carb with a pair of forceps I have, that didn't work. So I figured I woudl get better clamps than forceps (made for this) and then empty that one carb, clean it with a Spary Can of Carb Cleaner (through the drain) and hope that works.