[wacio]

Sorry Vegas I need to chime in. The Petcock vacuum valve is only for on/off. The very moment engine is running it just opens fuel and it stays open until engine is shut off. I replaced petcock with manual as many did with no ill effects. You need to cap the nipple at carb to eliminate vacuum leak.
Also this vacuum line is sadly stepped so replacing with generic hose is not straight forward. Its a lot bigger on petcock side.