jetpack,

It depends on wether you are using a pump or regulator or not. Muffler pressure will work fine with no pump or regulator on most 2 cycle and 4 cycle engines. If using a pump (VP-30) or a cline type regulator with a 2 cycle engine you use crankcase pressure with a one-way valve to keep a pressure head in the tank and prevent back-flow. A 4 cycle uses muffler pressure with the valve if using a regulator,( I am not sure about a VP-30 using muffler pressure as I only use a cline regulator on my engines).

Phil