Lonnie - the "air" you see [absence of liquid, sometimes a vacuum], is the void caused by the pulsating action of the fuel pump and the intermittent opening of the carb needle - the pump has springs and valves that react to movement of the liquid and pressure change caused by fuel use, none of the flow is instantaneous or continuous - the input side of the pump is suction, the output side is pressure, the "lag" introduces the voids and the "air" comes from the tank, entrained with the fuel
Vern - I think the only time that "air" is really vapor is when the supply line is over heated, eg: vapor lock, maintaining a pressure that prevents the fuel pump from pumping
Bill
