It's probably not "air"- - -it's vaporized fuel. A little vacuum on a very volatile liquid like gasoline is bound to make a little of it evaporate. That's what happens when a pump "vapor locks"- - - -there's not enough pressure provided by a weak pump to overcome the tendency of the fuel to boil in the pump and the line to the carburetor as the engine gets hot. Once a large volume of vapor accumulates, the pump stops moving liquid- - - -it's just pulsating vapor. The old practice of putting wooden clothespins on the fuel line and/or wrapping the lines with aluminum foil is simply adding an insulator to help keep the lines cool.
Jerry
No, its air. The fuel pump is really a fuel vacuum AND a pump.. it sucks fuel from the tank.. so it would be very difficult for any substantial pressure to develop in the pulsator dome as any excess pressure would vent through the outlet.... also air is compressible (its actually that very principle that makes the pulsator work.
