1986 Onan BFA 4 - Starts every time, but runs just 5 sec.

A test you can try for the fuel pump is take a gas can put a hose into it connected to the fuel pump of the generator. Start the generator and if the filter fills up with gas and the generator keeps running then you have a problem along the fuel line to the tank. By the way most gas tanks have a hose between the tank and the hard line along the framerail. That way the tank can be dropped some to disconnect the lines.