If the fuel is old change it. That may be the cause in these days of ethanol in fuel
I have had ethanol destroy fuel hoses. It is easy to remove and replace. Also the cork in the fuel pump glass filter dries out and leaks air. NAPA has rubber ones that are better.
I have an electric pump wired in ready to hook up instead of the manual pump. I just plumb it in. BUT!!! go straight from the electric pump to the carbs. ByPass the manual pump. Bad things can happen if You do run the fuel thru the manual pump while using the electric pump.
Another way to insure it is a fuel problem is take off the cover of the SU float bowl and look inside. If there is fuel it is an ignition problem. No Fuel than a fuel problem.
