Check for perforations in the fuel pu or the wall of the pu tube has bubble on the inside that prevents flow at a rate to sustain optimum flow. Also check any fittings in the fuel line for hair line cracks . A guy I know blew up 6 motors due to a cracked piece of brass on the fuel cell. Try blowing air backwards thru the fuel line.....also teflon tape can plug up lines if it gets into the system.