[Tommy]

Your Holley pump has an internal pressure regulator that limits output pressure by diverting fuel coming out of the pump back to the input side. The valve that diverts the fuel is controlled by a spring that is supposed to move at about 7 PSI. If the spring is not just right, it could open at 5 PSI and that is all the pressure you would see.

Fuel circulating through the Holley pump's internal regulator is subject to heating from the pump and can lead to vapor lock. The usual solution is to install an external pressure regulator that controls pressure to the carb by diverting excess fuel back to the fuel tank. Of course the external pressure regulator can only do its job if the fuel leaving the pump is at a higher pressure than that set on the external regulator. If, as in your case, the pump is putting out less pressure than is set on the external regulator, the external regulator will never sense enough pressure to divert fuel back to the tank. Thus the external regulator is useless in this case.

I learned this by suffering vapor lock on my Holley red fuel pump. I solved the problem by adding the external regulator and return line, AND by replacing the stock spring in my pump with a much stronger one. The stronger spring effectively prevents the internal pressure regulator from working, allowing the external regulator to do its job. I suggest you either do the same or replace the Holley red pump with an unregulated pump.