Re: 1938 Super 8 1605 - adventures with a newbie

Posted by TxGoat On 2023/4/19 18:22:29

7 hg sounds low to me, but I don't know the specification. Gasoline weighs about 75% as much as water, so a vacuum of 7" hg might raise gasoline about 10" assuming no line friction. That's not much more than the difference in elevation of the top of the fuel in a full tank and the fuel pump, with the car on a level surface.

I'd think the fuel pump would pull more vacuum than that on a closed line of low volume, and I'd think it would reach a higher reading after a few strokes and then hold it for at least a few seconds or longer after the pump stopped operating. Of course, any minute air leak anywhere in the plumbing, or through the pump diaphragm, the pump valves, or in the gauge would work to limit the maximum vacuum produced and would quickly dissipate it when the pump stopped. If your fuel pump was rebuilt with old stock parts, the diaphragm may be porous or cracked. It's also possible that one of the pump valves is leaking or sticking, or that it isn't seated properly in the pump body, which could allow leakage past it. Gasoline can be difficult to pick up with a pump, and any small problem can reduce the pump's effectiveness or prevent it from working at all. Sometimes the puickuip line inside a gas tank will devolp a pinhole leak above the level of the gasoline. This will cause no end of headaches with hot stalling, hard starting after sitting, etc. Any air leak anywhere on the suction side of the pump all the way back to the tank and inside the tank will cause these issues, especially in warm weather.