Fuel Auto Shutoff

[hmrtyme]

I am building a 1936 Chevrolet pickup and it has a 261ci straight six with 3x2 Stromberg carburetors and I am installing an inline fuel pump and my dilemma is that the pump just keeps pumping when I have just the key on and so therefore it just keeps filling the bowls and intern just keeps dumping fuel into the engine which is obviously not safe for one but not good for the engine, so I'm trying to figure out a way to design or if it already has been done, a auto shutoff switch which works when it reaches 3 to 5 psi, like most modern vehicles there's are ran from the ecu, well this one definitely doesn't have one so yeah if anyone could help me out or point me in the right direction it will be much appreciated

 

[Diver300]

What sort of pump do you have?

On carburettors it usual to have a pump that only creates a few PSI. The flow of air pulls fuel from the float chamber. The fuel level in the float chamber is controlled by a valve which is closed when the float rises. Too much pressure will push the valve open and push the float down slightly.

Carburettors are normally fed by pumps that slow down or reduce their stroke when their maximum pressure is reached.

Something like this:- https://www.mgbits.com/contents/en-uk/p8020_Fuel-Pump-(New-S.U.).html is what I would suggest. Some cars had two of those to get a large enough fuel flow.

Pumps like that are diaphragm pumps, where the diaphragm is pulled back by a solenoid to suck the fuel into the pump. When the diaphragm is fully back, the solenoid is turned off and the diaphragm is pushed by a spring to push the fuel into the carburettor(s). When the float chambers are full, the float valves close so no fuel flows, and the spring movement is stopped by the pressure of the fuel, so the pump just stalls.

In an old car that I drove that was like that, when the ignition was turned on, the fuel pump could be heard to click rapidly a few times and then slow and stop as the float chamber filled up. During running, the click rate of the fuel pump varied as more or less fuel was used.

Modern engines have fuel injection of one sort or another, and they use higher pressures. On those it's common to have a pressure relief valve so the fuel pressure can't get too high, and the excess fuel goes back to the tank. That also helps to keep the fuel in the engine bay cool, and lets air in the fuel system be taken back to the tank.

 

[Nigel Goodwin]

In an old car that I drove that was like that, when the ignition was turned on, the fuel pump could be heard to click rapidly a few times and then slow and stop as the float chamber filled up. During running, the click rate of the fuel pump varied as more or less fuel was used.

Exactly, you could hear them ticking and then stop - it sounds like he's perhaps using a modern high pressure pump used for fuel injection.

 

[jpanhalt]

As implied above, check the floats. If they are really old, they my not be doing their job. I had an old CH-5 Jeep (early 1980's) with a bad float. It would flood out at stop lights, if you didn't keep your foot on the accelerator. It does seem odd that all 3 floats would fail. It is possible only one is causing the flooding. Careful examination (which one reeks of fuel odor) may help. Of course, too much pressure may also be the cause, but I would do the visual/smell check first. Actual fuel cut-off valves in carbureted cars are pretty rare, I believe.

EDIT: Since the fuel pump you are adding is electric, it would be easier simply to add a switch to that. Small aircraft have such switches. But, I would still check the carbs first.

 

[Nigel Goodwin]

As implied above, check the floats. If they are really old, they my not be doing their job.

Years ago (early/mid 70's) I had a Suzuki T350 two stroke twin motor bike, it had a repeating issue with the floats (in both carbs) stopping floating. The floats were copper, and started leaking, filling up with petrol until they didn't float any more - I 'cured' them by soldering the leaks, but more leaks eventually appeared, and I had to repair them a number of times.

 

[jpanhalt]

A lot would depend on how old the aftermarket Stromberg carbs are. I have had fewer problems with brass floats than with plastic foam floats in newer carbs. Of course, cars made this century probably don't have float carbs. Still, no one would design a car/pickup that would flood like that with an electric fuel pump, assuming its pressure is not too high. Alcohol in fuel is a killer. I had a new lawn mower carb (apparently magnesium/aluminum alloy) destroyed during one winter of storage. I switched to marine fuel, which in the US is alcohol free. Aviation gas is probably alcohol free too, but both are very expensive in the US. I only have 3 lawn tools that are 2-cycle (chain saw, hedge trimmer, and string trimmer) for which I only use marine gasoline. My other implements are diesel for with alcohol in fuel is not a concern.