Fuel Auto Shutoff

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.

Diver300 said:

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.

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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.

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.

jpanhalt said:

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

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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.

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.

Thank you for all your replies, little back sorry my grandpa use to drag race the truck in the 50's and 60's we'll my dad said the last time he'd seen it run was in 1965, my grandpa passed away in '95 really young unfortunately, and now that my dad is retired he's put the task on me to get it running, again thank you all for your help..

hmrtyme said:

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

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This article says your fuel pump shouldn't exceed 2.5psi on the Stromberg 97 (yours).

Even turning down the pumping rate doesn't turn down the maximum pressure you get - it only takes longer to reach max pressure. I'd start with swapping out your fuel pump. Make sure it can meet the volume/minute needed at full power but not exceed pressure to overpower your check valves in the float.

hmrtyme said:

I am installing an inline fuel pump

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As has been said, that style of pump is simply not compatible in any way with conventional carburettors.

Pumps for newer fuel injected engines give anything from 30 - 70 PSI, and they is no way you can control one to give a steady low pressure supply suitable for a needle valve and float system in a carb.

You must have the correct type of low pressure, diaphragm and spring pump - either the correct (from what I can find) AC type 421 mechanical pump for the 1936 engine, or an SU electric style as Diver linked in post #2.

The originals do appear occasionally, eg.

Rebuild kits are readily available if you find an old one.

You need the new diaphragm as the originals are rotted by ethanol on modern fuel.

rjenkinsgb said:

As has been said, that style of pump is simply not compatible in any way with conventional carburettors.

Pumps for newer fuel injected engines give anything from 30 - 70 PSI, and they is no way you can control one to give a steady low pressure supply suitable for a needle valve and float system in a carb.

You must have the correct type of low pressure, diaphragm and spring pump - either the correct (from what I can find) AC type 421 mechanical pump for the 1936 engine, or an SU electric style as Diver linked in post #2.

The originals do appear occasionally, eg.

1935 - 1936 CHEVROLET CHEVY AC FUEL PUMP TYPE 421 AUTO 609454 AUTOMOBILE OLD CAR | eBay UK

1935 - 1936 CHEVROLET CHEVY AC FUEL PUMP TYPE 421 AUTO 609454 AUTOMOBILE OLD CAR.

www.ebay.co.uk

Rebuild kits are readily available if you find an old one.

1935 1936 CHEVROLET 6 CYL 181+207 ENG. AC# 421 MODERN FUEL PUMP KIT NEW FUELS | eBay

Find many great new & used options and get the best deals for 1935 1936 CHEVROLET 6 CYL 181+207 ENG. AC# 421 MODERN FUEL PUMP KIT NEW FUELS at the best online prices at eBay! Free shipping for many products!

www.ebay.ca

You need the new diaphragm as the originals are rotted by ethanol on modern fuel.

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Those are mechanical fuel pumps. They have the advantage of not needing electricity and no danger of feeding fuel if the engine isn't running. However they won't prime if there is no fuel in the carburettors unless the engine is cranked for a long time, and you can get vapour lock problems by having the suction side in a hot area, and obviously there has to be provision on the engine for the fuel pump, where its lever is usually operated by an eccentric on the camshaft.

The diaphragm in the rebuild kit for the mechanical pump is similar to the diaphragm in the electric fuel pump that I linked to before.