Technical Can you adjust mechanical fuel pump pressure?

Wonder if the issue was ever solved 3 years later...

 

I asked this question some years ago and was told that it didn’t change the pressure, as the pressure is determined by the spring in the pump.
I would be interested in hearing more comments about it. - Thanks !

 

By shimming a pump you move the lever pivot farther away from the cam that drives it, effectively lengthening the lever. This shortens the lever's stroke, so the diaphragm would move a lesser volume of fuel per stroke. Less volume per stroke would reduce pressure, in the absence of any other pressure controls.

The lever preload spring only keeps the lever in contact with the cam so it doesn't clatter, it has no effect on pump output. An internal spring behind the diaphragm returns the diaphragm to refill the chamber.

 

Pressure is a result of resistance to flow. To change pressure you can either change the resistance, or change the flow.

There's not much opportunity to reduce pressure in the fuel system, except to use more fuel; but as @Halfdozen posted, you can change the volume of the flow, which should change the pressure.

Inside a mechanical fuel pump there is a spring which sets the maximum pressure the pump will distribute. That's the spring mentioned that controls the pump pressure. That is maximum pressure. When the pressure of the fuel being pumped reaches the pressure of the spring the pump simply stops pumping, which reduces flow, so it is still flow and resistance that determines the pressure.

 

I believe by shimming the pump outward, you'd reduce the stroke and compression of the spring and thus reduce the amount of pressure made. At least that's the logic in my pea-brain.

 

You are not alone , that’s how I understood it to be.

 

The spring in the fuel pump is what makes the pressure! The cam and lever only “ load” that spring! By Lessing the stroke of the lever with shims and stuff , lessens how much the spring is compressed, there by reducing pressure. Reducing the volume the pump pumps rarely comes into play, as most pumps have the ability to pump way more than most engines can use. They are built this way to ensure fuel delivery!





Bones

 

So, does the pressure control spring you're talking about hold the output valve closed when the downstream pressure reaches a certain point?

I gotta go take a pump apart and look at it, it's been a while...

 

In most fuel pumps I think it's just the diaphragm spring. Check out this cutaway drawing:


The pump arm moves the diaphragm down mechanically pulling fuel into the chamber through the inlet check valve, and the spring moves it back up pushing fuel out the discharge check valve. When the fuel pressure in the diaphragm chamber equals the pressure of the spring the diaphragm is kept from returning to the upper position, so it cannot push any fuel out. As fuel is consumed in the carburetor the needle valve opens and allows fuel into the float bowl, the pressure in the pump decreases and the spring begins moving the diaphragm up again causing fuel to flow again.

I don't think there is a separate pressure relief valve in this kind of pump, it's just controlled by the strength of the diaphragm spring.

 

Very well put! The spring is what makes the pressure, stronger spring, more pressure,weaker spring, less pressure! Simple!





Bones

 

The amount of fuel pump misunderstanding in this thread is quite entertaining. There are a few who understand that the purpose of the lever riding on the cam is to draw fuel into the diaphragm chamber, and the spring on the other side of that diaphragm pushes the fuel out of the outlet towards the carb(s). It is this spring that determines the outlet fuel pressure, assuming that the volume of fuel being drawn in and pushed out by the spring is greater than that being used by the engine.

The check valve in the outlet of the pump does not determine the pressure. It just becomes a less effective check valve when the spring created fuel pressure in the diaphragm and the fuel pressure beyond the check valve towards the carburetors is equal (ie. needle valves in the carb(s) are closed).

Further, the diaphragm in the pump does not exactly follow the motion of the lever at all times. When there is a full load of fuel in the diaphragm chamber, pressurized by the spring, and there is no fuel flowing to the carbs due to closed needle valves, the diaphragm remains motionless until the carbs allow some fuel to enter.

 

Very good!




Bones

 

This is a very timely thread to pop up. I have considered putting a regulator on my ‘50 Chevrolet with a 216. It seems that I can’t get the carburetor to quit puking gas all over the manifolds. I’ve tried two different carbs, two rebuilds on one, one on the other, new float in the double rebuild, adjusted the float low so there would be more resistance to overcome, that seemed to work for a while, but then again with the fuel everywhere. Scary! It seems that my next step should be a regulator. And possibly install the ‘49 model Carter W-1 that I found at the swap meet and toss the Rochester B 1bbl.

 

To address the oldorhidden look of the regulator For future Reference. When I put 2bbl & points on my efi 5.8 block I kept the efi in tank pump 40-60 psi, I used a small brass propane “shutoff” ball valve in my return line I keep it closed just enough to create the proper amount of back pressure in the carb fuel line. And it keeps the vintage equipment look. Any fuel shutoff valve in a return line will do as a regulator. The big shiny dial ones everyone uses are just glorified shutoffs as well.

 

Old Thread but I have to ask... How does a shutoff valve in the return line reduce fuel pressure? Seems like the restriction would need to be in the supply line to reduce pressure.

 

Restriction in the supply line wouldn't reduce pressure, just volume. I don't understand the shut off in the return line thing either.
Edit: Okay, after thinking about it, I do understand it, but I would much rather have the pressure regulated before it got to the carburetor.

 

In my case the return line is open flow straight to the tank, I connected the return line straight to the fuel pump line with the straight side of the tee, making a loop from the pump back to the tank. then the bottom barb of the tee to the carb. Then put the shutoff just past the tee (toward the tank) closing the valve results in adjustable back pressure to the carb line. If open the valve 100% there is not enough pressure to open the needle. I keep it around 75% open which gives about 5-7 psi.

 

Kind of a double breasted deal... like holding a garden hose wide open, then turning down/off the spigot. Instant loss of pressure and volume.

 

The line pressure between the pump & restriction will increase with More restriction & decrease with less restriction .