How would the diaphragm behave if the output of the stock mechanical fuel pump were restricted? So basically if it were blocked off whilst the engine was still turning over what would happen?

I know the diaphragm isn't the thickest material, would it rupture?

I need to drop the flow significantly.

Why do you want to reduce flow?

G'day Sean.

I'm running carbs that prefer to be drip fed or gravity fed. Rather than make a return and reservoir (surge) tank I'd take the punt on limiting the stock pump first (budget).






So people have made surge tanks with a return and I've heard of a float and valve system. I would like to skip all of that with a simpler more logical arrangement (maybe).


Here are some thoughts:

Does the pump share a cam lobe that also works a tappet? Or does it have it's own lobe (If I recall right it does?)?

If it did have it's own lobe I would simply carve the lobe down until it raised the fuel pump arm much lower (for longer??) so that the diaphram would be moving much less, therefore pumping much less fuel. Here is a pic that might help (obviously inaccurate but representative):






If the pump does share a lobe with a tappet then would it make sense to alter the pump arm in a manner that would reduce the diaphragm lift(and fall)?


Am I making sense??

In the Philipines they use these carbs on everything and I mean everything
absolute wackos but most have success with fuel regulator at 3psi.
That is all she cried.

pm sent, I can hook you up chris...

Well that's only dropping it by 1 or maybe less than one PSI. SO they might just behave on their own.

I think there may be too many inaccuracies to get it spot on, but I read somewhere that the stock pump moves 465ml a minute? I dunno, random figures always get stuck in my head.

I reckon I could do some math and determine just how much less the pump arm needs to move.....

Can anyone confirm that this could work out? Or that the pump does in fact have it's very own lobe?

Just change the spring in the stock pump.

You can cap the output of the stock pump and it won't damage it. The pressure is controlled by the spring inside.

You want all the volume you can get, just at the right pressure. Put a lighter spring in it to get lower pressure output.

If you restrict the output you risk not getting enough flow at higher horsepower levels (full throttle).

Thankyou dd, you're a bright spark aren't ya?!

Good thinking, tahnkyou.

hehe, let us know how my theory works in practice. The stock 1200 fuel pump comes apart, you can see the springs. Maybe Holley carb springs might fit, those come in different strengths.

Or just use a regulator, that's the easiest way.

L-series pumps are lower pressure if I remember right, I have a shagged one you can have the spring out of to try...