Biggus

iTrader: (1)

I've been trying to diagnose a fuel pressure issue. Twin 671 blown 468's. Holley 130 gph mechanical pumps with adjustable regulators.

One motor runs 9 lbs at idle and no less than 6 lbs wide open 6 lbs boost.

The other motor is 7 lbs idle (with the regulator adjusted all the way down) and 3-4 lbs wide open.

Both fuel systems are identical with 1/2"lines from the tanks and blow back valves removed.

I've changed fuel pumps, regulators, filters, swapped senders and guages. Checked for a restriction from the tank and still can't seem to get the pressure of the other motor.

Does float level have an effect on fuel pressure?

Thanks,

Kurt

GLH

Did you switch gauges to rule that out?

My pressures are the same as your first engine 9 and 6

Biggus

iTrader: (1)

Yes, I swapped guages from side to side, senders too. Installed new pump and regulator. Even with the regulater adjusted all the way down I can't get 9 lbs at idle. Infact adjusting the regulator when idling seems to have very little effect (about a pound or so) same with the old regulator.

The motor runs great and is not loading up/black transom.

I'm kinda stumped.

formula31

How about swapping the carbs? Could it be that you have a leaking float valve? Can you borrow someones electric fuel pump and use it to pressurize the carbs without the engine running and see it you have fuel running into the manifold. Just some ideas.

Big Boys Toys

Kirt regarding the fuel lines from the tank to the fuel pumps, is it possible thay you may have a couple of additional 90 degree fittings in one line verses the other? Another consideration is that I have seen different fittings identical on the outside but some have a smaller inside diameter, could this be a possibility? I have also seen different size 1/2 " tubing, some have a heavier wall meaning a smaller inside diameter.
Just tossing a few things at you seems you have covered all other bases. I worked on a boat once that had a plastic cap floating around in the gas tank, and every so often it would get caught on the inlet tube and cause the motor to be laking in fuel. We went through many items before we finally found the problem. It may be worth looking at the end of the tube and inspect the inside of the tank.
Good Luck
Brian

Big Boys Toys

Kirt regarding the fuel lines from the tank to the fuel pumps, is it possible that you may have a couple of additional 90 degree fittings in one line verses the other? Another consideration is that I have seen different fittings identical on the outside but some have a smaller inside diameter, could this be a possibility? I have also seen different size 1/2 " tubing, some have a heavier wall meaning a smaller inside diameter.
Just tossing a few things at you seems you have covered all other bases. I worked on a boat once that had a plastic cap floating around in the gas tank, and every so often it would get caught on the inlet tube and cause the motor to be laking in fuel. We went through many items before we finally found the problem. It may be worth looking at the end of the tube and inspect the inside of the tank.
Good Luck
Brian

Turbojack

1-Try mech. gauge. first on one engine & then on the other. That will take out any gauge/sender & wiring/ground problems.
2- Switch fuel lines from tanks to the other engne. If problem moves then tank/ fuel line related.

mcollinstn

Leaking float valve can affect fuel pressure.
Different float heights (within a range) should have no influence on the fuel pressure ( but fuel pressure has a tiny influence on float height... ).

Mbam

Due to the low volume of fuel used by the engine at idle, there is almost nothing that should affect the pressure aside from the pump output pressure and regulator. Absolutely use a mechanical gauge to verify. Would not hurt to put a piece of clear tubing just before the carb to look for air bubbles - this is for testing only, DO NOT LEAVE IT THERE as if it pops you will be in deep doo doo.