starts breaking up at 4500 rpm.

[west coast]

Are you changing pump setups between the 2 systems? Your pump my not be putting out enough flow at the higher pressure 40 and above, but put enough flow at 7 psi for a carb. I have run as fast as 11.60 with all the stock fuel lines in my car with just one size up pump. This is also a 5.0 mustang.

[Sean Marconette]

If it's an Aeromotive A1000 it can do either carb or EFI, with just a regulator change.
How is the fuel pump positioned and where is it relative to the cell sump? Aerated fuel is possible if the elevation of the pump has to draw the fuel out of the cell. The pump should always have a column of fuel sitting on it, otherwise the vacuum it will pull can make the pump cavitate.

With the foam removed, the return could also aerate the fuel in the cell, unless as others have stated it has a length of hose to eliminate fuel dumping above the liquid level in the cell. With the foam installed that eliminates this issue.

What pump do you have?

Sean

[partsbob67]

thanks for all the responses. was an aeromotive 750 pump and an Mallory regulator on a 94-95 fuel rail. changed to a aeromotive regulator on the rails. then changed to 87-93 rails and a aeromotive reg on that. fuel pressure starts at 40psi and drops to 36psi. then I put a holley pump on it and it did the same thing. voltage is steady at the pump. pump is mounted right below the cell. maybe to large a return line?

[buzzinhalfdozen]

You could try installing some sort of restrictor in the return line, just to see if that's the problem again -8 seems big for return. I have an -8 return on mine but it has a -6 to -8 adapter on the regulator when I'm using the EFI setup so it has a smaller hole in that fitting. Really should not be seeing a pressure drop, either your pump isn't up to the task or for some reason your injectors are flowing way more fuel than they should be. Have you verified your tune up?

[Dick Butler]

Where are you picking up the fuel pressure for the gauge? For control of pressure on Q jet car needed larger line for return than supply or the line created Higher pressure than regulator could reduce. Obviously with injection needs that is different but maybe where you are plumbed in with gauge isn't giving pressure at correct area for operation information. (I bet on return line air creation getting in pump.) Would explain drop in pressure by bubbles in line.

[MikeMoller]

You might also check your battery/connections/grounding etc. Low voltage will cause the injector pulse to be slow and dump to much fuel per pulse.

[Marty Buth]

Are your fuel injectors sized properly for the HP you are making? Have you had all of the injectors flowed to make sure one or more of them aren't flowing too low?