I'm probably going to get flamed for this on a Camaro site but I'm going crazy trying to get my engine running right. I transplanted a '88 305ci out of a Camaro into my '89 Mazda RX-7. It has a Edelbrock performer intake and a Edelbrock 1405 carb. I'm loosing fuel pressure. I started out using the stock Mazda electric in-tank fuel pump with the stock 5/16" fuel lines, both supply and return. I bought a holley bypass style pressure regulator to drop the 40+ psi down to 6psi for the carb. I ran a bigger 3/8" line from the regulator to my carb. I could set the pressure at 6psi, but the longer the engine ran the pressure gradually keep dropping to where the engine would stall. Thinking that the old Mazda pump might be going out I bought a Holley blue pump with regulator. Mounted it out side of the tank and left the in tank pump in place. Still using the existing 5/16" hard line up the the regulator and abandoning the return line. Same thing happened, fuel pressure still gradually dropped till engine stalled. Now my question, Is the carb being starved by the 5/16" lines? Do I need to run 3/8" line from inside the tank through the new holley blue and all the way to the regulator? or could something be wrong with the carb? Any Ideas??? Thanks -Jeff

Could be a cavitation issue since you're pulling through the stock pump.
I would personally remove the in-tank unit...

Are there any other filters between the pump and the regulator?

How close is the Holley pump to the fuel tank?

I have a filter 6" before the regulator on the 5/16" line.

The holley pump is mounted level with the bottom of the fuel tank with a foot long section of 5/16" hose.

I don't think it's a problem pulling through the intank pump because I got the same result running either of the pumps. I'm thinking that the 3/8" line from the regulator to carb is flowing more volume than the 5/16" line before the regulator, and the carb is sucking it dry the longer it runs, dropping the pressure, does that make any sense?

-Jeff

i doubt at idle that carb is sucking ANYTHING dry. Remove your gas cap and see if that makes a difference, if not check the lines because it sounds like vapor lock.

No it does not make any sense at all.

Leave mysticism and voodoo out of it; and stick with possible explanations that involve simple physical principles. See my signature for help with logical thinking.

To start with, there's NO WAY that having a short section of a larger diameter fuel line somewhere in the system, can possible "suck dry" anything. That would imply that the short section of larger line is somehow sending fuel somewhere besides the carb. If a large amount of fuel isn't ending up on the ground somewhere, I think we can safely rule that out.

Next thing is, whatever you've changed so far, hasn't affected your problem. That means, the root cause is something you HAVEN'T changed.Explain, logically, how you can possibly come to this conclusion from the data at hand. "I don't think it's this one thing because no matter how much I fool with this other unrelated thing, the problem stays the same". In fact, logic points you the OTHER way: you've tried 2 fuel pumps, the problem is unchanged, therefore all you've accomplished so far, is to establish that the cause probably isn't the fuel pump.

Fuel pumps are well and widely know to be unable to generate suction. ALL of them. They're good at generating pressure, terrible at the other. Meaning, if there's an obstruction in the fuel system between the fuel and the pump, they can't deal with it. Should be pretty easy to eliminatet he voodoo-type explanation you proposed, by unhooking the line from the pump output, and putting a short section of hose there that dumps into a gas can, and run the pump; and watch how much fuel the pump can move. I'm betting you'll see quite quickly that the pressure-side line is not a factor in the pump being unable to mve fuel.

It would seem that a great place to start working on this problem, would be to eliminate potential restrictions on the suction side of the active fuel pump, since that IS the simplest explanation that fits all of the facts at hand. First thing you need to do therefore, is to take that inactive in-tank pump OUT. As long as it's there, it's going to give other pumps a hard time sucking fuel through it. Nothing else you do, will overcome that. I'm afraid you're just going to have to bite the bullet and address the cause of the problem, if you want the symptom to go away.