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EFI To Carb Fuel Pressure Reduction Problem, Holley 12-880 Regulator

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  • EFI To Carb Fuel Pressure Reduction Problem, Holley 12-880 Regulator

    I've removed my HP EFI and have installed a new intake, distributor, and AED HO 750. I'm having trouble with the Holley 12-880 bypass regulator and cannot get the pressure below 11-12 psi. The EFI fuel pump is an AEM 50-1000 @ 74 GPH. Inlet & outlines are -6AN and I've put the smaller spring inside. Even when run with an oversize return line going into a bucket, I still get the 11-12 psi. Holley Tech Support suspects the diaphragm is stretched and a repair kit is on the way. A visual inspection of the diaphragm shows distortion so I hope this solves the problem. Is this regulator capable of an EFI to carb conversion? The return orifice in the regulator body is around 9/64". Hope someone can shed to positive light on this minor setback. Thanks!

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    Last edited by 355V8FieroSteve; 10-10-2021, 12:17 PM.

  • #2
    I would say 9/64" as a return restriction in combination with the high flow of that AEM pump, is why it still stays up that high. Also, your carburetor should be well and truly flooding at 11-12 PSI. What gauge are you using? Gary
    Regards, Gary

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    • #3
      Gary, you're right, the carb is flooding. So far, I'm not running the engine, but rather turning on different systems like electric water pump, fuel pump, etc.

      I'm using an AED 0-15 psi gauge that came with the carb and fuel line assembly. I've run the return into a fuel can twice and came up with a pressure of around 11-12 each time. I think the problem is either the distorted diaphragm is cocking and not exposing the return port fully or the 9/64 (measured with a drill bit) is a restriction.

      Removing the Fiero fuel tank is time consuming and I've had a lot of practice. Instead of changing the pump for this 427 HP dynoed engine, my backup idea is an Aeromotive dual stage regulator, PN 13220. Thanks for your input.
      Last edited by 355V8FieroSteve; 10-10-2021, 10:51 PM.

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      • #4
        Hello Steve. Your pump is more than capable of supplying your carb engine. While the engine is not running, all of the pump volume has to go back to the tank with a return style regulator. I run a Mallory 4307 on my 434. It has a much larger return seat area than your Holley regulator. Your Holley regulator return seat restriction area is fine for smaller pump capacities. I like your thinking on the Aeromotive regulator. You'll get the result you need. Gary
        Regards, Gary

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        • #5
          Gary, the Aeromotive unit arrived yesterday. Last night I connected everything and set both pressures. With no pressure set on the 1st stage, all the fuel returned to the tank with no back pressure. Aeromotive tells me the return orifice size is .313" diameter, whereas the Holley is only .141. A simple orifice pressure drop calculation with my pump flow showed around 12-13 psi drop across the Holley return. No wonder I had problems.

          This morning the car started right up with no leaks, no pressure problems, etc. Yes, the Aeromotive is pricey, but it solved all my fuel pressure problems with ONE unit.

          The Aeromotive bracket is not user friendly in tight quarters so I had to modify it.

          I would've had to buy a smaller pump just under $100 and spend maybe ten hours under the car. This took me about an hour and maybe I can sell the practically new Holley regulator. So for me, it was the perfect solution. Thanks, Steve Anderson

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          • #6
            You're welcome, good result Steve. So now you should have a rock solid fuel pressure of whatever you want, typically 5-7 psi, at ignition on, to redline. Nothing wrong with any of your parts, just a little mismatch of small regulator and large volume pump. Thanks for posting your regulator return orifice sizes. Gary
            Last edited by Gaz64; 10-13-2021, 08:28 PM.
            Regards, Gary

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