Which Holley Power Valves in a L78 ? - NCRS Discussion Boards

Which Holley Power Valves in a L78 ?

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  • Wayne M.
    Expired
    • March 1, 1980
    • 6414

    Which Holley Power Valves in a L78 ?

    Back in 1992, Bob Kunz told me that both primary and secondary metering blocks in my model 4150 Holley LIST 3124 take the "85" power valves (ie. they open when manifold vacuum reaches or falls below 8.5 inches Hg). GM info hard to find, as the Interim Shop Manual ST-71 (spring '65) only mentions the primary jets (#72) and the secondaries (#76).

    Well the last few times I started my car, it ran like (censored); poor idle, soot out the exhaust, etc. Some spark plugs were sooty and dry; others wet with smell of gasoline. Turns out the secondary metering block power valve diaphragm was ruptured, and fuel was draining into the intake manifold. That's what I get from driving it (say) 20 miles in the last decade .

    Just wondering if others are using "85" power valves on their 3124's. Also, do both PV's have to be the same rating ? What happens if you have [to use extremes] a "25" on the primary and "105" on the secondary block ? Is there a chance that the secondary power valve will open before the secondary barrels (throttle plates) open, as these activate by signal from the boost venturi (?), and not manifold vacuum AFAIK. Or should the primary always be an equal or numerically higher rating than the secondary ?
    Last edited by Wayne M.; June 7, 2016, 05:03 PM.
  • Joe L.
    Beyond Control Poster
    • February 1, 1988
    • 43219

    #2
    Re: Which Holley Power Valves in a L78 ?

    Originally posted by Wayne Midkiff (3437)
    Back in 1992, Bob Kunz told me that both primary and secondary metering blocks in my model 4150 Holley LIST 3124 take the "85" power valves (ie. they open when manifold vacuum reaches or falls below 8.5 inches Hg. GM info hard to find, as the Interim Shop Manual ST-71 (spring '65) only mentions the primary jets (#72) and the secondaries (#76).

    Well the last few times I started my car, it ran like (censored); poor idle, soot out the exhaust, etc. Some spark plugs were sooty and dry; others wet with smell of gasoline. Turns out the secondary metering block power valve diaphragm was ruptured, and fuel was draining into the intake manifold. That's what I get from driving it (say) 20 miles in the last decade .

    Just wondering if others are using "85" power valves on their 3124's. Also, do both PV's have to be the same rating ? What happens if you have [to use extremes] a "25" on the primary and "105" on the secondary block ? Is there a chance that the secondary power valve will open before the secondary barrels (throttle plates) open, as these activate by signal from the boost venturi (?), and not manifold vacuum AFAIK. Or should the primary always be an equal or a higher rating than the secondary ?

    Wayne------


    Well, I get a different story depending on where the information is coming from.

    It looks like Holley says that a 125-85 power valve was used for the primary and, possibly, also for the secondary (not clear about secondary)

    GM says the primary power valve was GM #3852470, aka Holley #25R-219-65. The secondary power valve was GM #3877167 with no reference to Holley number.

    In any event, if I were replacing a power valve today I'd definitely want to be sure it's a "blow-out proof" type.
    In Appreciation of John Hinckley

    Comment

    • Timothy B.
      Extremely Frequent Poster
      • April 30, 1983
      • 5186

      #3
      Re: Which Holley Power Valves in a L78 ?

      Wayne,

      It's manifold vacuum that controls the opening of the power valve and venturi vacuum that opens the vacuum secondary on a Holley. It's also venturi vacuum that signals fuel flow from the booster venturi's.

      To answer your question, the secondary power valve could open before the throttle blades provided the manifold vacuum drops below the P/V vacuum threshold . This power valve is another tuning tool, many Holley's don't use one and just have a larger secondary main jet. If the valve did open it would simply enrich the fuel mixture at booster start up and at worst create a over rich mixture. If it's removed and a plug installed the secondary jet size needs to be enlarged.

      I would think the power valve needs to be the same vacuum as the primary valve.

      Comment

      • Ken A.
        Very Frequent User
        • July 31, 1986
        • 929

        #4
        Re: Which Holley Power Valves in a L78 ?

        You need 2 #65 power valves. There is no such thing as a "blowout proof power valve", however, there is a check ball kit that Holley sells to prevent a backfire taking out the power valve.

        Comment

        • Joe L.
          Beyond Control Poster
          • February 1, 1988
          • 43219

          #5
          Re: Which Holley Power Valves in a L78 ?

          Originally posted by Ken Anderson (10232)
          You need 2 #65 power valves. There is no such thing as a "blowout proof power valve", however, there is a check ball kit that Holley sells to prevent a backfire taking out the power valve.

          Ken-----

          For some reason, I thought that Holley had introduced power valves that had integral protection from back-fire induced blowouts although I never actually sought one out. However, I see that I was incorrect. Perhaps Holley includes the blowout protection feature on later manufactured carbs.
          In Appreciation of John Hinckley

          Comment

          • Timothy B.
            Extremely Frequent Poster
            • April 30, 1983
            • 5186

            #6
            Re: Which Holley Power Valves in a L78 ?

            Joe,

            I believe you are correct in that all the new Holley's come with the blow out protection feature, the kit can be retro fitted to the old carburetors if desired. Another thing to not overlook with these power valves is the gasket leaking fuel. Manifold vacuum will find air or fuel anywhere it can.

            Comment

            • Ray K.
              Very Frequent User
              • July 31, 1985
              • 370

              #7
              Re: Which Holley Power Valves in a L78 ?

              Wayne

              I have used both #65 and #85 Holley power valves for my L78 over the years and although I prefer the #65 I have had good results with the #85 as well. A Holley " major " rebuild kit that I purchased came with #65's in it. Original power valves has the entire surface of the diaphragm exposed where as later units have a metal shield covering the diaphragm with a small hole in the center. I think this was Holley's interpretation of blow out prevention. I have had power valves fail and running poorly with lots of blue / black smoke is a good indication. However, as Tim pointed out, the gasket for the power valve is many times the source for leaking fuel into carb / manifold and also causing problems with staring and idling.

              Ray

              Comment

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