power valve block off

Fuel and Air Systems

  1. phaelax

    phaelax Well-Known Member

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    Was tearing down my holley 750 dp to rebuild it when I noticed the power valves were blocked off. From what the last owner told me, the guy who built the engine primarily used the car drag racing. I don't really go to strip, just drive around town for fun, so should I replace these? Would I notice much difference?
     
  2. rklein383

    rklein383 Well-Known Member

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    The power valve is part of the intermediate or part throttle circuit in your carb. For street driving, you absolutely need it. I think you will notice a considerable difference. You probably will need to change main jets as well.
     
  3. momoparman

    momoparman MOISMYHOME

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    IMO

    Yes you should install them. (Simple Explanation) They work off vacuum and aid in provide the fuel that helps idle and drive-ability at low RPM's. Figure out what came stock on the 750, and work from there. It's based on engine vacuum so you need a Vac guage. While your in there, I would check the jetting as well. Hook up an A/F gauge, if your able too, so you can tune it properly.

    Here is a link to some basic info on set up and how it works.

    Choosing the Right Power Valve for Your Holley Carburetor

    Good luck
     
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    • JBurch

      JBurch FABO Gold Member FABO Gold Member

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      If you are gonna be doing mostly street driving, put a power valve in the primary side. It's purpose is part throttle operation; street driving! I can't say with certainty what will be exactly what you need, however a start would be a 6.5 P/V and drop primary jets 4 sizes from where they are now. It would be helpful to know what your idle vacuum was before you started. That information would lead you to P/V selection. To be sure it's gonna be trial and error.

      Good luck.
       
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      • Locomotion

        Locomotion Well-Known Member

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      • phaelax

        phaelax Well-Known Member

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        For the jets, looks like I have 28 for the primary and 35 secondary. Based on info I found online, about 99% of 750 holleys were 28 and 31, so I'll drop down to the 31. (which I have from another main body I just pick up on ebay) As for the PV, I'll use the 6.5 the rebuild kit comes with and see how the car runs. This makes JBurch's info sound right on point. (I don't have a vacuum gauge on hand)

        I'm also installing a manual choke which it's lacked. Thanks for the all the info guys.
         
      • 70aarcuda

        70aarcuda Master Hoader of SBM FABO Gold Member

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        I think you are reading the squirter nozzle...not the jet size...primary jet 71 and secondary 80...that would be with a power valve in the primary side and the secondary side blocked..
         
      • phaelax

        phaelax Well-Known Member

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        Yes that's what I meant. I'm waiting for the car to charge, battery completely dead. I think I have a drain somewhere.
         
      • phaelax

        phaelax Well-Known Member

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        Well, looks like my first rebuild didn't go so well. Seemed pretty straight forward and simple to me. But I've got no fuel shooting from the primary, and the secondary is shooting fuel up the vent tube when I try to start. Only thing I can think to recheck is the float and needle, which so far appear to be fine.
         
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