power valve size

Discussion in 'Forced Induction Mopar' started by slowdown, May 25, 2018.

  1. slowdown

    slowdown Well-Known Member

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    Do you think 10.5 power seem like to much. The carbs both came with 10.5 power valves in them. Vac at idle cut in half is supposed to give you power valve size right. But what about if your running 2 carbs is that cut in half again? I just fouled my first set of plugs. Power valves don't come in till it's under load but but if there the wrong size could they dip into the idle circuit? 2# 750 dp holley vac ref power valves on a 6-71. It took awhile for it to foul out the plugs. I've been driving it around town without problems. When it did foul out it was on the way home from a car show in the rain. So I was driving slow and not really giving it to much pedel on wet roads. Anyway I was wondering if anyone has any thoughts on these power valves. Thanks.
     
  2. brian6pac

    brian6pac Well-Known Member

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    You need to drive it with a vacuum gauge and see if it drops below the 10.5" that will open the valves, with a blower it's always got vacuum sucking so it's not the same as a NA engine.
     
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    • slowdown

      slowdown Well-Known Member

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      Well I have 6 vac at idle. I ran around town with it an even took it out to thunder mountain to a car show before it started to foul the plugs. It ran super strong didn’t smoke but I kinda thought it still was a little rich. I decided to go back and Make sure the floats just trickled out the hole when I bumped the car an down jetted primaries, I just done this an put in a new set of plugs an holy cow it fouled them going around the block a couple of times. I’m wondering if my floats are to high now. My fuel pressure is at 6psi. The power valves are boost ref, so they get there signal under the blower. I’m going to start back at square one I think I just added to the problem with what ever I just did.
       
    • Abodybomber

      Abodybomber Breaking street machines , since 1983.....:)

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      Call the super charger company, I bet they have a basic tune..on your setup.
       
    • slowdown

      slowdown Well-Known Member

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      That’s a good idea. Being how they said 750 carbs are the way to go I bet your right, thanks I’ll call em on Tuesday after the holidays.
       
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      • brian6pac

        brian6pac Well-Known Member

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        How are they boost ref? If they are on top of the blower there is never pressure there.
         
      • slowdown

        slowdown Well-Known Member

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        They get there signal from manifold pressure.there made for roots style superchargers 80573S are the carburators I have. All I can figure is I must have messed up some were because it was running way better before. But like a body bomber said and I never thought about calling dyers, they probably do have a basic starting tune.
         
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        • Mopar-Man

          Mopar-Man Big Block Better Burnout

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          With a roots blower, you are running off of the idle circuit around town at part throttle. Try leaning the mixture screws. Do you have 4 corner idling? Pull both carbs and square up the butterflies with the transition slots, then reset your idle speed and mixture. Not sure why you need 10.5 power valves, I run 6.5 in my two Demons. My vacuum at idle is 9 inches. I am on my 2nd set of plugs in 7 years and the pistons are still in it. Must be doing something right.
           
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          • slowdown

            slowdown Well-Known Member

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            Yep you must be doing something right. Yes four corner, I looked at trans slot when I started. And I agree I don’t know why 10.5 pv. I will probably mess with it before next week when I call dyers. I’ll just go back to the start. And try again. I did have one of these carbs on a smaller supercharger and it ran great no problems. Here in Colorado I’m sure my carbs will be jetted completely different than yours when it’s worked out.
             
          • brian6pac

            brian6pac Well-Known Member

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            You are not getting the point. The carbs are still under vacuum and if you have 6 in vacuum your PV are open at all times, you need to drop to 3.5 or maybe 5.0 but you need to drive it with a vacuum gauge hooked up under the throttle plates not manifold pressure. Now you will know what your vacuum drops to and you can pick a power valve that matches your engine. It's the only way to do it.
             
          • slowdown

            slowdown Well-Known Member

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            I get what your saying. And I appreciate your input for sure. I am definitely a novice when it comes to messing with carburators. But when I bought this style of carburetor on the specs page it says these are designed to work specifically with roots style superchargers and have a boost ref pv that gets its signal from manifold pressure. So there a hose barb coming off the side that gets a hose that attaches to manifold vac. And that’s were I’m getting 6 vac at idle. Am I looking at this wrong?
             
          • brian6pac

            brian6pac Well-Known Member

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            Yes and no, I see where you getting your ref from, so now the power valve is still the same It opens at 10.5" vacuum so you have 6 " at idle the PV's are open, and it will run overly rich. I just talked to a buddy of mine that runs a lot of blower motors on the street and he is running 2 950 80577s and Holley told him to block the power valves. Hmm than why are they there? I guess I would have to pull the bowl and metering block and see if the port on the side that goes to the manifold is just over riding the port in the base plate for a vacuum signal or the one in the base is blocked. Ether way when the manifold is under pressure the PV is open and closed under vacuum so were still at the same point your PV is too big for the vacuum you have at idle.
             
          • slowdown

            slowdown Well-Known Member

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            Thanks Brian6pac that’s the kinda info I was looking for. I’m not sure why it ran for so long the first time an then fouled out so quick after I messed with the carbs. But I was wondering about the pv size I just wasn’t sure exactly how the ref pv manifold pressure thing was supposed to be working. I’ll order some new PVs before I try it again and go from there. Really thanks again for the info.
             
          • slowdown

            slowdown Well-Known Member

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            “Wahla” it’s running great. Now I just need to find the time to hook up the a/f gauge an do some tuning. Thanks again!
             
          • brian6pac

            brian6pac Well-Known Member

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            What did you change to fix it?
             
          • slowdown

            slowdown Well-Known Member

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            Looks like one of the power valves was damaged. I probably messed it up some how when I was trying to adjust it. I’m more like a monkey in a china shop then a bull. But orielley’s had a couple 3.5 power valves on hand. I swapped them out an adjusted the mix screws some and it’s back up and running again. I would actually like to take it in an have it professionally tuned. But I did get a a/f gauge so I will install it and try my hand at tuning it first. Thanks again
             
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