Convert to replaceable idle air bleeds?

Fuel and Air Systems

  1. DrCharles

    DrCharles Well-Known Member

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    Do I have enough metal here Dart Carb 3_zpstm1vhfla.jpg to drill and tap the primary idle air bleeds?
    (Not sure what the thread size is on the replaceable jets, but I could use a brass set screw and make my own).
     
  2. jbc426

    jbc426 Well-Known Member

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    I have not do it, but I've read of others doing it. I do know the sizes of the Power Valve Channel Restrictors I've changed are less than consistent. I used a decent quality drill index to gauge the size of the holes. Many were a size or 3 off or different than their markings indicated.

    Are you using a wide band gauge to target the A/F ratios you are going after?

    There is a lot of good carb tuning info on the site Taptalk. Much of it from military studies made for aircraft carb tuning starting in WW1.
     
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    • DrCharles

      DrCharles Well-Known Member

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      Yes, I have a wideband gauge on the dash ;)
      Once I get a clean transition, then I'll start going after the PVCRs. Cruise on the mains is good and lean (14.5-15.5).
      First step is to put an 8.5 or 9.5 PV in, since I now know that Holley's rule of thumb is completely wrong (there's a 4.5 or 3.5 in there now!)
       
    • jbc426

      jbc426 Well-Known Member

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      I am currently basing my power valve selection on 1/2 idle vacuum, but am going to change it based on 1/2 cruise vacuum.

      I have a good article regarding drilling, tapping and relocating the power valve channel restrictors to the bottom of the metering block, but I will have to dig for it. It made a noticable improvement in stabilizing my A/F ration at idle with the large cam I am running.
       
    • DrCharles

      DrCharles Well-Known Member

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      Interesting. Why would the PVCR have any effect at idle, since that circuit is not drawing fuel (through the boosters)?

      My idle vac is 7-8" but cruise is around 15. So I'll start with an 8.5 PV for now.
       
    • yellow rose

      yellow rose Doctor of Thinkology.

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      Yes there is enough metal there to do replaceable brass.

      I get mine from McMaster-Carr and if you are going to go all the way (I do) you'll need 6/32, 8/32 and 10/32 brass set screws 3/16 long.

      The 6/32 is for the emulsion holes, IFR and maybe one orther, the 8/32 in the PVCR and T slot restricters and the 10/32 will be your idle air and main air bleeds.

      I say go for it. You'll need a small drill set, and you should have some pin gauges wouldn't hurt and a tap set.

      I'm down for you working this through!!!!
       
    • mderoy340

      mderoy340 Well-Known Member

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      It only adds fuel when the boosters are following. If it adds at idle it is blown.
       
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      • mderoy340

        mderoy340 Well-Known Member

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        I would do both up front for adjust-ability. 8/32 brass set screws work great.
         
      • jbc426

        jbc426 Well-Known Member

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        Good catch. Sorry, I'm calling them the wrong name. The ones I changed are actually the Idle Feed Restriction of IFR. Here's a picture.


        IFR jets.jpg
         
      • yellow rose

        yellow rose Doctor of Thinkology.

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        Which block do you have??? The holley or the billet 5 emulsion block?
         
      • DrCharles

        DrCharles Well-Known Member

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        Me? The original Holley block, at least for now.
         
      • Mattax

        Mattax Just the facts, ma'am FABO Gold Member

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        Before drilling, are you measuring what is in there now? :poke:

        A reference for the locations on a 4150/60
        Holley Carb Passages Identification Pictures

        Map out the entire carb.
        Holley Carb Sheet
        not sure if that speadsheet is still downloadable. If not, find some posts from "GNTKllr" and cut and paste his maps for a start.

        Tapping the air bleeds is a pain with the choke tower in place. One reason I suggested experimenting with wires first.
        Machining main body for screw in air bleeds

        making set screws into restrictors
        drilling restrictors

        Alternatively, the air bleeds can be removed and an aluminum, brass or slteel restrictor can be pressed in. I've done that on carbs with the choke tower

        A demonstration of why PV enrichment should not effect idle mixture. This is the way it should be when all the restrictions etc are in good relation with each other.
        Effect of an open or closed PV on idle

        An explanation of why PV opening can cause a change in idle mixture is in Shrinker's posts here
        Power Valve Question

        Your older carb probbaly has the IFRs in the lower position.,
        If so, keep them there. Although YR I think I thiunk has done better with them high. But he also is further along in tinking with carbs.... I would keep it low in your case so you are making the least changes
        Relocating the Holley Idle Jet

        Lots of good stuff in here. Racingfuelsystems - Stickies
        You can join, but you do not have to. We all HATE the Tapatalk takeover of the hosting service. If you don't want to join, just close the box[X] and continue browsing like normal.
         
      • Mattax

        Mattax Just the facts, ma'am FABO Gold Member

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        Nearly worthless eyeball estimate of the primary IABs in the photo, they look to be on the larger side, maybe .074 to .078.
        If so, then the primary IFRs ought to be at .033 or larger.
        I assume you have pin gages or drill bits with shanks that aren't buggered up. Use them to measure.
         
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        • DrCharles

          DrCharles Well-Known Member

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          Yes, I know to measure what's there before I start drilling :poke:
          I do have appropriately sized drills to use as pin gauges.

          This afternoon I'll be putting the distributor back in (probably not for the last time!) so I need the carb to run the engine :D
           
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          • yellow rose

            yellow rose Doctor of Thinkology.

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            The link for your Holley carb mapping on the RFS forum says it's dead.

            Do you have another link to it???

            TIA
            YR
             
          • jbc426

            jbc426 Well-Known Member

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            I have the Promax billet block on my center carb, and was getting wide swings in idle A?F ratios. The IFRs were located up above the fuel level on that metering block.

            I read that moving them to the bottom of the passage below the fuel level would stabilize that. A fact that was discovered in WW1 airplane carburetor research. It worked, and my idle fuel level stabilized. I also went with a .035 restriction in place of the .033.
             
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            • Mattax

              Mattax Just the facts, ma'am FABO Gold Member

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              For the xls sheet? No. Tapatalk must have lost it as they did many attachments when they took over.
              "Admin" is Mark W. If your on any forum with him, maybe PM him for a copy.
              I actually have a copy of the entire backup for RFS. Problem is none of us have any real knowledge of the backend of php, and what we need to do to unpack it on a new host server. Of course we have no assuranec that all the attachments got properly saved with their links until we can unpack the backup. Sucks.
               
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              • DrCharles

                DrCharles Well-Known Member

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                Thanks for the Tapatalk links. Lots of good reading there.
                I don't think I'll have the guts to drill the passage for the 4-corner idle mod, either :rolleyes:
                 
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                • Mattax

                  Mattax Just the facts, ma'am FABO Gold Member

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                  The pics are gone but Bruce aka Booster just marked with a pen and eyeballed it. I'd was afraid of the bit walking especially when held on an angle, and also whether I was aiming at the down passage. Vizard wrote Brad Urban's shop had a jig to do it. So way backwhen I sent my 3310-3 to QF to have it converted (Urban had sold or closed his shop and before Innovate had a forum). Eventually I got it back, but they hadn't been able to convert because the outer wall isn't thick enough on -3 to do it the normal way. In the long run I was glad they hadn't been able to make it 4-corner idle. My combos aren't radical enough to make good use of it.
                   
                • 68 HEMI GTS

                  68 HEMI GTS Well-Known Member

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                  I do them all the time. Just take a little guts to get through the first time..

                  6-32 for IFR
                  8-32 for air bleeds

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                  013BB903-8F0F-49FF-BD49-7EAD8B2C9B0E.jpeg 4449B6A9-CC84-4F8D-8014-EA995D3AAC94.jpeg
                   
                  Last edited: Feb 16, 2020
                • 68 HEMI GTS

                  68 HEMI GTS Well-Known Member

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                  DrCharles, you are correct, Pv circuit has nothing to do with idle vacuum (or affecting the idle). I tune mine to come in at a vacuum where I need the enrichment. Say like going from cruise to passing. Engine in my roadrunner idles at 6.5”, I run a 10.5 in it. There is always somebody that has zero understanding of the circuits in a carb that will argue this.
                   
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                  • DrCharles

                    DrCharles Well-Known Member

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                    Actually there are some limited circumstances under which the PV can affect the idle mixture somewhat (Mattax also provided the racingfuelsystems link). But normally it does not, unless damaged (blown) :)
                     
                  • 68 HEMI GTS

                    68 HEMI GTS Well-Known Member

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                    Care to explain? Unless the boosters are active at idle it cannot affect the idle. (Providing it’s not blown or leaking)

                    but I’m all ears
                     
                  • DrCharles

                    DrCharles Well-Known Member

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                    • Mattax

                      Mattax Just the facts, ma'am FABO Gold Member

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                      That's all straight drilling and tapping.
                      The one that is risky for those of us with less skill is plugging the idle weeps and drilling the idle ports.

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