Holley stumble trouble

Fuel and Air Systems

  1. SSG_Karg

    SSG_Karg Stuck in the middle

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    I’m having a helluva time trying to get rid of a stumble in my tune up. It happens at very light throttle while cruising when I try to roll into it just a little. It’s like a little hiccup then it smooths right out.
    Setup:
    318, comp xe268, action plus, headers. Carb is OLD Holley 4777 650DP.
    71/82 jets
    31/31 shooters
    Blue pump cams in #2 position
    Floats are at the bottom of the sight hole
    Timing is 22/34 Vacuum pod is plumbed to spark port.
    IAB at 1.5 out
    10.5 PV
    10” hg @ idle , around 20” at cruise.
    Transfer slots are .045” exposed
    Idles at 800
    Where should I start? It runs good everywhere else and I don’t want to totally destroy the tune while chasing this damn stumble. I unhooked the vacuum pod and plugged it and the stumble was totally gone, but I would like to keep the vacuum advance.
    Should I start by drilling out the IFRs?
     
  2. 318willrun

    318willrun Utube channel 318willrun

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    sounds like a lean stumble. How much slack is in the fuel pump arm?
     
  3. RustyRatRod

    RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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    I would start by making SURE the accelerator pump is adjusted correctly. The correct adjustment will have .010" between the pump arm and the throttle arm at full throttle. From there, I would reposition the accelerator pump cam. If no luck, then proceed to different cams.
     
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    • SSG_Karg

      SSG_Karg Stuck in the middle

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      I assume you mean the accelerator pump arm and not the actual fuel pump?
      Zero slack in the pump arm. Shooters come in at the slightest tickle of the throttle.
       
    • RustyRatRod

      RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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      See post #3. LOTS of youtube videos, too.
       
    • SSG_Karg

      SSG_Karg Stuck in the middle

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      Accelerator pump arms are dead nuts. I did try the pink cams but made no difference.
       
    • RustyRatRod

      RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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      Where is your initial timing?
       
    • SSG_Karg

      SSG_Karg Stuck in the middle

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      I think I’ve watched them all haha! Seems like most say to drill out the IFRs and tap for adjustable ones.
       
    • SSG_Karg

      SSG_Karg Stuck in the middle

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      22* as stated in post #1. 34 total.
       
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      • RustyRatRod

        RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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        Oh I'm sorry. I overlooked that somehow. You might try a bigger pump shot. Normally, a bog is too much air.
         
      • SSG_Karg

        SSG_Karg Stuck in the middle

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        I was tempted to try a bigger shooter but the next biggest I have on hand is a 37. Quite a jump from the 31 that’s currently in it. Besides, isn’t that just camouflaging the root of the problem? Not sure. Seems to me that the transition is lean and that’s what leads me to think I need to open up the idle feed restriction.
         
      • RustyRatRod

        RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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        Not necessarily. Not if it needs it.....and it just may. If the bigger pump shot fixes the problem, what difference does it make? lol
         
      • SSG_Karg

        SSG_Karg Stuck in the middle

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        Should I replace both or just the primary?
         
      • Mattax

        Mattax Just the facts, ma'am FABO Gold Member

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        Most likely cause is
        Holley's Mike Urich gave the typical working range as .020 to .040"
        Secondaries of course usually not exposed at all.
        You mean the mix screws. Yes?
        That's a good start. But the important thing is that the idle changes when you adjust them.
        Reduce the throttle position, and readjust the mixture for strongest rpm in gear. Or go leaner until the rpm just begins to drop, then turn 1/4 back richer.
        NO
        Is that 22* with or without the vacuum advance hose plugged?
        I ask because at .045" t-slots that could be bringing on the vac advance a little. Probably not, but always good to be sure.
        Well here's the other possibility.
        18 to 22 at 800 rpm is a reasonable range for initial with your cam IMO.
        34 as the mechanical maximum should be OK.
        However the curve could be too quick, or the vacuum advance could be contributing at too low of vacuum.
        Measure the timing at 1200, 1800, 2400 rpm, (or more if you want).
         
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        • mopowers

          mopowers Well-Known Member

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          You can test it by backing out the mixture screws a tad and driving it to see if that helps things. If it does, it needs more fuel somewhere.
           
        • SSG_Karg

          SSG_Karg Stuck in the middle

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          Yes, the screws do affect the idle. I set them using a vacuum gauge to achieve highest reading in neutral (4 speed).
          22* idle timing is with hose disconnected and plugged.
          I will map timing curve and report back.
           
        • Mattax

          Mattax Just the facts, ma'am FABO Gold Member

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          Blue cam is already an aggressive pump shot. Hate to be washing down the cylinders chasing more pump shot.
          Small and gentle throttle inputs shouldn't need much shot if any.

          Is tis light throttle hiccup cruising at 25, 45, or 65 MPH?
           
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          • RustyRatRod

            RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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            It sounds to me like your problem is on the primary side.
             
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            • SSG_Karg

              SSG_Karg Stuck in the middle

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              I’m sure it’s a lean stumble and needs some fuel added to the transition, just not sure on best way to go about adding it.
               
            • Mattax

              Mattax Just the facts, ma'am FABO Gold Member

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              That's OK method. Maybe give them an 1/8 turn richer which should produce the same reading. It will be stronger when under load - which with the clutch is not as noticible as with the automagics but its still real.
               
            • SSG_Karg

              SSG_Karg Stuck in the middle

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              It’s more throttle position dependent than speed dependent. Mostly cruising between 2k-3k while just barely on the throttle. Above 3k is about 60mph and its not there.
              Agreed
               
            • SSG_Karg

              SSG_Karg Stuck in the middle

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              As in small main jets? I’ve increased from 68 to 71 on the primary side with no improvement.
               
            • RustyRatRod

              RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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              No. The jets are not on that circuit.
               
            • mopowers

              mopowers Well-Known Member

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              try a larger pump shooter on the primary side.
               
            • Mattax

              Mattax Just the facts, ma'am FABO Gold Member

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              Your reasoning is sound. Experminenting will prove whether its the actual issue.
              The transfer slot open to .045 at idle means the slot is contributing a large portion of the fuel at idle closed;
              And then runs short of fuel as the throttle opens past.

              See if this works.
              Holley

              IMO, experiment with the smaller t-slot first.
              Then depending on how that's going, measure the primary idle air bleeds. Then stick some .020 to .030 wires in them. Loop the wires down under the air cleaner gasket. That's a quick an effective way to make the idle -especially the transition - richer.
              You can do the math if you want to figure the new equivelent IAB diameter with the wires. Calculate the area of the current IABs. The area of the wire. Then subtract and figure the diameter.
               
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