Qft carburetor help

Fuel and Air Systems

  1. mopardemon340

    mopardemon340 Well-Known Member

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    Got an Qft ss-850 crab with mechanical secondary. The carb works great except for a slight hesitation off the line. Change the squirter size up 2 size but still have the issue. When I move the throttle the pump arm moves instantly and I get a shot of fuel right away. I'm running a pink pump cam in the number one slot. Any insight on what to try next?
     
  2. mderoy340

    mderoy340 Well-Known Member

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    Run a blue or green cam up front, pink cams are slow. A little more engine info would help.
     
  3. mopardemon340

    mopardemon340 Well-Known Member

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    I idle at 950 and slightly lower in gear. 408stroker, edelbrock rpm air gap intake, 904 trans, 3.73 rear. What else would you like to know. How do I know if I'm getting my shot to soon or to late for my engine
     
  4. yellow rose

    yellow rose Doctor of Thinkology.

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    What power valve are you running?
     
  5. mopardemon340

    mopardemon340 Well-Known Member

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    6.5 is what came in it and still the proper one for me.
     
  6. Mattax

    Mattax Just the facts, ma'am FABO Gold Member

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    Is this 'off the line' street driving or at the drag strip?
    And if its at the drag strip, give us the best description you can of the conditions.
    For example, Foot braking and bringing it up to 2500 rpm and then going full throttle at the light.
    Does it goes then falls a bit flat (probably used up to early)? or does it hesitate then go (too late)?
    First try to get the gentle acceleration as good as you can.
    Then experiment, possibly with secondaries disconnected if that helps reduce variables.

    There's a few graphs floating around on the internet illustrating pump cam lift vs. throttle angles.
    Take all, including Holley's, as useful but not always what you may find on a specific carburetor. One reason is the pump lever contact on the cam can be alterered. There's probably other reasons too.

    Here's the one from Holley catalog. I've got a couple more if anyone wants. But this shows why mderoy340 made the suggestion he did.

    Holley Cam chart.JPG
     
  7. mopardemon340

    mopardemon340 Well-Known Member

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    It has a hesitation off the line from a stop then gos. It's not a big hesitation but its enough to notice it. I seen the graphs but it just confuses me cause I dont understand how to read it.
     
  8. Mattax

    Mattax Just the facts, ma'am FABO Gold Member

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    Then you're talking about street driving.
    OK.
    Try to accelerate as gentle as possible and see if it still happens.
    If its does, then the beginning of transisition is too lean. Could be as simple as a slight adjustment of the throttle position at idle and a slight change in initial timing.

    If it does not happen on gentle acceleration from stop, then an accelerator pump cam change is probably the answer.
    So that gets us back to the chart.
    If the throttle is halfway opened, that's about 45*; and if its fully open, thats close to 90* or straight up and down.
    The throttle blade doesn't start at exactly horizontal, so the zero on the graph is the idle position, not true horizontal.

    To accelerate from a stop on a local street, the throttle isn't going to be opened very much. Lets say its 10 degrees.
    Look at the bottom of the graph, go to 10.
    Then take your finger and go up from the 10 degree mark to the pink line.
    Look all the way to the left. That's on the first line.
    Now keep going up from the 10 degree mark and we see most of the other cams are around the second line.
    What this means is in the first 10 degrees of the throttle opening, the pink cam lifts the pump lever 1/2 as much as the others. So the pink cam causes the least amount of diaphram movement and so pushes the least amount of fuel out of the nozzles at the smallest throttle opening.

    upload_2019-7-23_21-59-48.png

    Screenshot from the 2012 Catalog.
    You can see the shape of the pink cam is very gentle at the begining compared to the green and blue cams - both have a very abrupt initial lift.
    upload_2019-7-23_22-26-24.png
     
    Last edited: Jul 23, 2019
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    • mopardemon340

      mopardemon340 Well-Known Member

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      Thanks for explaining it to me. That helps alot. I'll let you know tomorrow what happened with a standard acceleration. Right now I'm positive it happens only when you get on it from the stop. And with slow acceleration it doesn't do it. But I'll verify it tomorrow.
       
    • Mattax

      Mattax Just the facts, ma'am FABO Gold Member

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      Great. Sounds to me like it may be accelerator pump timing.
      See what you find and there may be other opinions here.

      If its not, then maybe worth going back and investigating the initial throttle position.
      If this link still works, look at the illustrated side bar on that page. It explains it better than I can. It's in Urich & Fisher Holley Carburetors & Manifolds
      If you want to experiment with that, but not drill or chamfer, there's a few other ways to do it.
      1. Your QF may have an 'idle eze' or you can crack open the secondary idle position just a hair.
      2. Change the idle speed by bumping up or reducing the initial timing a couple degrees.
       
      Last edited: Jul 23, 2019
    • yellow rose

      yellow rose Doctor of Thinkology.

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      I can promise you that isn't the correct power valve opening. I guarantee it. May not be the whole issue but it's part of it.
       
    • perfacar

      perfacar Well-Known Member

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      just curious, what is the timing in the engine? where is it set at RPM? you running vac advance?
       
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      • mopardemon340

        mopardemon340 Well-Known Member

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        The hesitation happens both when you get on it from a stop and when you take it easy from a stop. It's just not as had when when you take it easy. If I shift the car manually it doesn't have the hesitation.
         
      • mopardemon340

        mopardemon340 Well-Known Member

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        I'm pulling between 12 and 13 for vacuum in park. But I pull 10 while in gear. So i guess i need to drop to 4.5 PV since its recommended to check it while in gear with an automatic. Just watched a Holley video that said check in gear
         
        Last edited: Jul 24, 2019
      • Mattax

        Mattax Just the facts, ma'am FABO Gold Member

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        Then first we have to fix the 'idle circuit'. It's in quotes because the 'idle circuit' supplies also supplies all of the fuel at off-idle, low speed cruising, and a significant portion of the fuel mix at moderate cruising.

        Without knowing more, the first thing I would suggest is checking the transfer slot openings on the primary side.
        See where they are at now (length visible under the throttle plates) and then note the how many turns of the screw from closed to open the throttles enough to show .020", , 0.030", and .040". If you read the sidebar linked above they say .020 to .040 is the typical working range. I think with a QF its likely .040 is too much.

        End of drill bit is next to the transfer slot.
        The amount of slot exposure is about the same as the bit's diameter. A wire feeler gage can also be used.

        upload_2018-12-23_10-37-4-png.png

        The question about ignition timing, idle rpm, (and cam, and compression) is relevant because it works together.
        If warmed up the engine wont' idle with .020 to .030 depending on the current timing, giving it a little more advance may be what's needed.
        Alternatively, cracking open the secondaries a little more at idle may be an answer (or the idle-eze if so equipped)
        Finally its possible a slightly larger Idle feed restriction(IFR) or smaller idle air bleed (IAB) is needed.
        That's getting into much more advanced work, so that's why I left that to last.
         
      • mopardemon340

        mopardemon340 Well-Known Member

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        Never had these problems until I went to a mechanical secondary. The vacuum secondary was flawless and set on the dino. Only thing that's changed was the carb. Went from a holley 770street advantage to a qft ss-850
        Transfer slot is set.
        950 for idle,
        10:1 compression,
        intake lift. 353, duration @ .050° 235°/288°
        Exhaust. 353, duration @ .050° 241°/294°
        Time 18 btdc
         
      • mopardemon340

        mopardemon340 Well-Known Member

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        never had this issue until I went from my Holley street avenger 770 to a qft ss-850.
        Nothing was changed since the engine was on the dyno except for the carb.
        Transfer slot properly by an engine builder.
        Timing 18 btdc
        Idle 950
        Intake lift .353 degree at .050° 235°/288°
        Exhaust lift .353. Degree at .050° 241°/294°
         
      • yellow rose

        yellow rose Doctor of Thinkology.

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        The Holley video is wrong. Always was wrong, always will be wrong. Shame on Holley for producing ignorant shit like that. It's moronic. Anyone who tells you to pick power valve opening by idle vacuum is wrong. And always will be wrong.

        Idle vacuum has ZERO affect on power valve opening. The later you open the power valve, the bigger the hole in the fuel curve you have to fill with pump shot.

        Set your power valve opening by CRUISE vacuum. I'm betting you can use a 10.5 power valve. Probably a 12.5 if you can get one.

        Again, don't drop to a 4.5 power valve. Start over, and forget everything Holley says. If they can't get that right, it's no wonder they had to buy Quick Fuel before they went broke.
         
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        • Mattax

          Mattax Just the facts, ma'am FABO Gold Member

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          There is no such thing.
          This is not a factory set up where everything has been worked out over weeks if not months of testing and tuning.
          Tuning a hot rod or race car requires systematic trials.
          You stop when you are satisfied or give up.
           
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