650cfm QFT Carb Tuning / 383 / 727

Discussion in 'Fuel and Air Systems' started by Nico383, May 13, 2018.

  1. Nico383

    Nico383 Well-Known Member

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    Hi Gents,

    I am looking for a set up for my QFT 650cfm double pumper carb as I can't find help over here in France. I would need a configuration of jets, squirters and power valve as a basis I could adjust afterward.

    I did some tests but I get lost with my AFR readings. When the carb was stock I thought the Jets were way too big. I changed them, now I'm back trying to adjust the squirter size but now, with secondaries disconnected I get AFR reading around 20...

    I did try to tune my carb last year and ended with this configuration :

    Primary jets : 62
    Secondary jets :74

    Primary Squirter : 42
    Secondary Squirter : 45

    PV: 6.5

    But honestly I don't find it runs well. When accelerating from zero at WOT, it stumbles.

    Engine is a 383, with this camshaft :
    Trans is a 727. The car is a 1970 Duster.

    Cheers,

    Nico

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    Last edited: May 13, 2018
  2. Mattax

    Mattax Just the facts, ma'am FABO Gold Member

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    Nico,
    Too much depends on the air bleed configuration to give a simple answer to a simple question.

    I'll answer two ways.
    If the only problem is 0 to WOT, then the problem is entirely in the idle set up and accelerator pump. If it still does this with secondaries disconnected, then its all in the primary side. That's a huge squirter for the situation. Try a quicker acting pump cam and smaller squirter. Make sure the cam moves the pump arm as soon as the throttle moves.

    It's probably not the only issue, so here's in general how I suggest performance tuning.
    1. Check Fuel level in bowls. Use the QF recommended starting level.
    2. Timing. Timing has to be close to what the engine wants or the fuel mixtures can't be properly tuned.
    With the cam you posted, initial timing probably should be around 16 to 19*, 700 - 800 rpm. Do a search for timing curves. I've posted a lot of stuff recently.
    3. Adjust the carb's idle setting so the amount of transfer slot visible underneath the primary plates is between .020 and .040" (roughly square). The first time you do this, it has to be done with the carb off of the engine. Take notes on how many turns of the screw is required and you won't have to remove it again for this purpose. I'd start with .040" and as the engine warms up you can reduce it a little.
    4. Engine fully warmed up, adjust the idle mixture. Set for highest vacuum or rpm when shifted into D.

    Test and tune for steady conditions:
    Off idle & Low speed cruise. Slowly open throttle (driving if you can) from stop. If there is hesitation, the idle circuit will need adjusting.
    It should get leaner as the throttle opens.

    High speed - steady: Driving steady at 60 - 70 mph. This is adjusted with the primary main jets. Just go for good driving for now. Later you can go as lean as it will tolerate and get better economy and part throttle accleration.

    Then work on the accelerator pump circuit if it hesitates.

    Test Wide open throttle acceleration. You can adjust the AFR with the main jets. But if you later find a big difference between best high speed steady and best WOT power, that can be addressed.

    Then work on part throttle power. If there's big hesitation around 2/3 throttle, try an earlier opening power valve, an 8.5 or a 10.5

    If you want to get deeper into it, or continue to have problems, then you may want to take the carb apart and check everything. You'll need pin gages or micro-drills to check all of the holes.
    QC at these companies sometimes isn't what it ought to be.
    See here for example: Q-850 metering block

    And if you're going that far, you can get help coverting it to be setup like an old school Holley 650 double pumper. But without knowing the air-fuel correction bleeds of your QF carb I hesitate to recommend copying the jetting of a Holley list 4777 or similar. That said, a 62 is pretty small primary jet for that size carb.*

    As far as AFR goes, please read this: Wideband

    The AFR meter is nice tool to help understand changes, but tune for performance. By that I mean things like power at idle (represented by vacuum in Drive), economy on the road, and max power by best speed in a 1/4 mile run or Hp on the dyno.

    *another factor in jet size will be the booster. Some QF carbs use annular, many use downleg, etc.
     
    Last edited: May 13, 2018
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    • Nico383

      Nico383 Well-Known Member

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      Hi Mattax,

      Many thanks for this great answer.
      I may not be that far from a good solution. I think that without knowing I've been trying to compensate a lean condition setting bigger pump nozzle whereas it may come from the pump shot cam.
      I'm going to go from 62 to 64 on primary jets and try to come back from 42 to 40 (or 37?) on the primary pump nozzle but changing the cam from a pink to a green or blue one. I'll keep you posted!

      Thanks again for your help!

      Nicolas
       
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      • Nico383

        Nico383 Well-Known Member

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        Hi guys,

        I did some tests, went from 62 to 64 on primary jets, from 42 to 37 on primary pump nozzle and changed from pink to green cam. Results are much better! I even gave it a go with the blue cam that I think I will stick with.
        Going from 0 to WOT as never been that fluent since I own the car ;)

        I still have one question : how can I know what pump cam and pump nozzle fits the best to secondaries?

        Many thanks for your help!

        Nico
         
      • mderoy340

        mderoy340 Well-Known Member

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        a 650 should only need a 31 or 33 nozzle. Single plain manifolds normally need more pump shot. Dialing in pump cams and nozzles is best done at the track using 60' times, or butt dyno if no track time. Use the smallest amount of pump shot required to transition, it's all test and tune.
        My car 340 Demon 650dp likes 31/31 white cam #2 hole throttle/cam up front, and pink #1 rear. I launch at 2k at the track.
         
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        • Mattax

          Mattax Just the facts, ma'am FABO Gold Member

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          Green cam is generally about the most early lift a 4150 needs. If it really needs the blue, so be it, but I would encourage testing different nozzles with the green before settling on the blue.
          Nothing wrong with experimenting with the secondaries disconnected to isolate the primaries. There is a slight disadvantage in that airflow and fuel distribution it's not going to be quite the same as when all four throttles are open. But its a useful tuning and learning technique.
           
          Last edited: May 16, 2018