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Quadrajet too lean at idle

Discussion in 'The Venerable Q-Jet' started by CJay, Sep 22, 2023.

  1. CJay

    CJay Supercar owner Staff Member

    On the Q-Jet on the vette, in order to get the mixture rich enough, I have the screws almost 4 turns out. It's running good at 13-1. still need to take a plug out to see what color thry are.

    My question is, do I need to increase the diameter of the idle feed restrictors in order to get screws screwed in more with the same afr?
     
    CJay, Sep 22, 2023
    #1
  2. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    Is it running smoothly? If so, I would look the other way.
     
    Smartin, Sep 22, 2023
    #2
    Max Damage, FLGS400 and SP4SPD like this.
  3. CJay

    CJay Supercar owner Staff Member

    Yeah, it runs well. You know I can't leave anything alone though
     
    CJay, Sep 22, 2023
    #3
    Max Damage likes this.
  4. Cliff R

    Cliff R Well-Known Member

    Not enough information to help at all.

    At a minimum :

    Carburetor part number?

    Engine specs?

    More is always better than less......
     
    Cliff R, Sep 27, 2023
    #4
    Dano likes this.
  5. CJay

    CJay Supercar owner Staff Member

    Thanks for the reply Cliff. Its a 74 corvette 454. Bone stock, 43k miles. Carb is a 7044221.
     
    CJay, Sep 27, 2023
    #5
  6. BadBrad

    BadBrad Got 4-speed?

    Tuning became much easier for me after I contemplated and found an out-of-the-box vacuum leak.
     
    BadBrad, Sep 27, 2023
    #6
  7. Cliff R

    Cliff R Well-Known Member

    The 1974 big block carb has a LOT of idle bypass air and pretty small holes under the mixture screws from what I can remember. They need to come out at least 4 turns to get decent idle quality. No need to modify it as long as you have control with the mixture screws and can get good idle quality.

    Has the carburetor been rebuilt or is it still unmolested?
     
    Cliff R, Oct 1, 2023
    #7
    Dano and timesublime like this.
  8. CJay

    CJay Supercar owner Staff Member

    Well, that makes sense! Ken at Everyday Performance restored it for me. Runs like a top. Just not used to having those screws out so much. I'll just have to obsess over something else!
     
    CJay, Oct 1, 2023
    #8
    Dadrider likes this.
  9. Cliff R

    Cliff R Well-Known Member

    I make several very minor changes to the Chevy Big Block carbs when I build them here. Otherwise you'll get customers calling you back saying they work really good BUT slightly lean on the primary side and at idle.

    Keep in mind that those big inefficient super low compression 454's in the early to mid-70's were NOT very efficient at idle and really low RPM's. Hence they have the most idle bypass air ever used in a Quadrajet. They have a couple other minor flaws in the basic calibration which I'll cover on another day. The factory was doing everything they could to get them thru emissions so you aren't going to have the most optimum idle and part throttle A/F ratios.

    Takes about a minute to correct all that during the rebuild and the end user will like the results........FWIW.......
     
    Cliff R, Oct 2, 2023
    #9
    Dano likes this.
  10. Cliff R

    Cliff R Well-Known Member

    I make several very minor changes to the Chevy Big Block carbs when I build them here. Otherwise you'll get customers calling you back saying they work really good BUT slightly lean on the primary side and at idle. The complaints could be as minor as they hearing a tiny "cracking" in the exhaust at very light engine load to being a little "anemic" until the engine is fully warmed up and well heat soaked.

    Keep in mind that those big inefficient super low compression 454's in the early to mid-70's were NOT very efficient at idle and really low RPM's. Hence they have the most idle bypass air ever used in a Quadrajet. They have a couple other minor flaws in the basic calibration which I'll cover on another day. The factory was doing everything they could to get them thru emissions so you aren't going to have the most optimum idle and part throttle A/F ratios.

    The builder also needs to factor in that they were originally calibrated on fuel with more specific gravity and BTU's plus a better burn rate. This new piss-water stuff we're feeding them isn't helping the situation much.

    Takes about a minute to correct all that during the rebuild and the end user will like the results........FWIW.......
     
    Cliff R, Oct 2, 2023
    #10
  11. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    You can use the baseplate gasget that blocks the idel air bypass holes. I did this ounce and made my airhorn frosty. It looked pretty neat.
     
    HotRodRivi, Feb 20, 2024
    #11

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