How much will my car benefit from going to an 850 Holley DP?

Discussion in 'The Mixing shop.' started by accelr8, Jul 29, 2021.

  1. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    If the mixture screws do nothing, that is an instant tip off that your throttle blades are too far open at idle. The primaries on a Q-jet are so efficient that it doesn't take much to initiate fuel flow from the main nozzles. This results in very rich at idle, and no response from the idle mixture screws. The carburetor is running on the mains, NOT the idle system. The two biggest causes of this are one, the wrong carburetor (350 carburetor on a big block), or two, not enough ignition timing at idle. More ignition timing at idle allows you to close the throttle blades. You should be able to stall the engine by turning the mixture needles all the way in.

    Make sure you have the right Q-jet to begin with. Then, temporarily increase you ignition timing, and try and back out the idle speed screw to close the throttle blades. See if you have response from the mixture screws. If that remedies the issue, you need to recurve the distributor for less mechanical advance so you can run more initial timing. Don't drive the car because when you increase your initial timing, you increase the advance at higher RPM. That can cause detonation.
     
    Lucy Fair likes this.
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Don't give up on the Q-jet. It's a great street carburetor when it is right. People seem to think all Q-jets are the same, and bolting on any Q-jet will result in satisfactory performance. That is wrong.
     
    Lucy Fair likes this.
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Just looked at your signature. That's a moderately sized cam. It will want at least 20* of initial timing. It will probably want a softer power piston spring. The Q-jet needs to be calibrated for the cam. The engine needs to be tuned as well.
     
    Lucy Fair likes this.
  4. accelr8

    accelr8 Well-Known Member

    Thanks for the info Larry. I’ll run through the checks you outline above.

    One question, do you know how to tell if it’s a big block carb? I checked the one set of numbers I found on the carb (7043240 LA 2622) and determined it’s a 1973 Buick 4 barrel for an automatic trans. This number did not indicated whether it is a small or big block carb. There is another number on the back of the carb (7047931), but I couldn’t figure out how to decode it.
     
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Lucy Fair and 69GS430/TKX like this.
  6. accelr8

    accelr8 Well-Known Member

    Thanks for the great info Larry!
     

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