Adjusting the Q-jet idle

  • Look at the front throttle plate, if you see two round caps then you will need to remove the carb and "chisel" (from the bottom) the caps out to gain access to the screws. These are "protective" caps. If the carb has been fooled with before odds are that these have already been removed. Note that these screws are in the same location as on the older Q-Jets but the caps are there as well. Removing the caps is not a bad thing.
  • You will need a special tool from GM to adjust the screws unless you are really creative.
  • It is necessary to have the screws open a few turns during closed loop idle.
  • Adjustment must be done with a cold engine. Really.
  • Adjustment isn't always necessary, check for clogged air bleed valves among other fairly obvious things. Also check for bad O2 sensor.
  • Note that adjusting these screws only has a certain amount of effect due to computer intervention.
  • As always, when possible, consult a technical manual.

    More adjustment info

    Date: Sat, 22 May 1999 00:05:51 -0500
    From: Robert Lyssy (rlyssy at
    Subject: More Q-Jet Adjusting Tips
    Following the information posted here concerning idle mixture and idle
    air bleed valve adjustment, I found the following to provide the best
    baseline for electronic Q-Jet adjustment for those like myself who had
    no prior settings to refer back to.  Completely close both idle mixture
    screws (going clockwise), then back each one out two full turns. This
    should allow the desired 30 degree 6-cylinder dwell meter reading to be
    achieved with idle air bleed valve adjustment only. This was on my
    unmodified '86 SS. One and a half turns out on the idle mixture screws
    left the car a little flat on mild acceleration out of the turns. Moving
    up to the two full turns and leaning out with the idle air bleed valve
    improved the overall driving feel of the car, thru the turns and
    straightline. Idle speed was set a bit over 600 rpm in gear.

    Post from: Richard Kwarciany (kwar at FNAL.FNAL.GOV)
            The "thingy" is the idle air bleed valve.  It doesn't affect the
    main circuit mixture, it only affects idle and part throttle.  Think of the
    idle adjuster screws as the coarse idle mixture adjustment, and the air
    bleed valve as the fine adjustment.  Connect an ignition dwell meter to the
    mixture control test terminal.  This is a green connector you'll find
    sticking out of the wiring harness near the front of the carb.  Start and
    warm up the motor.  Make sure it is at operating temperature.  Adjust the
    idle air bleed valve until the dwell reads 30 degrees on the 6 cylinder
    scale.  You may not be able to get the dwell to 30 degrees.  If you can't,
    and you've made changes to the motor (cam, headers, etc.) then you'll need
    to adjust the mixture screws before attempting to adjust the idle air bleed
    valve again.  Adjust the screws in 1/8 turn increments, both screws the same
    amount and direction, then try the idle air bleed valve again.  If the motor
    is stock and you can't get 30 degrees, then something is most likely broken.
    If the dwell is always 30 degrees exactly, and adjusting the idle air bleed
    valve makes no difference, then the motor is not running in closed loop.
    Run the motor at 1500 rpm for a few minutes.  It should go closed loop.  If
    it doesn't, something is wrong.
    >Post from: richardm at (Richard)
    >        O.K.  1988 LG4 with factory electronic feedback carb.
    >        Immediately above the mixture control solenoid is a screw-in piston
    >thingy with a spring loaded plunger that rides against the primary metering
    >rod hanger.  It appears that this assembly serves as both a rich-stop
    >adjustment for the M/C solenoid and as a "biasing" for the soleniod.
    >Adjusting it downwards applies more downforce to the M/C assisting the
    >computer in holding it down against spring pressure.  This, it seems to me,
    >should cause the computer to narrow its pulse widths in order to maintain
    >the same mixture.  Likewise, adjusting it upwards should decrease downforce
    >on the M/C plunger and cause the computer to increase duty-cycle in order to
    >keep the M/C at its proper position.  
    >        The small piece of a 1987 Buick service manual that I have talks
    >about using a dwell meter connected to the M/C connector to set idle
    >mixture.  It's done with the engine closed-loop and idling.  The adjustment
    >is performed by turning the idle-mixture screws at the base of the carb.
    >Fine and good.  
    >        What's the procedure for setting this plunger doo-dad (that used to
    >be protected by a plug)?  Despite the fact that it appears to only affect
    >the main-metering section of the carb, turning it has a very profound effect
    >on idle mixture as well for some reason.  I feel that I need to set this
    >thing to the right positon before I play with the idle-mixture screws...

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