GM Storm/Spectrum 1985-1993 Repair Guide

Idle Mixture

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ADJUSTMENT



Carbureted Vehicles

The carburetor has been calibrated at the factory and should not normally need adjustment. For this reason, the mixture adjustment should never be changed from original factory setting. However if during diagnosis, the check indicates the carburetor to be the cause of a performance complaint or emission failure, or if the carburetor is being overhauled, the idle mixture can be adjusted using the following procedure:

  1. Confirm the following prior to checking or adjusting the idle speed.
    1. Check that the lead wires and hoses of the engine emission control system are connected correctly.
    2.  
    3. Check that the accelerator cable has some play.
    4.  
    5. Check that the ignition timing is within specification.
    6.  
    7. Check that the valve lash is adjusted to specification.
    8.  
    9. Check that the fast-idle actuator does not operate.
    10.  
    11. Check that the cooling fan is not operating.
    12.  

  2.  
  3. If not already done, remove the carburetor from the intake manifold and remove the mixture adjusting screw cover pin. This can be done center punching, then drilling the center of the pin using a 4-4.5mm drill bit. The drill depth should be less than 10mm. A punch may be used to remove the pin.
  4.  


WARNING
When drilling the idle mixture screw plug from the carburetor, always cover the primary and secondary bores along with any other ports to prevent chips or other debris from entering the carburetor.

  1. Remove and inspect the idle mixture adjusting screw. Use compressed air (while wearing safety glasses) to blow chips from the screw bore. Inspect the screw and replace if warped or if damaged by the drill.
  2.  
  3. Install the screw and seat it lightly in the bore, then carefully back the screw out 3 turns from fully closed for manual transaxles or 2 turns from fully closed for automatic transaxles.
  4.  
  5. Reinstall the carburetor and air cleaner.
  6.  
  7. Place gear shift lever in N (MT) or P (AT) position, set the parking brake and block the drive wheels.
  8.  
  9. Start the engine and allow it to reach operating temperature.
  10.  
  11. Check and, if necessary, adjust the ignition timing.
  12.  
  13. Using a dwell meter, connect the positive lead to the duty monitor and the negative lead to ground. Place the meter dial on the 4 cylinder scale. Turn the idle mixture screw until the dwell meter reads 45 degrees (4 cylinder scale).
  14.  
  15. Check idle speed and adjust to specification as necessary.
  16.  
  17. Install a new mixture adjusting screw pin and remove the tachometer.
  18.  

Fuel Injected Vehicles

The function of the electronic fuel injection system is to deliver the correct amount of fuel to the engine under all operating conditions. Fuel delivery is controlled by the Engine Control Module (ECM) and can not be adjusted. If a fuel delivery problem is indicated, individual component inspection and electronic control system diagnosis must be performed to determine the proper corrective action. Refer to Chapters 4 and 5 of this guide for electronic engine control and fuel system diagnosis.

 
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