GM Metro/Sprint 1985-1993 Repair Guide

Idle Mixture



See Figures 1 and 2

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Fig. Fig. 1: Removing the mixture adjusting screw pin - carburetor

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Fig. Fig. 2: Connecting dwell meter to duty check connector - carburetor

1985-88 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.
    3. Check that the accelerator cable has some play.
    5. Check that the ignition timing is within specification.
    7. Check that the valve lash is adjusted to specification.
    9. Check that the idle-up actuator does not operate.
    11. Check that the cooling fan is not operating.

  3. If not already done, remove the carburetor from the intake manifold and remove the mixture adjusting screw cover pin. This can be done by drilling the pin out using a 4-4.5 mm drill bit, then using a punch to remove the pin. Reinstall the carburetor and air cleaner.
  5. Place gear shift lever in N position, set the parking brake and block the drive wheels.
  7. Start the engine and allow it to reach operating temperature.
  9. Remove the coupler on the duty check connector located in the engine compartment. Connect a dwell meter to the connector with the positive probe on the blue/red wire and the negative probe on the black/green wire.
  11. Set dwell meter to 6-cylinder position and make sure indicator moves.
  13. Check idle speed and adjust to specification as necessary.
  15. Run engine at idle speed and adjust mixture adjusting screw slowly, in small increments, allowing time for dwell to stabilize. Adjust until a dwell of 21-27° is obtained.
  17. If dwell is too high, back screw out. If dwell is too low, turn screw in.
  19. Recheck idle speed and adjust as necessary.
  21. If adjustment can not be made because dwell meter indicator does not deflect, check the feedback system for proper operation.
  23. Install a new mixture adjusting screw pin.

1987-93 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.