GM Full-Size Trucks 1980-1987 Repair Guide

Carbureted Engines

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In most cases, the mixture screws have limiter caps, but in later years the mixture screws are concealed under staked in plugs. Idle mixture is adjustable only during carburetor overhaul, and requires the addition of propane as an artificial mixture. For these reasons, mixture adjustments are not covered here for affected models.

See the emission control label in the engine compartment for procedures and specifications not supplied here. See Carburetor Identification in Chassis Electrical for carburetor I.D. specifics.

1980-87



See Figures 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 and 13

Idle mixture is not adjustable in these years, except for the heavy duty emission 292 6-cylinder equipped with the IME carburetor, and heavy duty emission V8s equipped with the four barrel M4MC.

6-250 Engine

  1. All adjustments should be made with the engine at normal operating temperature, air cleaner on, choke open, and air conditioning off, unless otherwise noted.
  2.  
  3. Set the parking brake and block the rear wheels.
  4.  
  5. Automatic transmissions should be set in Drive, manuals in Neutral, unless otherwise noted in the procedures or in the Tune-Up Specifications chart, or on the emission control label.
  6.  
  7. Check the emission control label for any special instructions.
  8.  
  9. Open the throttle slightly to allow the solenoid plunger to extend.
  10.  



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Fig. Fig. 1: Idle speed adjustment for 250 and 292 six cylinder, 1980 and later



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Fig. Fig. 2: 1980 and later in-line six cylinder idle speed adjustment, MV and ME-type 2-bbls,

  1. Turn the solenoid screw to adjust the curb idle to the figure given in the Tune-Up Specifications chart or on the emission control label.
  2.  
  3. Disconnect the electrical connector from the solenoid. The idle speed will drop.
  4.  
  5. Adjust the idle to the basic idle speed figure given on the emission control label by means of the idle speed screw.
  6.  
  7. Shut off the engine and reconnect the solenoid.
  8.  



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Fig. Fig. 3: 1 ME 1-bbl fast idle adjustment

6-292 Engine

IDLE SPEED
  1. All adjustments should be made with the engine at normal operating temperature, air cleaner on, choke open, and air conditioning off, unless otherwise noted.
  2.  

  1. Set the parking brake and block the rear wheels.
  2.  

  1. Automatics should be place in Drive, manuals in Neutral, except as noted in the Tune-Up Specifications chart.
  2.  
  3. Make certain that the fast idle follower is not on any of the steps marked H , 2 , or L on the cam. It should be resting against the first step below L .
  4.  
  5. Set the idle to specifications by turning the solenoid in or out. Do this with a wrench on the nut attached to the end of the solenoid body.
  6.  
  7. Disconnect the electrical connector from the solenoid. The engine speed will drop.
  8.  

  1. With a 1 /8 inch Allen head wrench, turn the screw located inside the nut attached to the solenoid body, and set the idle speed to the specification on the emission label
  2.  
  3. Reconnect the solenoid wire.
  4.  

IDLE MIXTURE
  1. Set the parking brake and block the rear wheels.
  2.  
  3. Remove the air cleaner but do not disconnect any of the hoses. Disconnect and plug the other hoses as directed on the emission control label.
  4.  
  5. The engine should be at normal operating temperature, choke open, and air conditioning off, if equipped. Connect an accurate tachometer to the engine.
  6.  
  7. Disconnect and plug the vacuum advance hose at the distributor and check the ignition timing. Correct as necessary. Reconnect the vacuum advance hose.
  8.  
  9. Carefully remove the limiter cap from the idle mixture screw. Lightly seat the screw, then back it out just enough to allow the engine to run.
  10.  
  11. Place the transmission in Neutral.
  12.  
  13. Back the mixture screw out 1 /8 turn at a time until the maximum idle speed is obtained. Adjust the idle speed to the figure given on the emission control label by means of the idle speed screw. Repeat this step until you are certain that the maximum speed has been obtained with the mixture screw.
  14.  
  15. Turn the mixture screw in 1 /8 turn at a time until the idle speed drops to the figure given on the emission control label.
  16.  
  17. Reset the idle speed to the figure given on the emission control label by means of the idle speed screw. Check and adjust the fast idle as directed on the emission control label. Reconnect any vacuum hoses removed in Step 2, install the air cleaner, and recheck the idle speed. Correct, if necessary, by means of the idle speed screw.
  18.  



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Fig. Fig. 4: 1980-81 V8 2-bbl (M2MC) idle speed adjustment with solenoid



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Fig. Fig. 5: 2GC 2-bbl slow idle adjustment, A/C equipped



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Fig. Fig. 6: 2SE 2-bbl idle speed and fast idle adjustments



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Fig. Fig. 7: E2SE 2-bbl idle speed adjustment with A/C, 1983



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Fig. Fig. 8: E2SE 2-bbl idle speed adjustment without A/C



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Fig. Fig. 9: E2SE fast idle adjustment

8-305 Engine

  1. All adjustments should be made with the engine at normal operating temperature, air cleaner on, choke open, and air conditioning off, unless otherwise noted.
  2.  
  3. Set the parking brake and block the rear wheels.
  4.  
  5. Automatic transmissions should be set in Drive, manuals in Neutral, unless otherwise noted in the procedures or in the Tune-Up Specifications chart, or on the emission control label.
  6.  
  7. Check the emission control label for any special instructions or to determine which hoses, if any, must be disconnected.
  8.  



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Fig. Fig. 10: Adjusting the slow idle speed on a 1984 4bbl carburetor. The adjustment should be made with air cleaner installed, but was removed in this picture for clarity



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Fig. Fig. 11: Adjusting the fast idle speed on a 1984 4bbl carburetor. The adjustment should be made with air cleaner installed, but was removed in this picture for clarity

  1. Make sure the idle speed screw is on the low ( L ) step of the fast idle cam.
  2.  
  3. For vehicles without air conditioning, do the following:
    1. Turn the idle speed screw to adjust idle speed to the specification found in the Tune-Up Specifications chart or on the underhood emissions sticker.
    2.  
    3. Open the throttle slightly to allow the solenoid plunger to extend.
    4.  
    5. Turn the solenoid screw to adjust the curb idle to specification, as given in the chart or on the emission control sticker in the engine compartment.
    6.  
    7. Then disconnect the electrical connector from the solenoid. The idle speed will drop.
    8.  
    9. Turn the idle speed screw to set the slow engine idle to the figure given on the emission control sticker.
    10.  
    11. Reconnect the solenoid and shut off the engine.
    12.  

  4.  
  5. For vehicles equipped with air conditioning, do the following:
    1. Turn the idle speed screw to set the idle to specifications.
    2.  
    3. Disconnect the air conditioner compressor electrical lead at the compressor, and turn the air conditioner on.
    4.  
    5. Open the throttle slightly to allow the solenoid plunger to fully extend.
    6.  
    7. Turn the solenoid screw and adjust to the specification on the engine emission label.
    8.  
    9. Reconnect the air conditioner compressor lead.
    10.  

  6.  
  7. Reconnect any hoses that were disconnected.
  8.  



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Fig. Fig. 12: 1980 and later 4-bbl adjustments without solenoid; 1980-81 V8 2-bbl (M2MC) models



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Fig. Fig. 13: 1980 and later 4-bbl adjustments with solenoid

8-350, 8-400, and 8-454 Engines

IDLE SPEED ADJUSTMENT
  1. All adjustments should be made with the engine at normal operating temperature, air cleaner on, choke open, and air conditioning off, unless otherwise noted.
  2.  
  3. Set the parking brake and block the rear wheels.
  4.  
  5. Automatics should be place in Drive, manuals in Neutral, except as noted in the Tune-Up Specifications chart.
  6.  
  7. Check the underhood emissions sticker to determine which hoses, if any, must be disconnected.
  8.  
  9. For two barrel carburetors, make sure that the idle speed screw is on the low( L ) step of the fast idle cam.
  10.  
  11. For vehicles without air conditioning, do the following:
    1. Turn the idle speed screw to adjust idle speed to the specification found in the Tune-Up Specifications chart or on the underhood emissions sticker.
    2.  
    3. Open the throttle slightly to allow the solenoid plunger to extend.
    4.  
    5. Turn the solenoid screw to adjust the curb idle to specification, as given in the chart or on the emission control sticker in the engine compartment.
    6.  
    7. Then disconnect the electrical connector from the solenoid. The idle speed will drop.
    8.  
    9. Turn the idle speed screw to set the slow engine idle to the figure given on the emission control sticker.
    10.  
    11. Reconnect the solenoid and shut off the engine.
    12.  

  12.  
  13. For vehicles equipped with air conditioning, do the following:
    1. Turn the idle speed screw to set the idle to specifications.
    2.  
    3. Disconnect the air conditioner compressor electrical lead at the compressor, and turn the air conditioner on.
    4.  
    5. Open the throttle slightly to allow the solenoid plunger to fully extend.
    6.  
    7. Turn the solenoid screw and adjust to the specification on the emission control label and the Tune-Up Specifications chart.
    8.  
    9. Reconnect the air conditioner compressor lead.
    10.  

  14.  
  15. Reconnect any hoses that were disconnected.
  16.  

IDLE MIXTURE ADJUSTMENT

Mixture is adjustable on heavy duty emissions V8s with the four barrel M4MC carburetor. This procedure will not work on light duty emissions trucks.

  1. The engine must be at normal operating temperature, choke open, parking brake applied, and the transmission in Park or Neutral. Block the rear wheels and do not stand in front of the truck when making adjustments.
  2.  
  3. Remove the air cleaner. Connect a tachometer and a vacuum gauge to the engine.
  4.  
  5. Turn the idle mixture screws in lightly until they seat, then back them out two turns. Be careful not to tighten the mixture screw against its seat, or damage may result.
  6.  
  7. Adjust the idle speed screw to obtain the engine rpm figure specified on the emission control label.
  8.  
  9. Adjust the idle mixture screws equally to obtain the highest engine speed.
  10.  
  11. Repeat Steps 4 and 5 until the best idle is obtained.
  12.  
  13. Shut off the engine, remove the tachometer and vacuum gauge, and install the air cleaner.
  14.  

 
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