GM Buick/Oldsmobile/Pontiac/Full-Size 1975-1990 Repair Guide

Idle Mixture

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ADJUSTMENTS



2GC, GE, GV Models
  1. Adjust the slow idle as outlined above. Keep all hoses disconnected, and keep the tachometer connected. Make sure the parking brake is set and the drive wheels blocked.
  2.  
  3. Break off the mixture tabs, using care not to damage the mixture adjusting screws.
  4.  
  5. Turn out the mixture screws equally until the maximum idle speed is achieved. Turning the screws in richens the mixture. Reset the speed if necessary with the solenoid screw to 80 rpm above specified rpm. If the mixture screws were out of balance, lightly seat both screws then back both out 5 full turns to attain an equal adjustment point.
  6.  
  7. Equally lean (turn in) the mixture screws until the specified idle speed is reached. The carburetor solenoid must be energized for this procedure. Reset the slow idle speed if necessary with the air cleaner in place.
  8.  

All Other 2 Barrel Models To 1977

On models with the air conditioning idle solenoid, energize the solenoid (by opening the throttle slightly so the plunger extends) until the engine is turning the specified solenoid energized rpm. Turn off the A/C and adjust the carburetor idle speed screw to the specified curb idle rpm.

  1. Run the engine to normal operating temperature and turn the A/C off. Set the parking brake and block the wheels.
  2.  
  3. Remove the air cleaner for access to the carburetor, but leave the hoses connected. Follow instructions on the emission sticker in disconnecting other hoses.
  4.  
  5. Connect a tachometer to the engine. Disconnect the vacuum advance hose and plug it.
  6.  
  7. Make sure the timing is set correctly, then reconnect the hose.
  8.  
  9. Break off the mixture tabs from the caps.
  10.  
  11. Turn the screws in until they lightly seat themselves, then back both screws out until the engines just idles.
  12.  
  13. Place the transmission in DRIVE.
  14.  


CAUTION
When making adjustments with the vehicle in DRIVE, always have a helper in the car with his or her foot on the brake and the parking brake fully applied .

  1. Back mixture screws out a little at a time equally, until the highest possible idle speed is obtained.
  2.  
  3. Adjust the idle speed to obtain the specified curb idle.
  4.  
  5. Reconnect all hoses and install the air cleaner.
  6.  

4 Barrel Models To 1977
EXCEPT CALIFORNIA
  1. Run the engine to normal operating temperature. Remove the air cleaner, disconnect the vacuum hose at the intake manifold, and plug the fitting.
  2.  
  3. Make sure the choke plate is open and the air conditioning is off.
  4.  
  5. Block the rear wheels and firmly set the parking brake.
  6.  
  7. Disconnect the carburetor hoses from the vapor (charcoal) canister and E.G.R. valve and plug the hoses with golf tees or pencils. Leave the distributor vacuum hose connected.
  8.  
  9. Break off the mixture tabs from the caps, using care not to damage the screws. Connect a tachometer and an accurate vacuum gauge to the engine.
  10.  
  11. If the mixture screws appear out of balance or the carburetor is being overhauled, turn the screws in until they both lightly seat themselves, then back both screws out equally 3 full turns.
  12.  
  13. Equally richen (turn out) the mixture screws until maximum idle speed is achieved. Note the manifold vacuum reading.
  14.  
  15. With the transmission in DRIVE , adjust the idle speed screw until the engine is turning 580 rpm. Observe the cautions in above sections on tuning an engine while the transmission is in DRIVE .
  16.  
  17. Equally lean (turn in) the mixture screws until the idle speed is 550 rpm. Manifold vacuum should not be reduced more than 2 in. Hg from the reading obtained in Step 7. If the reading is reduced more than 2 in. Hg, repeat the procedure.
  18.  
  19. Reconnect the canister and E.G.R. hoses, and install the air cleaner and air cleaner vacuum hoses. Disconnect the tach.
  20.  

CALIFORNIA MODELS
  1. Follow Steps 1 through 5 of the above procedure.
  2.  
  3. Turn in the idle mixture screws until they seat themselves lightly. Equally richen (turn out) the screws 4 full turns. Note the manifold vacuum reading.
  4.  
  5. With the transmission in DRIVE (and observing the above Cautions), adjust the idle speed screw so the engine is turning 625 rpm.
  6.  
  7. Equally lean (turn in) the mixture screws until the idle speed is 600 rpm. Manifold vacuum should not be reduced by more than 2 in. Hg from the reading obtained in Step 2. If the reading is less than this, repeat the procedure.
  8.  
  9. Reconnect the distributor, canister, and E.G.R. hoses. Disconnect the vacuum gauge and tachometer.
  10.  
  11. Install the air cleaner and air cleaner vacuum hoses.
  12.  

1978-90 All V6 and V8

These models require extensive carburetor modifications to gain access to mixture adjustments and, in many cases, a number of special tools. Idle mixture on these models is constantly monitored and controlled by computer and only extreme carburetor misadjustment at the factory will necessitate service adjustment.

If your engine does not idle properly, and you have ruled out vacuum leaks and ordinary maintenance problems such as improperly adjusted ignition timing or worn spark plugs, we suggest you have idle mixture checked and adjusted by a competent professional who has the specialized tools and equipment required to do the job.

 
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