GM Cadillac 1967-1989 Repair Guide

Carbureted Engines

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1967-74

Adjust with the air cleaner removed.

Idle speed is adjusted at an anti-dieseling solenoid. The throttle must be opened slightly to allow the plunger to move out all the way, then it must be closed against the now-extended solenoid plunger before making the idle speed adjustment. The solenoid plunger will retract when the ignition is shut off.

  1. Disconnect and plug the distributor vacuum advance hose and parking break vacuum hose (at the release cylinder). If equipped with self-leveling, remove and plug the air leveling compressor hose at the air cleaner. Remove the air cleaner but keep the vacuum hoses connected.
  2.  
  3. Connect a tachometer and set the parking brake with transmission in Neutral.
  4.  
  5. Turn in the mixture screws until they seat gently, then turn the screws out approximately 6 turns through 1973 or 4 turns for 1974.
  6.  
  7. Start the engine and allow it to warm up. Make sure the choke is off, the parking brake is firmly set and the drive wheels are blocked.
  8.  
  9. Place the car in Drive with A/C off.
  10.  

Press down on the hot idle compensator pin while making adjustments to Fleetwood 75 and Commercial models only.

  1. Set idle speed to 550 rpm 1967-69, 620 rpm 1970-1973 or 640 rpm for 1974 by adjusting the anti-dieseling solenoid.
  2.  
  3. Turn each mixture screw clockwise 1 / 4 turn at a time alternately until an idle speed of 600 rpm is obtained.
  4.  
  5. Install the limiter caps, then disconnect the wire that energizes the solenoid. The plunger should retract to allow a slower idle speed of 350-400 rpm.
  6.  
  7. Shut off the engine, disconnect the tach, connect the vacuum lines and solenoid wire and install the air cleaner.
  8.  

1975-76

Adjust with the air cleaner removed.

Normal engine idle speed is adjusted with the idle speed screw located at the throttle lever side of the carburetor.

  1. Disconnect and plug the distributor vacuum advance hose and parking brake vacuum hose (at the release cylinder). Disconnect the air leveling compressor hose at the air cleaner and plug it. Remove the air cleaner, but keep the vacuum hoses connected.
  2.  
  3. Connect a tachometer to the engine, set the parking brake, and block the wheels. Place the transmission in Neutral.
  4.  
  5. Turn in the mixture screws until they seat gently, then turn them out 5 turns.
  6.  
  7. Start and warm the engine to normal operating temperature. Be sure that the choke is off and that the throttle level stop tang is contacting the carburetor idle speed screw (slow idle position).
  8.  
  9. Place the transmission in Drive with A/C off.
  10.  
  11. Set the idle speed to the higher of the two figures on the underhood sticker by adjusting the idle speed screw located at the throttle level side of the carburetor.
  12.  

Do not depress the brake pedal on cars equipped with the Hydro-boost brake system as engine speed will be decreased.

  1. Alternately turn each mixture screw inward 1 / 4 turn at a time until 600 rpm is reached.
  2.  
  3. Install replacement mixture screw limiter caps and recheck idle speed.
  4.  
  5. Stop the engine, remove the tachometer, connect all vacuum lines, and install the air cleaner.
  6.  

1977

Adjust with the air cleaner removed.

Normal engine idle speed is adjusted with the idle speed screw located at the throttle level side of the carburetor.

  1. Disconnect and plug the distributor vacuum advance hose and parking brake vacuum hose (at the release cylinder). Disconnect the air leveling compressor hose at the air cleaner and plug it. Remove the air cleaner, but keep the vacuum hoses connected.
  2.  
  3. Connect a tachometer to the engine, set the parking brake, and block the wheels. Place the transmission in Neutral.
  4.  
  5. Turn in the mixture screws until they seat gently, then turn them out 5 turns.
  6.  
  7. Start and warm the engine to normal operating temperature. Be sure that the choke is off and that the throttle lever stop tang is contacting the carburetor idle speed screw (slow idle position).
  8.  
  9. Place the transmission in Drive with A/C off.
  10.  
  11. Set the idle speed to the higher of the two figures on the underhood sticker by adjusting the idle speed screw located at the throttle lever side of the carburetor.
  12.  

Do not depress the brake pedal on cars equipped with the Hydro-boost brake system as engine speed will be decreased.

  1. Alternately turn each mixture screw inward 1 / 4 turn at a time until 675 rpm is reached.
  2.  
  3. Install replacement mixture screw limiter caps and recheck idle speed.
  4.  
  5. Stop the engine, remove the tachometer, connect all vacuum lines, and install the air cleaner.
  6.  

1978 AND LATER



Changes have been made in the carburetors for these model years that make idle speed and mixture adjustments impossible without the use of a propane enrichment system which is not readily available to the general public.

Most 1979 and later carburetors have mixture needles concealed under staked-in screws. Mixture adjustments are possible only during carburetor overhaul.

Vehicles equipped with the Computer Command Control System can't use the propane enrichment or lean drop methods of idle mixture adjustment.

See Figure 1



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Fig. Fig. 1: Common 4-barrel adjustment with solenoid

 
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