Chevrolet Full Size Cars 1979-1989

Carburetor

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Idle mixture and speed adjustments are critical aspects of exhaust emission control. It is important that all tune-up instructions be carefully followed to ensure satisfactory engine performance and minimum exhaust pollution. The different combinations of emission systems application on the different engines have resulted in a great variety of tune-up specifications. All vehicles covered by this guide should have a decal conspicuously placed in the engine compartment giving tune-up specifications.

ADJUSTING IDLE SPEED AND MIXTURE



In the following adjustment procedures the term lean roll means turning the mixture adjusting screws in (clockwise) from optimum setting to obtain an obvious drop in engine speed (usually 20 rpm).

For more details on carburetor adjustment, not given below, please refer to of this guide.

1979-80 Vehicles

These cars have sealed idle mixture screws; in most cases these are concealed under staked-in plugs. Idle mixture is adjustable only during carburetor overhaul, and requires the addition of propane as an artificial mixture enrichener. See the emission control label in the engine compartment for procedures and specifications not supplied here. Prepare the car for adjustment (engine warm, choke open, fast idle screw off the fast idle cam) as per the label instructions.

1-BBL


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Fig. Fig. 1 Idle speed adjustment 1979 1bbl carburetor

  1. Run the engine to normal operating temperature.
  2.  
  3. Make sure that the choke is fully opened.
  4.  
  5. Turn the A/C off and disconnect the vacuum line at the vapor canister. Plug the line.
  6.  
  7. Set the parking brake, block the drive wheels and place the transmission in Drive. Connect a tachometer to the engine according to the manufacturer's instructions.
  8.  
  9. Turn the solenoid assembly to achieve the solenoid-on speed.
  10.  
  11. Disconnect the solenoid wire and turn the 1 / 8 in. hex screw in the solenoid end, to achieve the solenoid-off speed.
  12.  
  13. Remove the tachometer, connect the canister vacuum line and shut off the engine.
  14.  

2-BBL AND 4-BBL EXCEPT V8-350 ENGINE


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Fig. Fig. 2 Idle speed adjustment on a 2BBL, with solenoid 1979



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Fig. Fig. 3 Idle speed adjustment on a 2BBL, without solenoid 1979



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Fig. Fig. 4 Idle speed adjustment on a 4BBL, without solenoid 1979 and later vehicles



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Fig. Fig. 5 Idle speed adjustment on a 4BBL, with solenoid 1979 and later vehicles



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Fig. Fig. 6 Idle speed adjustment on a 2BBL, with solenoid 1980



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Fig. Fig. 7 Idle speed adjustment on a 2BBL, without solenoid 1980

  1. Run the engine to normal operating temperature.
  2.  
  3. Make sure that the choke is fully opened, turn the A/C off, set the parking brake, block the drive wheels and connect a tachometer to the engine according to the manufacturer's instructions.
  4.  
  5. Disconnect and plug the vacuum hoses at the EGR valve and the vapor canister.
  6.  
  7. If equipped with an automatic transmission, place the transmission in Park. If equipped with a manual transmission, place transmission in Neutral.
  8.  
  9. Disconnect and plug the vacuum advance hose at the distributor. Check and adjust the timing.
  10.  
  11. Connect the distributor vacuum line.
  12.  
  13. If equipped with A/C: set the idle speed screw to the specified rpm. Disconnect the compressor clutch wire and turn the A/C on. Open the throttle momentarily to extend the solenoid plunger. Turn the solenoid screw to obtain the specified rpm. Finally, adjust the fast idle speed using the choke screw on the opposite side of the carburetor assembly.
  14.  
  15. For automatic transmission cars without A/C, momentarily open the throttle to extend the solenoid plunger. Turn the solenoid screw to obtain the specified rpm. Disconnect the solenoid wire and turn the idle speed screw to obtain the slow engine idle speed.
  16.  

V8-350 ENGINE
  1. Run the engine to normal operating temperature.
  2.  
  3. Set the parking brake and block the drive wheels.
  4.  
  5. Connect a tachometer to the engine according to the manufacturer's instructions.
  6.  
  7. Disconnect and plug the purge hose at the vapor canister, then disconnect and plug the EGR vacuum hose at the EGR valve.
  8.  
  9. Turn the A/C off.
  10.  
  11. Place the transmission in Park.
  12.  
  13. Disconnect and plug the vacuum advance line at the distributor. Check and adjust the timing.
  14.  
  15. Connect the vacuum advance line. Place the automatic transmission in Drive.
  16.  
  17. If equipped with A/C: with the A/C Off, adjust the idle stop screw to obtain the specified rpm. Disconnect the compressor clutch wire and turn the A/C On. Open the throttle slightly to allow the solenoid plunger to extend. Turn the solenoid screw to obtain the solenoid rpm listed on the underhood emission sticker.
  18.  
  19. Connect all hoses and remove the tachometer.
  20.  

1981-82 Vehicles


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Fig. Fig. 8 Common Idle Speed Control (ISC) assembly

Most of the E2ME (two barrel) and E4ME (four barrel) carburetors used on these models are equipped with an Idle Speed Control (ISC) assembly, monitored by the ECM, which controls engine idle speed. The curb idle is programmed into the ECM and is not adjustable. Some models with A/C may be equipped with an Idle Speed Solenoid (ISS), if so, refer to the 1978-80 procedure.

1983-88


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Fig. Fig. 9 Common Idle Load Compensator (ILC)

The E2ME (two barrel) and the E4ME (four barrel) carburetors used on these models all are equipped with an Idle Speed Control (ISC) assembly, monitored by the ECM, which controls engine idle speed. The curb idle is programmed into the ECM and is not adjustable.

On the E4MC models, an Idle Load Compensator (ILC) mounted on the float bowl is used to control curb idle speeds. No attempt should be made to adjust this since it is controlled by the ECM.

On vehicles that do not include an ISC or ILC but are equipped with air conditioning, an Idle Speed Solenoid (ISS) is used to maintain curb idle speed any time the air conditioner compressor clutch is engaged. If so refer to the 1978-80 procedures.

 
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