GM Firebird 1967-1981 Repair Guide

Idle Speed and Mixture

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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 minimal exhaust pollution. The different combinations of emission systems application on the different engine models have resulted in a great variety of tune-up specifications. See the Tune-Up Specifications chart. Beginning in 1968, all models have a decal conspicuously placed in the engine compartment giving tune-up specifications.

When adjusting a carburetor with two idle mixture screws, adjust them alternately and evenly, unless otherwise stated.

INSPECTION & ADJUSTMENT



See Figures 1 through 12

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Fig. Fig. 1: Idle speed and mixture screw locations on a common 4-bbl. Rochester carburetor (without an idle stop solenoid)



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Fig. Fig. 2: Idle stop solenoid (idle speed) adjustment bolt location on a typical 4-bbl. Rochester carburetor the idle mixture screws are in the same position regardless of whether equipped with a solenoid or not



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Fig. Fig. 3: Idle mixture and speed screw locations for a common 4-bbl. Holley carburetor



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Fig. Fig. 4: Locations for the idle speed and mixture screws on Carter AFB 4-bbl. carburetors



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Fig. Fig. 5: Idle speed and mixture screw locations on 4-bbl. Carter WCFB carburetors



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Fig. Fig. 6: Idle speed adjustment procedure for 1976-79 2-bbl. carburetors



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Fig. Fig. 7: Idle speed adjustment procedure for 2-bbl. carburetors with idle stop solenoids used on 1976-79 models



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Fig. Fig. 8: Idle speed procedure for 2-bbl. carburetors used on 1976-79 models with air conditioning



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Fig. Fig. 9: Adjustment procedure for the idle speed on 4-bbl. carburetors without idle stop solenoids used on 1977-81 models



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Fig. Fig. 10: Idle speed adjustment procedure for 1977-81 4-bbl. carburetors with idle stop solenoids



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Fig. Fig. 11: Idle speed adjustment procedure for 1980-81 2-bbl. carburetors used on models without air conditioning



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Fig. Fig. 12: Idle speed adjustment procedure for 2-bbl. carburetors used on 1980-81 models with air conditioning

See Fuel System for illustrations and adjustment specifications of Carter and Rochester carburetors. 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).

1967 Models

WITHOUT AIR CONDITIONING

Adjust with air cleaner removed.

  1. Remove the air cleaner.
  2.  
  3. Connect a tachometer to the engine, set the parking brake, and place the transmission in Neutral.
  4.  
  5. Turn in the idle mixture screws until they gently seat, then back out 1 1 / 2 turns.
  6.  
  7. Start the engine and allow it to come to the normal operating temperature. Make sure that the choke is fully open, then adjust the idle speed screw to obtain the specified idle speed (automatic in Drive, manual in Neutral).
  8.  
  9. Adjust the idle mixture screw(s) to obtain the highest steady vacuum at the specified idle speed, except for the Rochester BV. For this carburetor, adjust the idle mixture screw out 1 / 4 turn from lean "drop off," the point where a 20-30 rpm drop is achieved by leaning the mixture.
  10.  

On carburetors having a hot idle compensator valve (A/C models), hold the brass valve down with a pencil while making the mixture adjustment.

  1. Repeat Steps 4 and 5 as necessary.
  2.  
  3. Turn off the engine, remove the gauge, and install the air cleaner.
  4.  

WITH AIR CONDITIONING

Adjust with the air cleaner removed.

  1. Remove the air cleaner.
  2.  
  3. Connect a tachometer to the engine, set the parking brake, and place the transmission in Neutral.
  4.  
  5. Turn in the idle mixture screw(s) until they gently seat, then back them out 3 turns.
  6.  
  7. Start the engine and allow it to reach normal operating temperature. Make sure that the choke is fully open, then adjust the idle speed screw(s) to obtain the specified idle speed (automatic in Drive, manual in Neutral).
  8.  
  9. Turn the idle mixture screw(s) clockwise (in) to the point where a 20-30 rpm drop in speed is achieved this is the lean "drop off" point. Back out the screws 1 / 4 turn from this point.
  10.  

During this adjustment, air conditioning should be turned OFF on V8 engines.

  1. Repeat Steps 4 and 5 as necessary.
  2.  
  3. Turn off the engine, remove the tachometer and install the air cleaner.
  4.  

1968-69 Models

6-CYLINDER ENGINES

Make these adjustments with the air cleaner installed.

  1. Turn in the mixture screws until they lightly seat, then back them out 5 turns.
  2.  
  3. Start the engine, connect the tachometer, and allow the engine to warm up to normal operating temperature. On automatic transmission cars, place the selector in Drive and turn off the A/C, if so equipped.
  4.  
  5. Adjust the idle stop solenoid screw to obtain 610 rpm for automatic transmission (1-bbl. and 4-bbl.), 730 rpm for 1968 manual transmission (4-bbl.), or 880 rpm for 1969 manual transmission (4-bbl.).
  6.  
  7. Turn the mixture screws clockwise to obtain 600 rpm for automatic transmission (1-bbl. and 4-bbl.), 700 rpm for manual transmission (1-bbl.), 800 rpm for 1968 manual transmission (4-bbl.), or 850 rpm for 1969 manual transmission (4-bbl.).
  8.  
  9. Disconnect the idle stop solenoid and adjust the idle speed screw on the carburetor to obtain 600 rpm for manual transmission (4-bbl.) and 500 rpm for all others.
  10.  

Don't disturb the idle mixture screws or the stop solenoid after this point.

  1. Reconnect the solenoid and adjust the fast idle speed.
  2.  

8-CYLINDER ENGINES WITHOUT IDLE STOP SOLENOID
  1. Turn in the idle mixture screws until they are lightly seated, then back them out 4 turns (2-bbl.) or 6 turns (4-bbl.).
  2.  
  3. Connect a tachometer, start the engine, and allow it to warm up to normal operating temperature. On cars with automatic transmission and A/C, turn off the A/C.
  4.  
  5. Turn in the idle mixture screw(s) until they gently seat, then back them out 3 turns.
  6.  
  7. Start the engine and allow it to reach normal operating temperature. Make sure that the choke is fully open, then adjust the idle speed screw(s) to obtain the specified idle speed (automatic in Drive, manual in Neutral).
  8.  
  9. Turn the idle mixture screw(s) clockwise (in) to the point where a 20-30 rpm drop in speed is achieved this is the lean "drop off" point. Back out the screws 1 / 4 turn from this point.
  10.  

During this adjustment, air conditioning should be turned off on V8 engines.

  1. Repeat Steps 4 and 5 as necessary.
  2.  
  3. Turn off the engine, remove the tachometer and install the air cleaner.
  4.  

8-CYLINDER ENGINES WITH IDLE STOP SOLENOID
  1. Turn in the idle mixture screws until they are lightly seated, then back them out 4 turns (2-bbl.) or 6 turns (4-bbl.).
  2.  
  3. Connect a tachometer, start the engine, and allow it to warm up to normal operating temperature. On cars with automatic transmissions and A/C, turn off the A/C.
  4.  
  5. Place the car in Drive (automatic) or Neutral (manual). With the idle stop solenoid energized, adjust the mixture screws for the best lean idle speed.
  6.  
  7. Adjust the idle stop solenoid screw to obtain the specified idle speed for all 1968 and only 1969 Ram Air models; use the idle screw for all other 1969 cars.
  8.  
  9. Disconnect the idle stop solenoid, then adjust the idle speed screw on the carburetor to obtain 650 rpm idle for manual transmission (4-bbl.), 500 rpm for all others.
  10.  
  11. Place the fast idle lever on the top step of the cam and adjust the fast idle speed.
  12.  

1970 Models

INITIAL ADJUSTMENTS

Make this adjustment with the air cleaner installed.

  1. On California cars, remove the fuel filler cap.
  2.  
  3. Disconnect and plug the distributor vacuum advance hose.
  4.  
  5. Plug the hot idle compensator on all automatic transmission V8's with the Quadrajet (4MV) carburetor, except for the Ram Air III and IV. Also plug the compensator on all 6-cylinder and V8 2-bbl. engines with automatic transmissions and A/C.
  6.  

With the car in Drive (automatic) or Neutral (manual), adjust the curb idle speed as follows:

6-CYLINDER AND RAM AIR IV ENGINES
  1. With the idle stop solenoid energized, adjust the solenoid screw to obtain 830 rpm for the manual 6-cylinder, 1,000 rpm for the manual Ram Air IV, 630 rpm for the automatic 6-cylinder, and 750 rpm for the automatic Ram Air IV.
  2.  
  3. Adjust the mixture screws equally to obtain lean best idle at 1000 rpm for the Ram Air IV (manual), 750 rpm for the automatic Ram Air IV, 750 rpm for the manual 6-cylinder, and 600 rpm for the automatic 6-cylinder.
  4.  
  5. Disconnect the solenoid wire and adjust the carburetor idle speed screw to obtain 400 rpm for the 6-cylinder, 500 rpm for the Ram Air IV (automatic) and 650 rpm for the Ram Air (manual).
  6.  

350, 400, 455 ENGINES (EXCEPT RAM AIR IV)
  1. Back out the mixture screws 3-5 turns from the lightly seated positions.
  2.  
  3. Adjust the carburetor idle speed screw to obtain 850 rpm for the manual 350 and 400 2-bbl., 1050 rpm for the manual 400 and 455 4-bbl., or 675 rpm for all automatic 350, 400, 455 engines.
  4.  
  5. Lean the mixture screws equally (turn in) to obtain 800 rpm for manual 350 and 400 2-bbl., 950 rpm for manual 400 and 455 4-bbl., or 650 rpm for all automatic 350, 400, 455 engines.
  6.  

1971-72 Models

INITIAL ADJUSTMENTS

Adjust with air cleaner installed.

The following initial idle adjustments are part of the normal engine tune-up. There is a tune-up decal placed conspicuously in the engine compartment outlining the specific procedure and settings for each engine application. Follow all of the instructions when adjusting the idle. These tuning procedures are necessary to obtain the delicate balance of variables for the maintenance of both reliable engine performance and efficient exhaust emission control.

All engines have limiter caps on the mixture adjusting screws. The idle mixture is preset and the limiter caps installed at the factory in order to meet emission control standards. Do not remove these limiter caps unless all other possible causes of poor idle condition have been thoroughly checked out. The solenoid used on 1971 carburetors is different from the one used on earlier models. The Combination Emission Control (CEC) System solenoid valve regulates distributor vacuum as a function of transmission gear position. The CEC solenoid is adjusted only after: 1) replacement of the solenoid, 2) major carburetor overhaul, or 3) after the throttle body is removed or replaced.

All initial adjustments described below are made:

  1. With the engine warmed up and running.
  2.  
  3. With the choke fully open.
  4.  
  5. With the fuel tank line disconnected from the Evaporative Emission canister on all models.
  6.  
  7. With the vacuum hose disconnected at the distributor and plugged.
  8.  

Be sure to reconnect the distributor vacuum hose and to connect the fuel tank to evaporative emission canister line or install the gas cap when idle adjustments are complete.

IDLE SPEED 6-CYLINDER ENGINES
  1. Disconnect the fuel tank "EVAP" hose from the vapor storage canister.
  2.  
  3. Disconnect and plug the distributor vacuum advance hose.
  4.  
  5. Set the dwell and timing (in that order) at the specified idle speed.
  6.  
  7. Adjust the carburetor idle speed screw to obtain 550 rpm for manual, 500 rpm for automatic (in Drive). Do not adjust the solenoid screw.
  8.  
  9. Place automatic in Park and manual in Neutral, then place the fast idle tang on the top step of the fast idle cam and check the fast idle speed. Adjust to obtain 2400 rpm.
  10.  

IDLE SPEED 8-CYLINDER ENGINES
  1. Disconnect the carburetor "EVAP" hose from the vapor storage canister.
  2.  
  3. Disconnect and plug the carburetor-to-vacuum (distributor vacuum) solenoid hose at the solenoid. Disconnect the throttle solenoid wire on 4-bbl. manual transmission engines.
  4.  
  5. Set the dwell and timing (in that order) at specified idle speed.
  6.  
  7. Adjust the carburetor speed screw to obtain the specified idle speed, automatic in Drive, manual in Neutral.
  8.  
  9. On 4-bbl. manual transmission models, reconnect the throttle solenoid wire, manually extend the solenoid screw and adjust to specified idle rpm.
  10.  
  11. Place automatic in Park, manual in Neutral and check the fast idle speed with the screw on top step of the cam. Adjust the fast idle screw to obtain 1700 rpm.
  12.  

2-bbl. carburetor fast idle cannot be adjusted.

  1. Reconnect the distributor vacuum and vapor storage hoses.
  2.  

IDLE MIXTURE 6- AND 8-CYLINDER ENGINES

If the carburetor has been overhauled, or the plastic caps removed from the mixture screws, the following procedure must be used to adjust the mixture. It must be emphasized that the manufacturer does not recommend this procedure as a substitute for the preceding methods, in that exhaust emission quality can be adversely affected unless the proper test equipment is available.

  1. Turn in the mixture screw(s) until lightly seated, then back them out 3 1 / 2 turns.
  2.  
  3. Start the engine and adjust the carburetor idle speed screw to obtain a speed 25 rpm above the specified idle (automatic), 75 rpm higher for 6-cylinder and 2-bbl. V8 (manual), or 100 rpm higher for 4-bbl. V8 (manual).
  4.  
  5. Turn the mixture screw(s) in equally until specified idle speed is obtained. At this point, a CO meter should be employed to adjust the mixture. A reading of 1.0% or less must be maintained.
  6.  
  7. Shut off the engine and install new limiter caps.
  8.  
  9. Adjust the fast idle speed, as previously described.
  10.  

1973-74 Models

All 6-cylinder engines, V8s with manual transmissions, and all 455 cu. in. Super Duty carburetors are equipped with an idle stop solenoid. Idle adjustments must be made with the engine at normal operating temperature, choke open, air conditioning OFF, all vacuum lines disconnected and plugged, and automatic transmissions in Drive.


CAUTION
Ensure that the parking brake is set and the drive wheels blocked.

6-CYLINDER ENGINES
  1. Disconnect the fuel tank evaporative hose from the canister.
  2.  
  3. Disconnect the distributor vacuum hose and plug it.
  4.  
  5. Dwell and ignition timing must already be set and correct.
  6.  
  7. Disconnect the idle stop solenoid wire. Using a 1 / 8 in. allen wrench, adjust the screw in the idle stop solenoid for the lower idle speed figure in the Tune-up Specifications chart (solenoid is not energized).
  8.  
  9. Reconnect the idle stop solenoid wire. Set the idle rpm to the higher figure in the chart by rotating the body of the idle stop solenoid (solenoid energized).
  10.  
  11. On manual transmission models, use a pair of needlenose pliers to extend the plunger on the CEC valve to the end of its travel. Use an open-end wrench to adjust the hex on the plunger until it just touches the throttle lever and idle speed is 850 rpm.
  12.  
  13. With automatic transmission in Park, or manual in Neutral, check the fast idle speed with the fast idle tang on the top step of the cam. If the fast idle speed is not at 2400 rpm on 1973 models or 1800 rpm on 1974 models, bend the tang to correct it.
  14.  

8-CYLINDER ENGINES
  1. Disconnect the carburetor hose from the vapor canister.
  2.  
  3. Disconnect the distributor and EGR valve vacuum hoses, plug the hoses to the carburetor, and disconnect the wire from the idle stop solenoid (if so equipped).
  4.  
  5. Check the dwell and ignition timing, if you haven't already done so.
  6.  
  7. Adjust the carburetor idle speed screw to the figure specified in the Tune-up Specifications chart (solenoid not energized).
  8.  
  9. Reconnect the idle stop solenoid wire, if so equipped. Extend the solenoid screw by hand and adjust the idle speed to the higher figure in the Tune-up Specifications chart (solenoid energized).
  10.  
  11. On 4-bbl. carburetors, with automatic transmission in Park or manual transmission in Neutral, check the fast idle speed with fast idle speed screw on the top step of the cam. Turn the screw to adjust the fast idle to 1500 rpm (except 455 SD, which should be 2000 rpm).
  12.  

Fast idle is not adjustable on 2-bbl. carburetors.

1975-76 Models

ALL ENGINES
  1. This adjustment must be made with the engine at normal operating temperature, with the air conditioner off and with the air cleaner removed. The air cleaner vacuum fitting in the manifold should be plugged. Automatic transmissions should be in Drive and manual transmissions should be in Neutral.
  2.  
  3. Set the parking brake and block the wheels.
  4.  
  5. Disconnect and plug the hose which runs from the carburetor to the vapor canister.
  6.  
  7. On 1975 350 (2-bbl.), 400 and 455 engines (except HO models) with manual transmissions and all California 250 6-cylinder engines, disconnect and plug the distributor vacuum hose.
  8.  
  9. Using pliers, break off the plastic idle mixture screw limiter caps (6-cylinder engines have only one cap). Turn the mixture screws clockwise until they seat lightly, then back them out five (5) complete turns (six turns on the 455 HO).
  10.  
  11. Hook up a tachometer and adjust the idle speed screw or the idle solenoid screw to get the "before lean drop idle" speed which is found on the underhood sticker.
  12.  
  13. Adjust the mixture screws equally ( 1 / 8 -turn increments) to obtain the highest possible idle speed.
  14.  

Check this adjustment by shifting into Neutral, running the engine at 2,000 rpm for 5-10 seconds, returning to idle, shifting back to Drive and letting the speed stabilize for 10 seconds.

  1. Return the idle speed to that which was set in Step 6.
  2.  
  3. Repeat Steps 7 and 8 until no further speed increase is possible.
  4.  
  5. Turn in the mixture screws equally until the normal idle speed is reached.
  6.  
  7. Place the automatic transmission in Park and the manual in Neutral. Check the underhood specifications sticker and adjust the fast idle speed to specifications. If there is no figure on the sticker for a fast idle setting, you need not adjust it.
  8.  

When adjusting the fast idle on a car equipped with the 4MC or the 1MV carburetor, make sure that the fast idle speed screw is on the high step of the fast idle cam.

  1. On cars equipped with an idle speed-up solenoid, place the transmission in Drive, detach the terminal connector at the air conditioner compressor clutch and adjust the solenoid to get 675 rpm; when finished, reconnect the terminal.
  2.  
  3. On cars equipped with a dashpot, adjust it so that at idle there is 0.040 in. clearance between the tip of the plunger (compressed) and the throttle lever.
  4.  
  5. Install and connect the air cleaner. Use the idle mixture screw to make any slight idle speed correction, if necessary.
  6.  
  7. Replace any vacuum hoses that were previously removed.
  8.  

1977 Models

IDLE SPEED 231 V6 AND 305 V8 ENGINES
  1. Run the engine until it reaches normal operating temperature. Make sure that the choke is open and the air conditioning is OFF.
  2.  
  3. Connect a tachometer and a timing light as detailed earlier in this section.
  4.  
  5. Set the parking brake and block the wheels.
  6.  
  7. Tag, disconnect and plug all vapor canister and EGR vacuum hoses.
  8.  
  9. Start the engine and place the transmission in Drive or Neutral.
  10.  
  11. Disconnect the vacuum advance hose and then set the timing as detailed earlier in this section.
  12.  
  13. Reconnect the vacuum advance hose and set the idle speed to specifications.
  14.  
  15. Connect all hoses and stop the engine.
  16.  

IDLE SPEED 301 V8 ENGINE
  1. Run the engine until it reaches normal operating temperature. Make sure that the choke is open and the air conditioning is OFF.
  2.  
  3. Connect a tachometer and a timing light as detailed earlier in this section.
  4.  
  5. Set the parking brake and block the wheels.
  6.  
  7. Tag, disconnect and plug all carbon canister and EGR vacuum hoses.
  8.  
  9. Disconnect the air conditioner compressor clutch connector.
  10.  
  11. Start the engine and place the transmission in Drive or Neutral.
  12.  
  13. Disconnect and plug the distributor vacuum advance line. Set the ignition timing.
  14.  
  15. Unplug and reconnect the distributor vacuum line.
  16.  
  17. On cars with A/C, adjust the idle stop screw to the specified rpm, turn the A/C ON and disconnect the electrical connector at the A/C compressor. Open the throttle momentarily to ensure that the solenoid plunger is fully extended. Adjust the solenoid screw to the specified rpm and then reconnect the electrical connector at the A/C compressor.
  18.  
  19. On cars without A/C, turn the idle screw to obtain the specified rpm.
  20.  
  21. Remove the tachometer, timing light and all hoses and connections.
  22.  
  23. Remove the tachometer and the timing light and connect all hoses and connections that were disconnected.
  24.  

IDLE SPEED 350, 400 AND 403 V8 ENGINES
  1. Follow Steps 1-4 of the 301 V8 procedure.
  2.  
  3. Disconnect the air conditioning compressor clutch lead wire.
  4.  
  5. Start the engine and place the transmission in Park or Neutral.
  6.  
  7. Check and adjust the ignition timing as detailed earlier in this section.
  8.  
  9. Place the transmission in Drive or Neutral.
  10.  
  11. On models with air conditioning, turn the system ON. This will energize the solenoid. Open the throttle momentarily to extend the plunger.
  12.  
  13. Adjust the solenoid to the specified rpm.
  14.  
  15. Reconnect the compressor and turn the A/C OFF.
  16.  
  17. On cars with manual transmissions, adjust the idle speed screw to the specified rpm.
  18.  
  19. Reconnect all hoses and shut off the engine.
  20.  

MIXTURE ADJUSTMENT ALL ENGINES
  1. Run the engine until it reaches normal operating temperature. Make sure that the choke is fully open and the air conditioning is OFF.
  2.  
  3. Set the parking brake and block the wheels.
  4.  
  5. Remove the air cleaner, but leave the vacuum hoses connected.
  6.  
  7. Disconnect and plug any hoses listed on the underhood specifications sticker.
  8.  
  9. Connect a tachometer as detailed earlier in this section.
  10.  
  11. Check and adjust the ignition timing.
  12.  
  13. With pliers, carefully remove the idle mixture screw limiter caps.
  14.  
  15. Turn the screws clockwise until they are just seated and then back them out just enough so that the engine will run.
  16.  
  17. Turn each screw counterclockwise 1 / 8 turn at a time until the maximum idle speed is obtained.
  18.  
  19. Set the idle speed to the higher of the two figures shown on the underhood sticker. Repeat Step 9 to be sure that you have the maximum idle speed.
  20.  
  21. Turn each screw clockwise 1 / 8 turn at a time until you reach the lower of the two figures given on the sticker.
  22.  
  23. Reset the idle to the specified rpm.
  24.  
  25. Reconnect all hoses and install the air cleaner.
  26.  

1978-79 Models

IDLE SPEED 231 V6 AND 305 V8 ENGINES

Adjustment procedures for idle speed and mixture on these engines correspond with those previously given for the model year 1977.

IDLE SPEED 301 V8 ENGINES
  1. Run the engine until it reaches normal operating temperature. Make sure that the choke is fully open and the air conditioning is OFF.
  2.  
  3. Connect a tachometer and a timing light as detailed earlier in this section.
  4.  
  5. Set the parking brake and block the wheels.
  6.  
  7. Disconnect the air conditioner compressor clutch lead wire.
  8.  
  9. Start the engine and place the transmission in Drive or Neutral.
  10.  
  11. Check and adjust the ignition timing.
  12.  
  13. Disconnect the purge hose at the vapor canister.
  14.  
  15. On cars with air conditioning: set the idle speed screw to obtain the specified rpm. Turn the A/C ON. Open the throttle momentarily to ensure that the solenoid plunger is fully extended. Adjust the idle speed solenoid to the speed given on the underhood sticker. Turn the A/C OFF.
  16.  
  17. On cars without air conditioning, turn the idle speed screw until you obtain the specified rpm.
  18.  
  19. Place the automatic transmission in Park.
  20.  
  21. Disconnect and plug the vacuum hose running from the EGR valve.
  22.  
  23. Adjust the fast idle screw on the second step of the fast idle cam until you obtain the specified rpm.
  24.  
  25. Stop the engine and reattach the EGR vacuum hose, the vapor canister hose and the A/C compressor clutch connector.
  26.  

IDLE SPEED 350, 400 AND 403 V8 ENGINES
  1. Follow Steps 1-3 of the preceeding 301 V8 procedure.
  2.  
  3. Disconnect the purge hose from the vapor canister. On the 350 engine, you must plug the hose.
  4.  
  5. Disconnect and plug the vacuum hose leading from the EGR valve.
  6.  
  7. Start the engine and place the transmission in Park or Neutral.
  8.  
  9. Check and adjust the ignition timing.
  10.  

After checking the timing on the 400 engine with a manual transmission, do not reconnect the vacuum advance line.

  1. Place the automatic transmission in Drive.
  2.  
  3. On models equipped with air conditioning, turn the A/C ON and disengage the electrical connector at the compressor clutch. Open the throttle momentarily to ensure that the solenoid plunger is fully extended. Adjust the solenoid screw to the specified rpm. Reconnect the compressor clutch connector and turn the A/C OFF.
  4.  
  5. On models without air conditioning, turn the idle screw to obtain the specified rpm.
  6.  
  7. Reconnect any hoses previously disconnected, then shut off the engine.
  8.  

MIXTURE ADJUSTMENT ALL ENGINES

A change has been made in GM carburetors which limits the effect of the mixture screw for rich adjustment. In other words, backing out the screw will now have little or no effect. 1978 and later models 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 enrichener. The equipment necessary for this procedure is not readily available to the general public.

1980-81 Models

IDLE SPEED 231 V6 AND 265 V8 ENGINES

Adjustment procedures for idle speed and mixture on these engines correspond with those previously given for the 1978-79 301 V8.

IDLE SPEED 301 AND 305 V8 ENGINES

Adjustment procedures for idle speed and mixture on these engines correspond with those previously given for the 1978-79 350, 400 and 403 V8 engines.

MIXTURE ADJUSTMENTS ALL ENGINES

As stated earlier, all 1978 and later models utilize a sealed idle mixture screw. Because it is sealed, idle mixture is adjustable only during carburetor overhaul and should be adjusted only by a qualified service technician.

 
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