GM Corvette 1963-1982 Repair Guide

Idle Speed and Mixture

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ADJUSTMENT



See Figures 1 through 8

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Fig. Fig. 1: Carter WCFB idle mixture and speed adustment screw location



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Fig. Fig. 2: Carter AFB idle mixture and speed adjustment screws



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Fig. Fig. 3: Holley 4150 idle mixture and speed adjustment screws



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Fig. Fig. 4: Rochester Quadrajet idle mixture and speed adjustment screw locations



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Fig. Fig. 5: Holley 2300 idle mixture and speed adjustment screws



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Fig. Fig. 6: Rochester Quadrajet (with idle solenoid) idle speed screw



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Fig. Fig. 7: Holley 4150 (with idle solenoid) idle mixture and speed adjustment locations



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Fig. Fig. 8: Rochester Quadrajet (with idle solenoid) idle mixture screws

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 engine models have resulted in a great variety of tune-up specifications. See the "Tune-Up Specifications" chart at the beginning of this information. 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.

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).

Due to the camshaft design of some special high performance engines (350/LT1, 427/L88, 454/LS6, etc.) it is normal for these engines to have a "lopey" idle.

1963-67 Vehicles Without A.I.R.

Adjust with air cleaner removed.

  1. Connect a tachometer and vacuum gauge to the engine, then set the parking brake and shift the manual transmission into Neutral, automatic into Drive.
  2.  
  3. Turn the idle mixture screw(s) in until lightly seated, then back out 1 1 / 2 turns.
  4.  
  5. With engine running, adjust the idle speed screw to obtain the specified rpm.
  6.  
  7. Adjust the idle mixture screw(s) to obtain the highest steady manifold vacuum at the specified speed. If necessary, reset the idle speed screw while adjusting mixture.
  8.  

On air conditioned models, the air conditioner is turned on and the hot idle compensator valve is held closed while adjusting idle speed and mixture.

  1. Final adjustment should be made with the air cleaner installed.
  2.  
  3. Remove tachometer and vacuum gauge.
  4.  

1966-67 Vehicles With A.I.R.

Adjust with air cleaner removed.

  1. Connect a tachometer to the engine, place manual transmission in Neutral, automatic in Drive.
  2.  
  3. Turn idle mixture screw(s) in until lightly seated, then back out 3 turns.
  4.  
  5. With engine running, adjust the idle speed screw to obtain the specified idle speed.
  6.  
  7. Adjust the idle mixture screw(s) in to "lean roll" position, then back them out (rich) 1 / 4 turn. Readjust the idle speed screw to keep the engine at the specified idle speed while adjusting the mixture.
  8.  

On air conditioned cars, turn the air conditioner off with 327 cu in. engines. Air conditioner must be on and hot idle compensator held closed with 427 cu in. engines.

  1. Final adjustment should be made with the air cleaner installed.
  2.  
  3. Remove the tachometer.
  4.  

1968-69 Vehicles

Adjust with air cleaner installed.

  1. Turn the idle mixture screw(s) in until lightly seated, then back out 3 turns.
  2.  
  3. With engine at operating temperature, adjust idle speed screw to obtain specified rpm, manual transmission in Neutral and automatic in Drive.
  4.  

On all 1968 models except 350 H.P. 327 cu. in. with manual transmission, the air conditioner is turned off. On the above-mentioned vehicles the air conditioner is left on. On 1969 models, turn the air conditioner either on or off according to the instructions on the tune-up decal.

  1. Adjust one idle mixture screw to obtain the highest steady idle speed.
  2.  
  3. Adjust the idle speed screw to the speed specified on the tune-up decal.
  4.  

On models equipped with an idle solenoid, adjust the solenoid plunger hex to obtain 600 rpm. Disconnect the wire at the solenoid to deenergize it, allowing the throttle lever to contact the carburetor idle speed screw. Adjust the carburetor idle speed screw. Adjust the carburetor idle screw to obtain 400 rpm.

5.Adjust the mixture screw in to "lean roll" position, then back out (rich) 1 / 4 turn.

  1. Repeat Steps 3, 4, and 5 for the other idle mixture screw.
  2.  
  3. Readjust the idle speed screw to obtain final specified rpm, if necessary.
  4.  

1970 Vehicles

Adjust with air cleaner installed.

If the vehicle is equipped with Evaporative Emission, disconnect the fuel tank line from the vapor canister while making the idle speed and mixture adjustments. Warm up the engine and leave it running while adjusting. The choke valve and, if applicable, air cleaner damper door should remain open. Leave the air conditioning off.

350 (300, 350 AND 370 HP) ENGINES
  1. Adjust the idle mixture screws equally to obtain maximum idle speed.
  2.  
  3. On the 300 H.P. engine with the manual transmission in Neutral adjust the idle speed screw to obtain 700 rpm. On the 300 H.P. engine with automatic transmission in Drive, adjust the idle speed screw to obtain 600 rpm.
  4.  
  5. On the 350 and 370 H.P. engines, adjust the idle speed screw to obtain 750 rpm with the manual transmission in Neutral.
  6.  

454 (450 HP) ENGINE
  1. Remove the air cleaner.
  2.  
  3. Disconnect the distributor vacuum hose at the distributor and plug the hose.
  4.  
  5. Adjust the mixture screws for maximum idle speed.
  6.  
  7. With manual transmission in Neutral, adjust the carburetor idle speed screw to obtain 750 rpm. With automatic transmission in Drive, adjust the carburetor idle speed screw to obtain 700 rpm.
  8.  
  9. Turn one idle mixture screw to obtain a 20 rpm drop in idle speed, then back the screw out 1 / 4 turn. Repeat for the second idle mixture screw.
  10.  
  11. Repeat Step 4 above.
  12.  
  13. Reconnect the distributor vacuum hose and install the air cleaner.
  14.  

454 (345 HP) and 454 (390 HP) ENGINES
  1. Disconnect the distributor vacuum hose at the distributor and plug the hose.
  2.  
  3. Turn the idle mixture screws in until they are lightly seated, then back them out 4 turns.
  4.  
  5. With the automatic transmission in Drive, adjust the carburetor idle speed screw to obtain 630 rpm. Adjust the idle mixture screws in equally to obtain 600 rpm.
  6.  
  7. With manual transmission in Neutral, adjust the carburetor idle speed screw to obtain 700 rpm. Turn one of the mixture screws in until the engine speed drops 40 rpm. Readjust the idle speed screw to obtain 700 rpm. Turn in the other mixture screw until the engine speed drops 40 rpm. Readjust the idle speed screw to obtain 700 rpm.
  8.  
  9. Reconnect the distributor vacuum hose.
  10.  

1971 Vehicles

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 except the 350 (330 hp) and 454 (425 hp) 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. Combination Emission Control System (C.E.C. solenoid) valve regulates distributor vacuum as a function of transmission gear position.


CAUTION
The C.E.C. 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 gas cap removed.
  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.

350 (4-BBL QUADRAJET) ENGINES

Adjust the carburetor idle speed screw (NOT the solenoid plunger) to obtain 600 rpm (manual transmission in Neutral with the air conditioner off) or 550 rpm (automatic transmission in Drive with the air conditioner on).

350 AND 454 (4-BBL HOLLEY) ENGINES
  1. Adjust the carburetor idle speed screw (NOT the solenoid plunger) to obtain 700 rpm (manual transmission in Neutral or automatic transmission in Drive).
  2.  
  3. Adjust the idle mixture screws alternately to obtain the maximum smooth idle speed.
  4.  
  5. Adjust one of the idle mixture screws to obtain a 20 rpm drop ("lean roll"), then back it out 1 / 4 turn.
  6.  
  7. Repeat Step 4 above for the other idle mixture screw.
  8.  
  9. Readjust the carburetor idle speed screw to obtain 700 rpm if necessary.
  10.  

454 (4-BBL QUADRAJET) ENGINES

Turn the air conditioner off. Adjust the carburetor idle speed screw (NOT the solenoid plunger) to obtain 600 rpm (manual transmission in Neutral or automatic transmission in Drive).

1972 Vehicles

All carburetors are equipped with idle limiter caps and idle mixture is preset at the factory and should not require adjustment.

  1. Remove the fuel filler cap but do not remove the vapor line.
  2.  
  3. Detach the distributor vacuum hose and plug the hose.
  4.  
  5. Set the parking brake and turn the air conditioner (if so equipped) off. On cars equipped with an automatic transmission, check the wheels.
  6.  
  7. Allow the engine to reach normal operating temperature. Be sure that the choke is open.
  8.  
  9. If the car has an automatic transmission, set the selector in Drive. If the car has a manual transmission keep the transmission in Neutral.
  10.  
  11. Adjust the anti-dieseling solenoid to the higher of the two rpm figures given in the specifications.
  12.  


CAUTION
Do not turn the solenoid more than one complete turn unless the electrical lead is disconnected (solenoid deenergized).

  1. Disconnect the solenoid lead and set the idle speed to the lower of the two figures given in the specifications. Use the normal idle speed adjusting screw.
  2.  

If no lower figure is given, adjust the idle to 450 rpm.

  1. Reconnect all of the wires and hoses which were disconnected in order to perform these adjustments.
  2.  

1973 Vehicles

All models are equipped with idle limiter caps and idle solenoids. Disconnect the fuel tank line from the evaporative canister. The engine must be running at operating temperature, choke off, parking brake on, and rear wheels blocked. Disconnect the distributor vacuum hose and plug it. After adjustment, reconnect the vacuum and evaporative hoses.

FOUR-BARREL 350 AND 454 CU IN. V8 ENGINES
  1. Adjust the idle stop solenoid screw for 900 rpm on manual, 600 rpm on automatic.
  2.  
  3. Connect the distributor vacuum hose and position the fast idle cam follower on the top step of the fast idle cam (turn air conditioning off) and adjust the fast idle to 1,300 rpm on manual transmission 350 engines; 1,600 on manual 454 engines and all automatics (in Park).
  4.  

OPTIONAL 350 CU IN. (L82) V8 ENGINE
  1. Adjust the idle stop solenoid screw (air conditioning off) for a speed of 900 rpm on manual transmission; 700 rpm on automatic (in Drive).
  2.  
  3. Connect the distributor vacuum hose and position the fast idle cam follower on the top step of the cam (turn air conditioning off) and adjust the fast idle to 1,300 rpm on manual; 1,600 rpm on automatic.
  4.  

1974 Vehicles

The same preconditions as for 1973 apply.

FOUR-BARREL 350 CU IN. V8
  1. Turn the air conditioning off. Adjust the idle stop solenoid screw for 900 rpm on manual transmission models; 600 rpm on automatic (in Drive).
  2.  
  3. Connect the distributor vacuum hose. Position the fast idle cam follower on the top step of the fast idle cam and adjust the fast idle speed to 1,300 rpm on manual; 1,600 on automatic (in Park).
  4.  

OPTIONAL 350 CU IN. (L82) V8 ENGINE
  1. Turn the air conditioning off. Adjust the idle stop solenoid for 900 rpm on manual; 700 rpm on automatic (in Drive).
  2.  
  3. Connect the distributor vacuum hose. Position the fast idle cam follower on the top step of the cam and adjust the fast idle to 1,300 rpm on manual; 1,600 rpm on automatic (in Park).
  4.  

454 CU IN. V8 ENGINE
  1. Shut off the air conditioning. Adjust the idle stop solenoid screw for 800 rpm on manual; 600 rpm on automatic (in Drive).
  2.  
  3. Connect the distributor vacuum hose and position the fast idle cam follower on the top step of the cam and adjust the fast idle to 1,600 rpm on manual; 1,500 rpm on automatic (in Park).
  4.  

1975-76 Vehicles

The same preconditions as for 1973 apply.

  1. Turn the air conditioning off.
  2.  
  3. Disconnect the idle speed solenoid. Turn the idle speed screw to adjust for the lower of the two idle speeds specified on the underhood tune-up decal. Adjust automatic transmission cars in Drive with wheels blocked; manual transmission cars in Neutral.
  4.  
  5. Connect the idle speed solenoid. Open the throttle to extend the solenoid plunger.
  6.  
  7. Use the solenoid plunger to adjust the idle speed to the higher of the two speeds on the underhood tune-up decal.
  8.  

1977 Vehicles-Idle Speed See Figure 9

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Fig. Fig. 9: Idle speed adjustment-1977-80 M4MC carburetors

Run the engine to normal operating temperature, A/C off, vacuum advance line disconnected and plugged, FUEL TANK line at the canister disconnected. Place the manual transmission in neutral; automatic transmission in drive. Connect a tachometer to the engine.

Make sure the parking brake is on and the drive wheels are sufficiently blocked when placing the transmission in drive.

WITHOUT A/C
  1. Turn the idle speed screw to achieve the rpm specified on the underhood tune-up decal.
  2.  

WITH A/C
  1. Turn the idle speed screw to the lower of the two idle speeds specified on the underhood tune-up decal.
  2.  
  3. Turn the A/C system on.
  4.  
  5. Disconnect the A/C compressor lead at the compressor.
  6.  
  7. Open the throttle slightly to allow the solenoid plunger to fully extend.
  8.  
  9. Turn the solenoid screw to higher of the two idle speeds specified on the underhood tune-up decal.
  10.  
  11. Reconnect the A/C compressor lead.
  12.  

1977 Vehicles-Idle Mixture

Idle mixture screws have been preset at the factory and capped and should not require adjustment, however if the need arises follow the procedure listed below.

  1. Set the parking brake and block the drive wheels.
  2.  
  3. Remove the air cleaner to gain access to the carburetor, but keep the vacuum hoses connected.
  4.  
  5. Run the engine to normal operating temperature with the choke open.
  6.  
  7. Turn the A/C off.
  8.  
  9. Disconnect and plug the vacuum advance hose at the distributor.
  10.  
  11. Check the ignition timing and adjust as necessary.
  12.  
  13. Using a sharp knife, carefully remove the plastic caps from the idle mixture screws. Use extreme caution to avoid bending the mixture screws.
  14.  
  15. Lightly seat the screws then back out equally just enough so the engine will run.
  16.  
  17. Place the automatic transmission in drive or the manual transmission in neutral.
  18.  
  19. Back out each screw (richen) 1 / 8 turn at a time until the maximum idle speed is obtained. Then set the idle speed screw to: Standard engine (180 hp) M.T.-800 rpm, A.T.-550 rpm; Optional engine (210 hp) M.T.-900 rpm, A.T. (low altitude) 750 rpm, A.T. (high altitude), 650 rpm.
  20.  
  21. Turn in each screw (lean) 1 / 8 turn at a time until the idle reaches the following rpm; Standard engine (180 hp)-M.T.-700 rpm, A.T.-500 rpm. Optional engine (210 hp)-800 rpm, A.T. (low altitude), 600 rpm for A.T., high altitude.
  22.  
  23. Reset the idle speed to specification.
  24.  
  25. Reconnect the vacuum hoses and install the air cleaner.
  26.  

1978-80 Vehicles-Idle Speed

This procedure applies only to those models which DO NOT have the computer controlled emissions system. Vehicles having the computer system have 13 screws retaining the air horn of the carburetor; vehicles without the computer system have 9 screws retaining the carburetor air horn. Also, computer equipped vehicles do not have the familiar vacuum advance unit installed on the distributor, as the spark advance is controlled by the computer.


CAUTION
If your vehicle has the computer emissions system, DO NOT attempt to adjust either the idle speed or mixture, as the fuel system calibration could become severely upset.

Run the engine to normal operating temperature, A/C off, the purge hose at the vapor canister and the vacuum hose at the EGR valve disconnected and plugged. Place the manual transmission in Neutral and the automatic transmission in Drive.

Make sure the parking brake is on and the drive wheels are safely blocked before the transmission is placed in Drive.

WITHOUT A/C
  1. Turn the idle speed screw to achieve the rpm specified on the underhood specifications decal.
  2.  

WITH A/C
  1. Turn the idle speed screw to the lower of the two idle speeds listed on the underhood specifications decal.
  2.  
  3. Turn the air conditioning on.
  4.  
  5. Disconnect the A/C compressor lead at the compressor (two-wire connector).
  6.  
  7. Open the throttle slightly to allow the solenoid plunger to extend fully.
  8.  
  9. Turn the solenoid screw to adjust the engine speed to the higher of the two speeds listed on the specifications decal under the hood. Use the higher idle speed, NOT the fast idle speed specification.
  10.  
  11. Reconnect the A/C compressor lead.
  12.  

1978-80 Vehicles-Idle Mixture

Changes in the idle systems of these models make it impossible to properly adjust the mixture without the use of a propane enrichment system, not available to the general public.

In some 1978 and all 1979-80 models, the idle mixture screws are sealed within the throttle body of the carburetor. Idle mixture adjustments should be left to the professional technician with the proper equipment and experience.

1981-82 Vehicles-Idle Mixture

Idle speed and mixture are controlled strictly by the computer emissions system. Do not attempt to adjust the idle speed or mixture as these functions are controlled by the computer.

For all throttle body injection adjustments, see the Fuel Injection information.

 
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