GM Full-Size Trucks 1970-1979 Repair Guide

Carburetor Idle Speed And Mixture Adjustments

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In most cases, the mixture screws have limiter caps, but in later years the mixture screws 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. For these reasons, mixture adjustments are not covered here for affected models.

See the emission control label in the engine compartment for procedures and specifications not supplies here.

1970 VEHICLES



On all vehicles, disconnect the FUEL TANK line from the vapor canister. Remember to reconnect the line after setting the idle speed and mixture. The engine should be at operating temperature with the choke valve and air cleaner damper door fully open, air conditioning OFF and park brake ON.



250 and 292 Engines, 1 / 2 ton models. Disconnect and plug the vacuum advance line. Turn the mixture screw in until it lightly contacts the seat, then back out 4 turns. Adjust the solenoid screw to obtain 800 rpm with manual transmission in drive. Adjust the mixture screw to obtain 750 rpm with manual transmission in Neutral or automatic in Drive. Electrically, disconnect the solenoid and set the carburetor idle speed screw to obtain 400 rpm and connect the solenoid. Reconnect the vacuum line.
 
292 Engine, 3 / 4 and 1 ton Series: Disconnect and plug the distributor vacuum line. Turn the mixture screws in until they lightly contact the seats and back the screw(s) out 4 turns. On manual transmission models, adjust the carburetor idle speed screw to obtain 600 rpm in Neutral. Then adjust the mixture screw to obtain 550 rpm in Neutral. On automatic transmission models, adjust the solenoid screw to obtain 550 rpm with transmission in Drive. Adjust the mixture screw to obtain 500 rpm with transmission in Drive. Disconnect the solenoid and set the carburetor idle speed screw to obtain 400 rpm and connect the solenoid. Reconnect the distributor vacuum line on all models.
 
307 V8, 1 / 2 ton Series: Disconnect and plug the distributor vacuum line. Turn the mixture screws in until they lightly contact the seats then back them out 4 turns. Adjust the carburetor idle speed screw to obtain 800 rpm with manual transmission in Drive. Adjust the carburetor idle speed screw to obtain 800 rpm with manual transmission in Drive. Adjust the mixture screw to obtain 630 rpm with automatic transmission in Drive. Adjust the mixture screws in equally to obtain 700 rpm with manual transmission in Neutral or 600 rpm with automatic transmission in Drive. Disconnect the solenoid and set the carburetor idle speed screw to obtain 450 rpm and reconnect the solenoid. Reconnect the vacuum line.
 
307 V8, 3 / 4 and 1 ton Series: Disconnect and plug the distributor vacuum line. Set the mixture screws for maximum idle rpm and adjust the idle speed screw to obtain 700 rpm with manual transmission in Neutral or 600 rpm with automatic transmission in Drive. Adjust the mixture screws equally to obtain a 200 rpm drop, then back the screws out 1 / 4 turn. As necessary, adjust the idle screw on manual transmission models to obtain 700 rpm with the transmission in Neutral. On automatic transmission models, adjust the solenoid screw to obtain 600 rpm with the transmission in Drive. Disconnect the solenoid electrically and set the carburetor idle screw to obtain 450 rpm and reconnect the solenoid. Reconnect the vacuum line.
 
350 V8, 1 / 2 ton Series: Disconnect and plug the distributor vacuum line. Turn the mixture screws in until they lightly contact the seats. Back the screws out 4 turns. Adjust the idle speed screw to obtain 650 rpm with manual transmission in Neutral or 550 rpm with automatic transmission in Drive. Adjust the mixture screws equally to obtain 600 rpm with manual transmission in Neutral or automatic transmission in Drive. Reconnect the vacuum advance line.
 
350 and 396 V8, 3 / 4 and 1 ton Series: Disconnect and plug the distributor vacuum line. Turn the mixture screws in until they lightly contact the seats and back them out 4 turns. Adjust the carburetor idle speed screw to obtain 775 rpm (manual transmission in Neutral) or 630 rpm (automatic transmission in Drive). Adjust the mixture screws equally to obtain 700 rpm (manual transmission in Neutral) or 600 rpm (automatic transmission in Drive). Reconnect the vacuum line.
 

1971 VEHICLES



The engine should be running at operating temperature, choke valve fully open, parking brake ON and drive wheels blocked.



250 and 292 Engines, 1 / 2 ton Series: Disconnect the FUEL TANK line from the evaporative canister. Disconnect the vacuum advance line and plug the vacuum source. Adjust the carburetor speed screw to obtain 550 rpm with manual transmission in Neutral or 550 rpm with automatic transmission in Drive. Do not adjust the solenoid screw.
 


CAUTION
Use the CEC solenoid screw to adjust the idle speed could result in a decrease in engine braking.

Reconnect the FUEL TANK line and the vacuum advance line.



307, 1 / 2 ton Series: Disconnect the FUEL TANK line from the evaporative canister. Disconnect the vacuum source opening. Adjust the carburetor speed screw to obtain 600 rpm with manual transmission in Neutral or 550 rpm with automatic transmission in Drive and air conditioner ON. Do not adjust the solenoid. See the previous Caution. Reconnect the FUEL TANK line and the vacuum advance line.
 
350 and 402, 1 / 2 ton Series: Disconnect the FUEL TANK line from the evaporative canister. Disconnect the vacuum advance line and plug the vacuum source opening. Turn the air conditioner OFF and adjust the carburetor speed screw to obtain 600 rpm with manual transmission in Neutral or 550 rpm (600 rpm on 8-402) with automatic transmission in Drive. Do not adjust the solenoid screw, or a decrease in engine breaking could result. Place the fast idle cam follower on the second step of the fast idle cam and turn the aid conditioner OFF. Adjust the fast idle to 1,350 rpm with manual transmission in Neutral or 1,500 rpm with automatic transmission in PARK. Reconnect the FUEL TANK and vacuum advance lines.
 
292 and 307, 3 / 4 and 1 ton Series: Disconnect the vacuum advance line and plug the vacuum source opening. Turn the air conditioner ON. Turn the mixture screws in until they lightly contact the seats and back them out 4 turns. Adjust the carburetor speed screw to obtain 600 rpm with manual transmission in Neutral and 500 rpm with automatic in Drive. Reconnect the vacuum advance hose.
 
350 and 402, 3 / 4 and 1 ton Series: Disconnect the distributor vacuum advance hose and plug the vacuum source opening. Turn the air conditioner off. Turn the mixture screws in until they contact the seats lightly. Back the screws out 4 turns. Adjust the carburetor speed screw to obtain 650 rpm with manual transmission in Neutral or 550 rpm with automatic transmission in Drive. Reconnect the vacuum advance hose.
 

1972 VEHICLES



The engine should be at normal operating temperature on the choke valve fully open, parking brake ON and the drive wheels blocked. All carburetors are equipped with idle mixture limiter caps, which provide for only a small adjustment range. Normally, these should not be removed. However, if they are removed, the CO content of the exhaust should be checked to be sure that it meets Federal Emission Control limits.


CAUTION
If the CEC solenoid screw is adjusted out of limits, a decrease in engine breaking may result.



250 and 292: Disconnect the FUEL TANK line from the vapor canister. Remember to reconnect it after making the adjustment. Disconnect the plug the vacuum line source. Adjust the idle stop solenoid to obtain 700 rpm with manual transmission in Neutral or 600 rpm with automatic transmission in Drive. Do not adjust the CEC solenoid screw.
 

Reconnect the vacuum line.



307: Disconnect the FUEL TANK line from the vapor canister and remove and plug the vacuum line source. Adjust the idle stop solenoid screw to obtain 900 rpm (950 on California trucks) with manual transmission in Neutral or 600 rpm with automatic transmission in Drive. On trucks without TCS, adjust speed screw for 600 rpm with transmission in Neutral. With transmission in Park or Neutral, adjust the fast idle speed to obtain 1850 rpm. Reconnect the FUEL TANK line and the vacuum line.
 
350: Disconnect the FUEL TANK line and disconnect and plug the vacuum line. On vehicles with TCS, turn the air conditioner OFF and adjust the idle solenoid screw to obtain 800 rpm with manual transmission in Neutral or 600 rpm with automatic transmission in Drive. On vehicles without TCS, adjust the carburetor speed screw to obtain 600 rpm with transmission in Neutral. Place the fast idle cam, turn the air conditioner OFF and adjust the fast idle to 1,350 rpm with manual transmission in Neutral or automatic transmission in Drive. Reconnect the FUEL TANK and vacuum lines.
 
402: Disconnect the FUEL TANK line from the evaporative canister. Disconnect the vacuum advance hose and plug the vacuum source opening. On trucks with TCS, turn the air conditioning OFF and adjust the idle stop solenoid screw to obtain 750 rpm with manual transmission in Neutral or 600 rpm with automatic in Drive. On trucks without TCS, adjust the carburetor speed screw to obtain 600 rpm with transmission in Neutral. Place the fast idle cam follower on the second step of the fast idle cam, turn the air conditioner OFF and adjust the fast idle speed to 1,350 rpm with manual transmission in Neutral or 1,500 rpm with automatic transmission in Drive. Reconnect the FUEL TANK line and the vacuum advance line.
 

1973-75 VEHICLES



Emission system requirements necessitate the division of trucks as follows:



Light Duty: All 1 / 2 ton Series
 
Heavy Duty: 3 / 4 and 1 ton
 

All adjustments should be made with the engine at operating temperature, choke fully open, air conditioner OFF, parking brake ON and drive wheels blocked.



250: Disconnect the FUEL TANK line and the vacuum source opening. Plug the vacuum line. Adjust the idle stop solenoid by turning the hex nut to obtain 700 rpm (1973) or 600 rpm (1974) on all heavy duty vehicles with manual transmission in Neutral; 600 rpm (1973-74) on all light duty vehicles with automatic transmission in Drive; 850 rpm (1974-75) on all light duty trucks with manual transmission.
 


CAUTION
Do not adjust the CEC solenoid on 1973 vehicles or a decrease in engine braking may result.

Place automatic transmissions in Park and manual transmission in Neutral and adjust the fast idle to 1800 rpm, on the top step of the fast idle cam.

Reconnect the FUEL TANK and vacuum lines.



292: Disconnect the vacuum advance line and plug the vacuum source opening. Adjust the idle stop solenoid screw to obtain 600 rpm on California (and all 1974-75) trucks or 700 rpm on all other trucks. Reconnect the vacuum advance line.
 
1973 307: On light duty vehicles, disconnect the FUEL TANK line from the vapor canister and plug the vacuum source opening. Adjust the idle stop solenoid to obtain 600 rpm with automatic transmission in Park or 900 rpm with manual transmission in Neutral. Disconnect the idle stop solenoid and adjust the low idle screw located inside the solenoid hex nut, to obtain 450 rpm in Neutral or Drive. Reconnect the idle stop solenoid, the FUEL TANK line, and the vacuum line.
 
350: On light duty vehicles, disconnect the FUEL TANK line from the vapor canister. Disconnect and plug the vacuum line. On heavy duty vehicles, adjust the carburetor idle speed screw to obtain 600 rpm with automatic transmission in Park or manual transmission in Neutral. On light duty vehicles, adjust the idle stop solenoid screw to obtain 600 rpm with automatic transmission in Drive or 900 rpm with manual transmission in Neutral. On light duty vehicles with automatic transmission, reconnect the vacuum line and adjust the fast idle to 1600 rpm on the top step of the fast idle cam. On light duty vehicles with manual transmission, adjust the fast idle screw to obtain 1300 rpm with the screw on the top step of the fast idle cam and the vacuum line disconnected. Reconnect the FUEL TANK line and the vacuum line.
 
454: On light duty trucks, disconnect the FUEL TANK line from the evaporative canister. Disconnect the vacuum advance line and plug the vacuum source opening. Adjust the idle speed with the idle stop solenoid screw to obtain: 700 rpm on all heavy duty trucks with automatic transmission in Park and manual transmission in Neutral. 600 rpm on light duty trucks with automatic transmission in Drive. 900 rpm (1973) or 800 (1974-75) on light duty trucks with manual transmission in Neutral.
 

On light duty trucks with automatic transmission, reconnect the vacuum advance line. Adjust the fast idle screw to obtain 1,600 rpm with the screw on the top step of the fast idle cam.

On light duty trucks with manual transmission, adjust the fast idle screw to obtain 1,600 rpm (1973) or 1,500 rpm (1974-75) with the screw on the top step of the fast idle cam and the vacuum hose disconnected.

Reconnect the FUEL TANK line and the vacuum advance line.

1976 VEHICLES



All adjustments should be made with engine at normal operating temperature, air cleaner on, choke open, and air conditioning off, unless otherwise noted. Set the parking brake and block the rear wheels.



250 and 292 Engines: Disconnect and plug the carburetor and PCV hoses at the vapor canister on the 250. On heavy duty emissions 250s and all 292s, disconnect the FUEL TANK hose at the vapor canister. If the engine has a solenoid located between the carburetor and the distributor on the vacuum hose line, disconnect and plug the vacuum line. Otherwise, leave the vacuum hose connected. On heavy duty 250s and all 292s turn the air conditioning on, if so equipped. With manual transmissions in Neutral, 250 automatics in Drive, and heavy duty 250 and 292 automatics in Neutral, turn the solenoid body in or out to set the idle speed to specified rpm. Disconnect the electrical wire from the carburetor solenoid, and turn the air conditioner off. Turn the 1 / 8 in. hex (Allen head) screw located in the end of the solenoid body to set the low idle to 425 rpm for 250s and 450 rpm for heavy duty 250s and 292s. Reconnect the electrical wire and any hoses that were disconnected.
 
350, 400 and 454 Engines: On California emissions engines, disconnect and plug the FUEL TANK hose at the vapor canister. On heavy duty emissions 350s and 400s, turn the air conditioning on. Place the automatic transmission in Drive, manuals in Neutral, unless otherwise noted in the Tune-Up Specifications chart. Adjust the carburetor idle speed to the specified rpm by turning the idle speed screw. Reconnect the FUEL TANK hose on California engines. On engines equipped with four barrel carburetors, place the transmission in Park for automatics, and leave manual transmission in Neutral. Disconnect and plug the vacuum hose at the EGR valve if so equipped. On 454 engines with electric chokes, disconnect and plug the vacuum hose to the front vacuum break unit located in front of the choke coil housing. Position the cam follower lever of the fast idle unit on the proper step on the cam, as specified on the underhood emissions sticker. Turn the fast idle screw to obtain the specified rpm on the sticker. Reconnect any hoses that were disconnected.
 

1977-78 VEHICLES



All adjustments should be made with the engine at normal operating temperature, air cleaner on, choke open, and air conditioning off, unless otherwise noted. Set the parking brake and block the rear wheels. Automatics should be place in Drive, manuals in Neutral, except as noted in the Tune-Up Specifications chart.



250 and 292 Engines: Make certain that the fast idle follower is not on any of the steps marked H , 2 , or L on the cam. It should be resting against the first step below L . Set the idle to specifications by turning the solenoid in or out. Do this with a wrench on the nut attached to the end of the solenoid body. Disconnect the electrical connector from the solenoid. The engine speed will drop. With a 1 / 8 in. hex (Allen head) wrench, turn the screw located inside the nut attached to the solenoid body, and set the idle speed to 425 rpm for light duty emissions trucks, 450 rpm for heavy duty 250s and 292s.
 

Reconnect the solenoid wire and check idle speed.



305, 350, 400 and 454 Engines: Check the underhood emissions sticker to determine which hoses, if any, must be disconnected. On carburetors not equipped with a solenoid: For two barrel carburetors, make sure that the idle speed screw is on the low (L) step of the fast idle cam. Then, for all engines except 1978 2-bbl models with a solenoid but without air conditioning: Turn the idle speed screw to adjust idle speed to the specification found in the Tune-Up Specifications chart or on the underhood emissions sticker.
 

On 1978 trucks with a two barrel carburetor which have a solenoid, but without air conditioning, open the throttle slightly to allow the solenoid plunger to extend. Turn the solenoid screw to adjust the curb idle to specification, as given in the chart or on the emission control sticker in the engine compartment. Then disconnect the electrical connector from the solenoid. The idle speed will drop. Turn the idle speed screw to set the slow engine idle to the figure given on the emission control sticker. Reconnect the solenoid and shut off the engine.

On carburetors equipped with a solenoid and air conditioning: Turn the idle speed screw to set the idle to specifications. Then, disconnect the air conditioner compressor electrical lead at the compressor, and turn the air conditioner on. Open the throttle slightly to allow the solenoid plunger to fully extend. Turn the solenoid screw and adjust to 650 rpm, except light duty emissions trucks with manual transmissions, which should be set to 700 rpm. Reconnect the air conditioner compressor lead.

1979 VEHICLES



Idle mixture is not adjustable in these years, except for the heavy duty emission 292 6-cylinder equipped with the IME carburetor, and heavy duty emission V8s equipped with the four barrel M4MC.

All adjustments should be made with the engine at normal operating temperature, air cleaner on, choke open, and air conditioning off, unless otherwise noted. Set the parking brake and block the rear wheels. Automatic transmissions should be set in Drive, manuals in Neutral, unless otherwise noted in the procedures or in the Tune-Up Specifications chart, or on the emission control label.



250 Engine: Check the emission control label for any special instructions. Open the throttle slightly to allow the solenoid plunger to extend. Turn the solenoid screw to adjust the curb idle to the figure given in the Tune-Up Specifications chart or on the emission control label. Disconnect the electrical connector from the solenoid. The idle speed will drop. Adjust the idle to the basic idle speed figure given on the emission control label by means of the idle speed screw. Connect the solenoid lead and shut off the engine.
 
292 Engine: See the procedure given for 1977-78. Check the idle figures given against the emission control label in the engine compartment; the label figures must be used if different.
 

Idle mixture is adjustable on this carburetor (model 1ME):

  1. Set the parking brake and block the rear wheels.
  2.  
  3. Remove the air cleaner but do not disconnect any of the hoses. Disconnect and plug the other hoses as directed on the emission control label.
  4.  
  5. The engine should be at normal operating temperature, choke open, and air conditioning off (if equipped). Connect an accurate tachometer to the engine.
  6.  
  7. Disconnect and plug the vacuum advance hose at the distributor and check the ignition timing. Correct as necessary. Reconnect the vacuum advance hose.
  8.  
  9. Carefully remove the limiter cap from the idle mixture screw. Lightly seat the screw, then back it out just enough to allow the engine to run.
  10.  
  11. Place the transmission in Neutral.
  12.  
  13. Back the mixture screw out 1 / 8 turn at a time until the maximum idle speed is obtained. Adjust the idle speed to the figure given on the emission control label by means of the idle speed screw. Repeat this step until you are certain that the maximum speed has been obtained with the mixture screw.
  14.  
  15. Turn the mixture screw in 1 / 8 turn at a time until the idle speed drops to the figure given on the emission control label.
  16.  
  17. Reset the idle speed to the figure given on the emission control label by means of the idle speed screw. Check and adjust the fast idle as directed on the emission control label. Reconnect any vacuum hoses removed in Step 2, install the air cleaner, and recheck the idle speed. Correct, if necessary, by means of the idle speed screw.
  18.  



305 Engine: Check the emission control label in the engine compartment to determine which hoses, if any, must be disconnected. Make sure the idle speed screw is on the low (L) step of the fast idle cam. Turn the idle speed screw to adjust the idle speed to the figure given in the Tune-Up Specifications chart, or on the emission control label.
 

On carburetors equipped with a solenoid (air conditioned trucks): turn the idle speed screw to set the idle to specifications, as in the previous paragraph. Then, disconnect the air conditioner compressor electrical lead at the compressor. Turn the air conditioning on. Open the throttle slightly to allow the solenoid plunger to fully extend. Turn the solenoid screw and adjust to 700 rpm with manual transmission (Neutral), or 600 rpm with automatic transmission (Drive). Reconnect the air conditioner electrical lead.

350, 400, and 454 Engines: The idle speed procedure is the same as given for 1977-78 models. Check the emission control label and the Tune-Up Specifications chart to determine the proper idle speeds.

Mixture is adjustable on heavy duty emissions V8s with the four barrel M4MC carburetor. This procedure will not work on light duty emissions trucks.

  1. The engine must be at normal operating temperature, choke open, parking brake applied, and the transmission in Park or Neutral. Block the rear wheels and do not stand in front of the truck when making adjustments.
  2.  
  3. Remove the air cleaner. Connect a tachometer and a vacuum gauge to the engine.
  4.  
  5. Turn the idle mixture screws in lightly until they seat, then back them out two turns. Be careful not to tighten the mixture screw against its seat, or damage may result.
  6.  
  7. Adjust the idle speed screw to obtain the engine rpm figure specified on the emission control label.
  8.  
  9. Adjust the idle mixture screws equally to obtain the highest engine speed.
  10.  
  11. Repeat Steps 4 and 5 until the best idle is obtained.
  12.  
  13. Shut off the engine, remove the tachometer and vacuum gauge, and install the air cleaner.
  14.  

 
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