GM Full Size Vans 1967-1986 Repair Guide

Carbureted Engines

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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 supplied here.

See Carburetor Identification in Fuel System for carburetor I.D. specifics.

ADJUSTMENT



These procedures require the use of a tachometer. In some cases, the degree of accuracy required is greater than that available on a hand-held unit; a shop tachometer would be required to follow the instructions exactly. If the idle speed screws have plastic limiter caps (1971-82), it is not recommended that they be removed unless a satisfactory idle cannot be obtained with them in place. If the caps are removed, exhaust emissions may go beyond the specified legal limits. This can be checked on an exhaust gas analyzer.

Most 1973 and later 4-bbl carburetors have an internal fuel passage restriction. Beyond a certain limited point, turning the idle mixture screws out has no further richening effect.

Idle speed and mixture are set with the engine at normal running temperature. The automatic transmission should be in Drive, except when specified otherwise. The air conditioner should be off for adjusting mixture and off unless otherwise specified in the text or specifications chart for setting idle speed.


CAUTION
Block the wheels, set the parking brake, and don't stand in front of the truck.

1967 Models

Turn the idle screw(s) slightly in to seat, and then back them out 2 turns (3 when equipped with air pump). Do not turn the idle mixture screws tightly against their seats or you could damage them. With the engine idling at operating temperature (air cleaner on and choke valve wide open), adjust the idle speed to the specified rpm (automatic transmission in Drive; manual in Neutral.

Adjust the mixture screw to obtain the highest steady idle speed, then adjust the idle speed screw to the specified rpm. Adjust the mixture screw in to obtain a 20 rpm drop, then back the screw out 1 / 4 turn. Repeat this operation on the second mixture screw, if so equipped. Readjust the idle speed screw as necessary until the specified rpm is reached.

1968-69 Models

Turn the idle mixture screw(s) in to seat and then back them out 3 turns. Don't turn the screws in tightly or you will damage them. Bring the engine to its operating temperature (air cleaner installed and choke valve open), and adjust the idle speed screw to obtain the specified rpm (automatic transmission in Drive, manual transmission in Neutral).

On air conditioned models, turn the air conditioner off except on 6-cylinder engines with automatic transmission. On these models idle speed is set with the air conditioning on.

Set the idle mixture screw(s) to give the highest steady idle speed. Adjust the idle speed to the specified rpm. Set the idle speed for engines with idle solenoids as follows: set the idle speed to 500 rpm by turning the idle solenoid hex bolt. Disconnect the solenoid wire and check the idle speed. De-energizing the solenoid will allow the throttle lever to seat against the carburetor idle screw. Turn the carburetor idle screw to obtain 400 rpm.

Turn the mixture screw(s) in to get a 20 rpm drop. Turn the mixture screw out 1 / 4 turn. Repeat for the second mixture screw, if so equipped. Readjust the idle speed screw, as necessary, to obtain the specified idle rpm.

1970-71 Models

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 parking brake ON.

On 6-cylinder engines, 10 Series: 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 Neutral or 630 rpm with automatic 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.

On 6-cylinder engines, 20 and 30 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.

On V8-307 engines, 10 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 Neutral. 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.

On 1970 V8-307, 20 & 30 Series: 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 20 rpm drop, then back 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.

On 1970-71 V8-350, 20 & 30 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.

SERVINFOSUB>1972 Models

The engine should be at normal operating temperature with 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, if they are removed, the CO content of the exhaust should be checked to be sure that it meets Federal Emission Control limits.

On 6 cyl-250 engines: Disconnect the FUEL TANK line from the vapor canister. Remember to reconnect it after making the adjustment. Disconnect and plug the vacuum line. 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.


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

Reconnect the vacuum line.

On V8-307 engines: Disconnect the FUEL TANK line from the vapor canister and remove and plug the vacuum line. With the air conditioner OFF, adjust the idle stop solenoid screw to obtain 900 rpm with manual transmission in Neutral or 600 rpm with automatic transmission in Drive. With transmission in Park or Neutral, adjust the fast idle speed to obtain 1850 rpm. Reconnect the FUEL TANK line and the vacuum line.

On V8-350 engines: Disconnect the FUEL TANK line and disconnect and plug the vacuum line. On vehicles with TCS (transmission controlled spark), 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 follower on the 2nd step of the fast idle cam, turn the air conditioner OFF and adjust the fast idle to 1350 rpm with manual transmission in Neutral or automatic transmission in Drive. Reconnect the FUEL TANK and vacuum lines.

1973-74 Models

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

On 6 cyl-250 engines: Disconnect the FUEL TANK line and the distributor vacuum line. Plug the vacuum line. Adjust the idle stop solenoid by turning the hex nut to obtain:



700 rpm (1973) or 850 rpm (1974) on manual transmission in Neutral
 
600 rpm (197374) on automatic transmission in Drive
 

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

On 1973 V8-307 engines: On light duty vehicles, disconnect the FUEL TANK line from the vapor canister and plug the distributor vacuum line. Adjust the idle stop solenoid to obtain 600 rpm with automatic transmission in Drive 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. Reconnect the idle stop solenoid, the FUEL TANK line, and the vacuum line.

On 8-350 engines: On the light duty vehicles, disconnect the FUEL TANK line from the vapor canister. Disconnect and plug the distributor 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 distributor vacuum line disconnected. Reconnect the FUEL TANK line and the vacuum line.

1975 Models

On 6-cylinder engines: Do not disconnect the distributor vacuum line. Disconnect the vapor canister FUEL TANK hose. With automatic transmission in Drive and manual in Neutral, adjust the solenoid to get the specified idle speed. Use a 1 / 8 in. (3.17mm) allen wrench in the end of the solenoid body to set the low idle speed to 450 rpm with the solenoid wire disconnected. Reset the idle speed with the air conditioning on, except on the 250 engine.

On V8 engines with the 2-bbl carburetor: Disconnect the vapor canister FUEL TANK hose. Leave the distributor vacuum advance hose in place. Adjust the idle speed screw to get the specified idle speed with automatic in Drive and manual in Neutral.

On light duty V8 engines with the 4-bbl carburetor: Disconnect the vapor canister FUEL TANK hose. Leave the distributor vacuum advance hose in place. Disconnect the solenoid wire. Place automatic in Drive and manual in Neutral. Turn the low idle speed screw on the carburetor to get about 450 rpm. Connect the solenoid wire and open the throttle slightly, so that the solenoid plunger can extend. Turn the plunger screw to get the specified idle speed.

On heavy duty V8 engines with the 4-bbl carburetor: This adjustment is the same as for 1974. Reset the idle speed with the air conditioner on.

1976 Models

On 6-cylinder engines: Disconnect and plug the CARBURETOR and PCV vapor canister hoses on the 250. Disconnect the canister FUEL TANK hose on the 292. If the engine has a vacuum advance hose running directly from the vacuum source to the distributor vacuum advance unit, disconnect and plug it. Turn the air conditioner on, only on the 292. Set the manual transmission in Neutral. Set the 250 automatic in Drive and the 292 in Neutral. Turn the solenoid to get the specified idle speed. Disconnect the solenoid wire and turn off the air conditioner. Use a allen 1 / 8 in. (3.17mm) wrench in the end of the solenoid body to set the low idle speed to 450 rpm.

On V8 engines with the two barrel carburetor: This procedure is the same as 1975, except that the canister hose can be left in place.

On light duty V8 engines with the 4-bbl carburetor: Place the automatic in Drive and manual in Neutral. Set the idle speed screw on the carburetor to obtain the specified rpm.

On heavy duty V8 engines with the 4-bbl carburetor: Disconnect the vapor canister FUEL TANK hose on California models. Leave the vacuum advance hose in place. Turn the air conditioner on. Set the automatic in Park and manual in Neutral. Set the idle speed screw on the carburetor to obtain the specified rpm.

1977 Models

See Figures 1, 2, 3 and 4



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Fig. Fig. 1: 1977-78 2bbl idle speed adjustment-without solenoid



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Fig. Fig. 2: 1977-78 2bbl idle speed adjustment-with solenoid



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Fig. Fig. 3: 1977 and later 4bbl idle speed adjustment-without solenoid



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Fig. Fig. 4: 1977 and later 4bbl and 1979 and later 2 bbl (V8 only) idle speed adjustment-with solenoid

See the underhood emission sticker for any hoses or wires that may need to be disconnected.

On 1 bbl equipped models: Start the engine and allow it to run until it reaches normal operating temperature. Be sure the choke is fully open and the cam follower is off the steps of the cam. Turn the nut on the end of the solenoid to obtain the specified rpm. See the Tune-up chart. Disconnect the wire from the solenoid and turn the 1 / 8 in. (3.17mm) allen head screw in the end of the solenoid to set the base idle to specification. Refer to the Tune-up chart or the underhood emission sticker. Reconnect the wire.

On 2 and 4 bbl equipped models: Be sure the ignition timing is correct. Refer to the underhood emission sticker in order to prepare the vehicle for adjustment.

On carburetors without a solenoid: Be sure the idle speed screw is on the low step of the fast idle cam. Turn the screw to obtain the idle specified in the Tune-Up chart.

On carburetors with a solenoid: Turn the idle screw to obtain the idle speed specified in the Tune-Up chart. Disconnect the electrical lead from between the solenoid and the A/C compressor at the compressor and turn the A/C On. Place the automatic transmission in Drive. Open the throttle momentarily to fully extend the solenoid plunger. Turn the solenoid screw to obtain the base idle speed as specified in the Tune-Up chart or on the emission sticker. Reconnect the electrical lead at the compressor.

1978 Models

See Figure 5



Click image to see an enlarged view

Fig. Fig. 5: 1978 and later idle speed adjustment-six cylinder engine

On 1-bbl equipped models: The idle speed adjusting procedure is the same as 1977. Refer to the Tune-Up chart for the correct idle speed.

On 2-bbl equipped models: Be sure the ignition timing is correct. Refer to the underhood emission sticker in order to prepare the vehicle for adjustment.

On carburetors without a solenoid: This procedure is the same as 1977. See the Tune-Up chart for the correct idle speed.

On models with a solenoid and without air conditioning: Rev the engine momentarily to fully extend the solenoid plunger. Turn the solenoid screw to obtain the curb idle speed listed in the Tune-Up chart. Disconnect the wire from the solenoid. Turn the idle speed screw to obtain the solenoid idle speed listed on the underhood emission sticker. Reconnect the wire at the solenoid.

On models with air conditioning: Turn the idle speed screw to obtain the idle speed listed in the Tune-Up chart. Disconnect the wire at the A/C compressor and turn the A/C On. Rev the engine momentarily to fully extend the solenoid plunger. Turn the solenoid screw to obtain the solenoid idle speed listed on the underhood emission sticker. Reconnect the wire at the compressor.

On 4-bbl equipped models: Refer to the underhood emission sticker and prepare the vehicle for adjustment as specified on the sticker.

On models without a solenoid, turn the idle speed to obtain the idle speed listed in the Tune-Up chart.

On models with a solenoid, turn the idle speed screw to obtain the idle speed listed in the Tune-Up chart. Disconnect the wire at the A/C compressor and turn the A/C On. Rev the engine momentarily to fully extend the solenoid plunger. Turn the solenoid screw to obtain the solenoid idle speed listed on the underhood emission sticker. Reconnect the A/C wire at the compressor.

1979 and Later Idle Speed Adjustment

Idle mixture is not adjustable in these years, except for the heavy duty emission V8s engines equipped with the 4-bbl M4MC carburetors.

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 on the emission control label.



On 6-cyl-250 engines: 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.
 
On 6 cyl-262 engines and V8-305 engines: 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 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.



On V8-350 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 V8 engines with the 4-bbl 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 from 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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