GM Full-Size Trucks 1970-1979 Repair Guide

Carburetor Adjustments

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1970-74 ROCHESTER MV



Fast Idle

See Figure 1



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Fig. Fig. 1: Fast idle adjustment - Rochester MV carburetors

The fast idle adjustment must be made with the transmission in Neutral.

  1. Position the fast idle lever on the high step of the fast idle cam.
  2.  
  3. Be sure that the choke is properly adjusted and in the wide open position with the engine warm. Disconnect the vacuum advance on 1974 manual transmission models.
  4.  
  5. Bend the fast idle lever until the specified speed is obtained.
  6.  

Choke Rod (Fast Idle Cam)

See Figures 2 and 3



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Fig. Fig. 2: Fast idle cam adjustment - Rochester MV carburetor



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Fig. Fig. 3: Choke rod adjustment - Rochester MV carburetor

Adjust the fast idle before making choke rod adjustments.

  1. Place the fast idle cam follower on the second step of the fast idle cam and hold it firmly against the rise to the high step.
  2.  
  3. Rotate the choke valve in the direction of a closed choke by applying force to the choke coil lever.
  4.  
  5. Bend the choke rod, to give the specified opening between the lower edge of the choke valve and the inside air horn wall.
  6.  

Measurement must be made at the center of the choke valve.

Choke Vacuum Break

The adjustment of the vacuum break diaphragm unit insures correct choke valve opening after engine starting.

  1. Remove the air cleaner on vehicles with Thermac air cleaner; plug the sensor's vacuum take off port.
  2.  
  3. Using an external vacuum source, apply vacuum to the vacuum break diaphragm until the plunger is fully seated.
  4.  
  5. When the plunger is seated, push the choke valve toward the closed position.
  6.  
  7. Holding the choke valve in this position, place the specified gauge between the lower end of the choke valve and the air horn wall.
  8.  
  9. If the measurement is not correct, bend the vacuum break rod.
  10.  

Choke Unloader
  1. Apply pressure to the choke valve and hold it in the closed position.
  2.  
  3. Open the throttle valve to the wide-open position.
  4.  
  5. Check the dimension between the lower edge of the choke plate and the air horn wall; if adjustment is needed, bend the unloader tang on the throttle lever to adjust to specification.
  6.  

Choke Coil Rod
  1. Disconnect the coil rod from the upper choke lever and hold the choke valve closed.
  2.  
  3. Push down on the coil rod to the end of its travel.
  4.  
  5. The top of the rod should be even with the bottom hole in the choke lever.
  6.  
  7. To make adjustments, bend the rod as needed.
  8.  

Float
  1. Hold the float retainer in place and the float arm against the top of the float needle by pushing down on the float arm at the outer end toward the float bowl casting.
  2.  
  3. Using an adjustable T-scale, measure the distance from the toe of the float to the float bowl gasket surface.
  4.  

The float bowl gasket should be removed and the gauge held on the index point on the float for accurate measurement.

  1. Adjust the float level by bending the float arm up or down at the float arm junction.
  2.  

Metering Rod
  1. Hold the throttle valve wide-open and push down on the metering rod against spring tension, then remove the rod from the main metering jet.
  2.  
  3. In order to check adjustment, the slow idle screw must be backed out and the fast idle cam rotated so that the fast idle cam follower does not contact the steps on the cam.
  4.  
  5. With the throttle valve closed, push down on the power piston until it contacts its stop.
  6.  
  7. With the power piston depressed, swing the metering rod holder over the flat surface of the bowl casting next to the carburetor bore.
  8.  
  9. Insert a specified size drill between the bowl casting sealing bead and the lower surface of the metering rod holder. The drill should slide smoothly between both surfaces.
  10.  
  11. If adjustment is needed, carefully bend the metering rod holder up or down. After adjustment, reinstall the metering rod.
  12.  

C.E.C. Solenoid Adjustment

Do not set the C.E.C. valve to the idle rpm. This adjustment should only be made after replacement of the solenoid, carburetor overhaul, or after the throttle body is replaced.

  1. Establish the following:

    Engine running
     
    Transmission in Neutral (manual) or Drive (automatic)
     
    Air conditioner off
     
    Distributor vacuum hose removed and plugged
     
    Fuel tank vapor hose disconnected
     

  2.  

  1. Manually extend the C.E.C. valve plunger to contact the throttle lever.
  2.  
  3. Adjust the plunger length to obtain the proper rpm.
  4.  
  5. Reconnect the vapor hose and vacuum hose.
  6.  

1975-76 ROCHESTER MV AND 1977-79 ME



Fast Idle
  1. Connect a tachometer.
  2.  
  3. Check and adjust the idle speed.
  4.  
  5. Establish the following:

    Engine at normal operating temperature
     
    Air cleaner installed
     
    EGR valve signal line disconnected and plugged
     
    Air conditioning off
     

  6.  

  1. Disconnect the vacuum advance hose at the distributor and plug the line.
  2.  
  3. With the transmission in Neutral, start the engine and set the fast idle cam follower on the high step of the cam.
  4.  
  5. Bend the tank in or out to obtain the fast idle speed.
  6.  

Fast Idle Cam
  1. Check and adjust the fast idle speed.
  2.  
  3. Set the fast idle cam follower on the second step of the cam.
  4.  
  5. Apply force to the choke coil rod and hold the choke valve toward the closed position.
  6.  
  7. Measure the clearance between the lower edge of the choke valve and the inside of the air horn wall. For 1976-77 models, insert the gauge between the upper edge of the choke valve and the inside of the air horn wall.
  8.  
  9. Bend the cam to choke rod to obtain clearance.
  10.  

Choke Unloader
  1. Hold the choke valve down by applying light force to the choke coil lever.
  2.  
  3. Open the throttle valve to the wide-open position.
  4.  
  5. Measure the clearance between the upper edge of the choke valve and the air horn wall. Measure at the lower edge of the choke valve on 1978 and later models only.
  6.  
  7. If adjustment is necessary, bend the tang on the throttle lever.
  8.  

Choke Coil Rod
1975 VEHICLES

See Figure 4



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Fig. Fig. 4: Choke coil rod adjustment - 1975 MV carburetors

  1. Pull the rod up to the end of its travel to completely close the choke valve.
  2.  
  3. The bottom of the rod should be even with the top of the lever.
  4.  
  5. If adjustment is necessary, bend the rod.
  6.  

1976 VEHICLES
  1. Disconnect the upper end of the choke coil rod at the choke valve.
  2.  
  3. Completely close the choke valve.
  4.  
  5. Push up on the choke coil rod to its end of travel.
  6.  
  7. Bottom of rod should be even with the top of the lever.
  8.  
  9. Bend the rod for adjustment.
  10.  
  11. Connect rod to choke valve.
  12.  

1977-79 VEHICLES
  1. Place the cam follower on the high step of the cam.
  2.  
  3. Hold the choke valve completely closed.
  4.  
  5. A 0.120 in. plug gauge must pass through the hole in the lever attached to the choke coil housing and enter the hole in the casting.
  6.  
  7. Bend link to adjust.
  8.  

Primary Vacuum Break
1975 VEHICLES

See Figure 5



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Fig. Fig. 5: Primary vacuum break adjustment - 1975 MV carburetors

  1. With an outside vacuum source, apply vacuum to the primary vacuum break diaphragm until the plunger is fully seated.
  2.  
  3. Measure the clearance between the choke valve and the air horn wall.
  4.  
  5. Bend the vacuum break rod to adjust the clearance. Be sure that there is no binding or interference.
  6.  

1976-79 VEHICLES
  1. Place cam follower on high step of cam.
  2.  
  3. Plug purge bleed hole with masking tape over vacuum break and cover. Not all 1977 and later models will have this hole.
  4.  
  5. Using an outside vacuum source, apply vacuum to primary vacuum break diaphragm until the plunger is fully seated.
  6.  
  7. Push up on choke coil lever rod in the end of the slot. Push down on the choke valve for 1978 and later models.
  8.  
  9. Insert a specified gauge between the upper edge of the choke valve and the air horn wall. Measure at the lower edge of the choke valve, 1978 and later models only.
  10.  
  11. Bend vacuum break rod for adjustment.
  12.  
  13. After adjustment, check for binding or interference. Remove tape.
  14.  

Auxiliary Vacuum Break
1975-76 VEHICLES ONLY

See Figure 6



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Fig. Fig. 6: Auxiliary vacuum break adjustment - 1975-76 MV carburetors

  1. With an outside vacuum source, apply vacuum to the auxiliary vacuum break diaphragm until the plunger is seated.
  2.  
  3. Place the cam follower on the high step of the fast idle cam.
  4.  
  5. Measure the clearance between the upper edge of the choke valve and the air horn wall. Bend the link between the vacuum break and the choke valve to adjust.
  6.  

Metering Rod Adjustment

See Figure 7



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Fig. Fig. 7: Metering rod adjustment

  1. Remove metering rod by holding throttle valve wide open. Push downward on metering rod against spring tension, then slide metering rod out of slot in holder and remove from main metering jet.
  2.  
  3. Back out idle stop solenoid-hold throttle valve until completely closed.
  4.  
  5. Hold power piston down and swing metering rod holder over flat surface (gasket removed) of bowl casting next to carburetor bore.
  6.  
  7. Insert specified gauge.
  8.  
  9. Bend the metering rod holder as necessary.
  10.  

1970-74 ROCHESTER 2GV



These procedures are for both the 1 1 / 4 in. and 1 1 / 2 in. models. Where there are differences these are noted. The 1 1 / 2 in. model has larger throttle bores and an additional fuel feed circuit to make it suitable for use on the 350 V8.

Fast Idle Cam
  1. Turn the idle screw onto the second step of the fast idle cam.
  2.  
  3. Hold the choke valve toward the closed position and check the clearance between the upper edge of the choke valve and the air horn wall.
  4.  
  5. If this measurement varies from specifications, bend the tang on the choke lever.
  6.  

Choke Vacuum Break
  1. Apply vacuum to the diaphragm to fully seat the plunger.
  2.  
  3. Push the choke valve in toward the closed position and hold it there.
  4.  
  5. Check the distance between the lower edge of the choke valve and the air horn wall.
  6.  
  7. If this dimension is not within specifications, bend the vacuum break rod to adjust.
  8.  

Choke Unloader
  1. Hold the throttle valves wide-open and use a rubber band to hold the choke valve toward the closed position.
  2.  
  3. Measure the distance between the upper edge of the choke valve and the air horn wall.
  4.  
  5. If this measurement is not within specifications, bend the unloader tang on the throttle lever to correct it.
  6.  

Choke Coil Rod
  1. Hold the choke valve completely open.
  2.  
  3. With the choke coil rod disconnected from the upper lever, push downward on the end of the rod to the end of its travel.
  4.  
  5. With the rod pushed fully downward, the bottom of the rod should be even with the bottom of the slotted hole in the lever.
  6.  
  7. To adjust the lever, bend the lever as needed.
  8.  

Accelerator Pump Rod
  1. Back the idle stop screw out and close the throttle valves in their bores.
  2.  
  3. Measure the distance from the top of the air horn to the top of the pump rod.
  4.  
  5. Bend the pump rod at a lower angle to correct this dimension.
  6.  

Float Level

Invert the air horn, and with the gasket in place and the needle seated, measure the level as follows:



On nitrophyl floats, measure from the air horn gasket to the lip on the toe of the float.
 
On brass floats, measure from the air horn gasket to the lower edge of the float seam.
 
Bend the float tang to adjust the level.
 

Float Drop

Holding the air horn right side up, measure float drop as follows:



On nitrophyl floats, measure from the air horn gasket to the lop at the toe of the float.
 
On brass floats, measure from the air horn gasket to the bottom of the float.
 
Bend the float tang to adjust either type float.
 

1975-78 ROCHESTER 2GC



Pump Rod

See Figure 8



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Fig. Fig. 8: Pump rod adjustment - 2GC carburetor

  1. Back out the idle speed adjusting screw.
  2.  
  3. Hold the throttle valve completely closed.
  4.  
  5. Measure the distance from the top of the air horn ring to the top of the pump rod.
  6.  
  7. If necessary, bend the pump rod to adjust.
  8.  

Fast Idle Cam

See Figure 9



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Fig. Fig. 9: Fast idle cam adjustment - 2GC carburetor

  1. Turn the idle speed screw in until it contacts the low step of the fast idle cam. Then turn the screw in one full turn.
  2.  
  3. Place the idle speed screw on the second step of the fast idle cam against the highest step.
  4.  
  5. Measure the clearance between the upper edge of the choke valve and the air horn wall.
  6.  
  7. Bend the choke lever tang to adjust.
  8.  

Choke Unloader

See Figure 10



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Fig. Fig. 10: Choke unloader adjustment - 2GC carburetor

  1. With the throttle valves wide-open, place the choke valve in the closed position.
  2.  
  3. Measure the clearance between the upper edge of the choke valve and the air horn casting.
  4.  
  5. Bend the throttle lever tang to adjust.
  6.  

Intermediate Choke Rod

See Figure 11



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Fig. Fig. 11: Intermediate choke rod adjustment - 2GC carburetor

  1. Remove the thermostatic cover coil, gasket, and inside baffle plate.
  2.  
  3. Place the idle screw on the high step of the fast idle cam.
  4.  
  5. Close the choke valve by pushing up on the intermediate choke lever.
  6.  
  7. The edge of the choke lever inside the choke housing must align with the edge of the plug gauge.
  8.  
  9. Bend the intermediate choke lever to adjust.
  10.  
  11. Replace the cover and set as in the following adjustment.
  12.  

Automatic Choke Coil

See Figure 12



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Fig. Fig. 12: Automatic choke coil adjustment - 2GC carburetor

  1. Place the idle screw on the high step of the fast idle cam.
  2.  
  3. Loosen the thermostatic choke coil cover retaining screws.
  4.  
  5. Rotate the choke cover against coil tension until the choke valve begins to close. Continue rotating it until the index mark aligns with the specified point on the choke housing. On models with slotted coil pick-up lever, make sure coil tang is installed in slot in lever. This will have to be checked with the choke coil cover removed.
  6.  
  7. Tighten the choke cover retaining screws.
  8.  

Vacuum Break

See Figure 13



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Fig. Fig. 13: Vacuum break adjustment - 2GC carburetor

  1. Disconnect the vacuum hose. Using an outside vacuum source, seat the vacuum diaphragm.
  2.  
  3. Cover the vacuum break bleed hole with a small piece of tape so that the diaphragm will be held inward.
  4.  
  5. Place the idle speed screw on the high step of the fast idle cam.
  6.  
  7. For 1975-76 vehicles, remove the thermostatic coil and cover; hold the choke coil lever inside the choke coil housing towards the closed choke position.
  8.  
  9. For 1977-78 vehicles, pull out the stem from the vacuum diaphragm to the vacuum break rod until seated.
  10.  
  11. Measure the clearance between the upper edge of the choke valve and the air horn wall.
  12.  
  13. Bend the vacuum break rod to adjust.
  14.  
  15. After adjustment, remove the piece of tape and reconnect the vacuum hose.
  16.  

Float Level

See Figure 14



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Fig. Fig. 14: Float level adjustment - 2GC carburetor

Invert the air horn, and with the gasket in place and the needle seated, measure the level as follows:



On nitrophyl floats, measure from the air horn gasket to the lip on the toe of the float.
 
On brass floats, measure from the air horn gasket to the lower edge of the float seam.
 
Bend the float tang to adjust the level.
 

Float Drop

See Figure 15



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Fig. Fig. 15: Float drop adjustment - 2GC carburetor

Holding the air horn right side up, measure float drop as follows:



On nitrophyl floats, measure from the air horn gasket to the lop at the toe of the float.
 
On brass floats, measure from the air horn gasket to the bottom of the float.
 
Bend the float tang to adjust either type float.
 

1979 ROCHESTER 2SE



Float Level

See Figure 16



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Fig. Fig. 16: Float adjustment - 2SE carburetors

  1. Remove the air horn from the throttle body.
  2.  
  3. Use your fingers to hold the retainer in place, and to push the float down into light contact with the needle.
  4.  
  5. Measure the distance from the toe of the float (furthest from the hinge) to the top of the carburetor (gasket removed).
  6.  
  7. To adjust, remove the float and gently bend the arm to specification. After adjustment, check the float alignment in the chamber.
  8.  

Pump Lever

See Figure 17



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Fig. Fig. 17: Pump adjustment - 2SE carburetor

  1. With the throttle closed and the fast idle screw off the steps of the fast idle cam, measure the distance from the air horn casting to the top of the pump stem.
  2.  
  3. To adjust, remove the retaining screw and washer and remove the pump lever. Bend the end of the lever to correct the stem height. Do not twist the lever or bend it sideways.
  4.  
  5. Install the lever, washer and screw and check the adjustment. When correct, open and close the throttle a few times to check the linkage movement and alignment.
  6.  

Fast Idle
  1. Set the ignition timing and curb idle speed, and disconnect and plug hoses as directed on the emission control decal.
  2.  
  3. Place the fast idle screw on the highest step of the cam.
  4.  
  5. Start the engine and adjust the engine speed to specification with the fast idle screw.
  6.  

Choke Coil Lever
  1. Remove the three retaining screws and remove the choke cover and coil. On models with a riveted choke cover, drill out the three rivets and remove the cover and choke coil.
  2.  

A choke stat cover retainer kit is required for reassembly.

  1. Place the fast idle screw on the high step on the cam.
  2.  
  3. Close the choke by pushing in on the intermediate choke lever. The intermediate choke lever is behind the choke vacuum diaphragm.
  4.  
  5. Insert a drill or gauge of the specified size (.085) into the hole in the choke housing. The choke lever in the housing should be up against the side of the gauge.
  6.  
  7. If the lever does not just touch the gauge, bend the intermediate choke rod to adjust.
  8.  

Fast Idle Cam (Choke Rod)

A special angle gauge should be used.

  1. Adjust the choke coil lever and fast idle first.
  2.  
  3. Rotate the degree scale until it is zeroed.
  4.  
  5. Close the choke and install the degree scale onto the choke plate. Center the leveling bubble.
  6.  
  7. Rotate the scale so that the specified degree is opposite the scale pointer.
  8.  
  9. Place the fast idle screw on the second step of the cam (against the high step). Close the choke by pushing in the intermediate lever.
  10.  
  11. Push on the vacuum break lever in the direction of opening choke until the lever is against the rear tang on the choke lever.
  12.  
  13. Bend the fast idle cam rod at the "U" to adjust angle to specifications.
  14.  

Air Valve Rod

See Figure 18



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Fig. Fig. 18: Air valve rod adjustment - 2SE carburetor

  1. Seat the vacuum diaphragm with an outside vacuum source. Tape over the purge bleed hole if present.
  2.  
  3. Close the air valve.
  4.  
  5. Insert the specified gauge between the rod and the end of the slot in the air valve.
  6.  
  7. Bend the rod to adjust the clearance.
  8.  

Primary Side Vacuum Break
  1. Follow Steps 1-4 of the Fast Idle Cam Adjustment.
  2.  
  3. Seat the choke vacuum diaphragm with an outside vacuum source.
  4.  
  5. Push in on the intermediate choke lever to close the choke valve, and hold closed during adjustment.
  6.  
  7. Adjust by bending the vacuum break rod until the bubble in centered.
  8.  

Secondary Vacuum Break
  1. Follow Steps 1-4 of the Fast Idle Cam Adjustment.
  2.  
  3. Seat the choke vacuum diaphragm with an outside vacuum source.
  4.  
  5. Push in on the intermediate choke level to close the choke valve, and hold closed during adjustment. Make sure the plunger spring is compressed and seated, if present.
  6.  
  7. Bend the vacuum break rod at the "U" next to the diaphragm until the bubble is centered.
  8.  

Electric Choke Setting

This procedure is only for those carburetors with choke covers retained by screws. Riveted choke covers are preset and nonadjustable.

  1. Loosen the three retaining screws.
  2.  
  3. Place the fast idle screw on the high step of the cam.
  4.  
  5. Rotate the choke cover to align the cover mark with the specified housing mark.
  6.  

Choke Unloader

See Figure 19



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Fig. Fig. 19: Unloader adjustment - 2SE carburetor

  1. Follow Steps 1-4 of the Fast Idle Cam Adjustment.
  2.  
  3. Install the choke cover and coil, if removed, aligning the marks on the housing and cover as specified.
  4.  
  5. Hold the primary throttle wide open.
  6.  
  7. If the engine is warm, close the choke valve by pushing in on the intermediate choke lever.
  8.  
  9. Bend the unloader tang until the bubble is centered.
  10.  

Secondary Lockout

See Figure 20



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Fig. Fig. 20: Secondary lockout adjustment - 2SE carburetor

  1. Pull the choke wide open by pushing out on the intermediate choke lever.
  2.  
  3. Open the throttle until the end of the secondary actuating lever is opposite the toe of the lockout lever.
  4.  
  5. Gauge clearance between the lockout lever and secondary lever should be as specified.
  6.  
  7. To adjust, bend the lockout lever where it contacts the fast idle cam.
  8.  

1979 ROCHESTER M2MC AND M2ME



Float Level

See Figure 21



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Fig. Fig. 21: Float adjustment - M2MC carburetor

The M2ME carburetor is the same as the M2MC except that the choke is now electronically operated.

  1. Remove the air horn from the throttle body.
  2.  
  3. Use your fingers to hold the retainer in place, and to push the float down into light contact with the needle.
  4.  
  5. Gauge from the top of the casting to the top of the float, at a point 3 / 16 in. back from the end of the toe of the float (gasket removed).
  6.  
  7. To adjust, remove the float and gently bend the arm to specification. After adjustment, check the float alignment in the chamber.
  8.  

Fast Idle Speed

See Figure 22



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Fig. Fig. 22: Fast idle adjustment on the bench - M2MC carburetor

  1. Place the fast idle lever on the high step of the fast idle cam.
  2.  
  3. Turn the fast idle screw out until the throttle valves are closed.
  4.  
  5. Turn the screw in to contact the lever, then turn it in two more turns. Check this preliminary setting against the sticker figure.
  6.  

Fast Idle Cam

See Figure 23



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Fig. Fig. 23: Fast idle cam adjustment - M2MC carburetor

  1. Adjust the fast idle speed.
  2.  
  3. Place the cam follower lever on the second step of the fast idle cam, holding it firmly against the rise of the high step.
  4.  
  5. Close the choke valve by pushing upward on the choke coil lever inside the choke housing.
  6.  
  7. Gauge between the upper edge of the choke valve and the inside of the air horn wall.
  8.  
  9. Bend the tang on the fast idle cam to adjust.
  10.  

Pump Lever

See Figure 24



Click image to see an enlarged view

Fig. Fig. 24: Pump adjustment - M2MC carburetor

  1. With the fast idle cam follower off the steps of the fast idle cam, back out the idle speed screw until the throttle valves are completely closed.
  2.  
  3. Place the pump rod in the proper hole of the lever.
  4.  
  5. Measure from the top of the choke valve wall, next to the vent stack, to the top of the pump stem.
  6.  
  7. Bend the pump lever to adjust.
  8.  

Choke Coil Lever

See Figure 25



Click image to see an enlarged view

Fig. Fig. 25: Choke coil lever adjustment - M2MC carburetor

  1. Remove the choke cover and thermostatic coil from the choke housing.
  2.  
  3. Push up on the coil tang (counterclockwise) until the choke valve is closed. The top of the choke rod should be at the bottom of the slot in the choke valve lever. Place the fast idle cam follower on the high step of the cam.
  4.  
  5. Insert a 0.120 in. plug gauge in the hole in the choke housing.
  6.  
  7. The lower edge of the choke coil lever should just contact the side of the plug gauge.
  8.  
  9. Bend the choke rod to adjust.
  10.  

Front Vacuum Break
  1. Seat the choke vacuum diaphragm, using an outside vacuum source. If there is an air bleed hole on the diaphragm, tape it over.
  2.  
  3. Remove the choke cover and coil. Rotate the inside coil lever counterclockwise.
  4.  
  5. Check that the specified gap is present between the top of the choke valve and the air horn wall.
  6.  
  7. Turn the vacuum break adjusting screw to adjust.
  8.  

Automatic Choke Coil
  1. Place the cam follower on the highest step of the fast idle cam.
  2.  
  3. Loosen the three choke cover retaining screws.
  4.  
  5. Rotate the cover and coil assembly counterclockwise until the choke valve just closes.
  6.  
  7. Align the mark on the choke cover with the specified mark on the housing. Make sure the slot in the lever is engaged with the coil tang. Tighten the cover retaining screws.
  8.  

Unloader

See Figure 26



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Fig. Fig. 26: Unloader adjustment - M2MC carburetor

  1. With the choke valve completely closed, hold the throttle valves wide open.
  2.  
  3. Measure between the upper edge of the choke valve and the air horn wall.
  4.  
  5. Bend the tang on the fast idle lever to obtain the proper measurement.
  6.  

Air Conditioning Idle Speed-Up Solenoid

See Figure 27



Click image to see an enlarged view

Fig. Fig. 27: A/C idle speed-up solenoid adjustment - M2MC carburetor

  1. With the engine at normal operating temperature and automatic transmissions in drive, manual transmissions in neutral, adjust the curb idle to specifications by means of the idle speed screw (air conditioning off).
  2.  
  3. Turn the air conditioning on and disconnect the compressor clutch electrical lead at the connector. The solenoid should be energized.
  4.  
  5. Open the throttle slightly to allow the solenoid plunger to fully extend.
  6.  
  7. Turn the solenoid screw (plunger) to adjust the engine idle to the specified rpm. See your underhood specifications sticker for proper idle speed.
  8.  

1970-78 ROCHESTER 4MV



Fast Idle
  1. Position the fast idle lever on the high step of the fast idle cam.
  2.  
  3. Be sure that the choke is wide-open and the engine warm. On 1974 models, with manual transmission, disconnect the vacuum advance. On 1976 California 454 engines and all 1977-78 models, disconnect and plug vacuum hose at EGR valve.
  4.  
  5. Turn the fast idle screw to gain the proper fast idle rpm.
  6.  

Choke Rod (Fast Idle Cam)
  1. Place the cam follower on the second step of the fast idle cam.
  2.  
  3. Close the choke valve by exerting counterclockwise pressure on the external choke lever.
  4.  
  5. Insert a gauge of the proper size between the lower edge of the choke valve and the inside air horn wall. For 1976-78 models, measure between the upper edge of the choke valve and the inside air horn wall.
  6.  
  7. To adjust, bend the choke rod.
  8.  

Vacuum Break
  1. Fully seat the vacuum break diaphragm using an outside vacuum source.
  2.  
  3. Open the throttle valve enough to allow the fast idle cam follower to clear the fast idle cam.
  4.  
  5. The end of the vacuum break rod should be at the outer end of the slot in the vacuum break diaphragm plunger.
  6.  
  7. The specified clearance should register from the lower end of the choke valve to the inside air horn wall. For 1976-78 models, measure between the upper edge of the choke valve to the inside air horn wall.
  8.  
  9. If the clearance is not correct, bend the vacuum break line at the point shown in the illustration.
  10.  

Secondary Vacuum Break

This procedure applies to 1970-75 models only.

  1. Using an outside vacuum source, seat the auxiliary vacuum break diaphragm plunger.
  2.  
  3. Rotate the choke lever in the closed position until the spring-loaded diaphragm plunger is fully extended.
  4.  
  5. Holding the choke valve closed, check the distance between the lower edge of the choke valve and the air horn wall.
  6.  
  7. To adjust to specifications, bend the vacuum break link.
  8.  

Choke Unloader
  1. Push up on the vacuum break lever and fully open the throttle valves. On 1976-78 models, push up or down to close choke.
  2.  
  3. Measure the distance from the lower edge of the choke valve to the air horn wall. On 1976-78 models, measure from the upper edge of choke valve.
  4.  
  5. To adjust, bend the tang on the fast idle lever.
  6.  

Choke Coil Rod

Before making this adjustment, check choke mechanism for free operation. Any binding caused by gum on shaft or linkage should be cleaned off. Do not oil linkage.

  1. Close the choke valve by rotating the choke coil lever counterclockwise.
  2.  
  3. Disconnect the thermostatic coil rod from the upper lever.
  4.  
  5. Push down on the rod until it contacts the bracket of the coil.
  6.  
  7. The rod must fit in the notch of the upper lever.
  8.  
  9. If it does not, it must be bent on the curved portion just below the upper lever.
  10.  

Secondary Closing

This adjustment assures proper closing of the secondary throttle plates.

  1. Set the slow idle as per instructions in Routine Maintenance . Make sure that the fast idle cam follower is not resting on the fast idle cam. The choke valve should be wide open.
  2.  
  3. There should be the specified clearance between the secondary throttle actuating rod and the front of the slot on the secondary throttle lever with the closing tang on the throttle lever resting against the actuating lever.
  4.  
  5. Bend the tang on the primary throttle actuating rod to adjust.
  6.  

Secondary Opening
  1. Open the primary throttle valves until the actuating link contacts the upper tang on the secondary lever.
  2.  
  3. With two point linkage, the bottom of the link should be in the center of the secondary lever slot.
  4.  
  5. With three point linkage, there should be the specified clearance between the link and the middle tang.
  6.  
  7. Bend the upper tang on the secondary lever to adjust as necessary.
  8.  

Float Level

With the air horn assembly upside down, measure the distance from the air horn gasket surface (gasket removed) to the top of the float at the toe.

Make sure that the retaining pin is firmly held in place and that the tang of the float is firmly against the needle and seat assembly.

Accelerator Pump
  1. Close the primary throttle valves by backing out the slow idle screw and making sure that the fast idle cam follower is off the steps of the fast idle cam.
  2.  
  3. Bend the secondary throttle closing tang away from the primary throttle lever, if necessary.
  4.  
  5. With the pump in the appropriate hole in the pump lever, measure from the top of the choke valve wall to the top of the pump stem.
  6.  
  7. To adjust, bend the pump lever.
  8.  
  9. After adjusting, readjust the secondary throttle tank and the slow idle screw.
  10.  

Air Valve Spring

To adjust the air valve spring windup, loosen the allen head lockscrew and turn the adjusting screw counterclockwise to remove all spring tension. Hold the air valve closed, and turn the adjusting screw clockwise the specified number of turns after the torsion spring contacts the pin on the shaft. Hold the adjusting screw in this position and tighten the lockscrew.

1975-78 ROCHESTER M4MC, M4MCA AND M4ME



Pump Rod

See Figure 28



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Fig. Fig. 28: Pump rod adjustment - M4MC and M4ME carburetors

  1. Take the fast idle cam follower off the fast idle cam steps.
  2.  
  3. Back out the idle speed screw until the throttle valves are completely closed.
  4.  
  5. Be sure that the secondary actuating rod is not preventing the throttle from closing completely. If the primary throttle valves do not close completely, bend the secondary closing tang out of position, then readjust later.
  6.  
  7. Place the pump rod in the specified hole in the lever.
  8.  
  9. Measure the clearance from the top of the choke valve wall, next to the vent stack, and the top of the pump stem.
  10.  
  11. To adjust the dimension, support the pump lever and bend the pump lever.
  12.  
  13. Adjust the idle speed.
  14.  
  15. If necessary, readjust the secondary actuating rod.
  16.  

Fast Idle
1975-77 VEHICLES

See Figure 29



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Fig. Fig. 29: Fast idle adjustment - M4MC and M4ME carburetors

This procedure is to be used only when the carburetor is off the truck, as a means of roughly setting the fast idle speed before the carburetor is reinstalled. For normal adjustment of fast idle speed, refer to the idle adjustment portion in Routine Maintenance .

  1. Hold the cam follower on the high step of the fast idle cam.
  2.  
  3. Turn the fast idle screw out until the primary throttle valves are closed (1975 models), or until it pulls away from the fast idle cam follower (1976-77 models).
  4.  
  5. Turn the fast idle screw in to contact the lever, then turn the screw in three turns.
  6.  
  7. Recheck the fast idle speed.
  8.  

Choke Coil Lever

See Figure 30



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Fig. Fig. 30: Choke coil lever adjustment - 1975-78 M4ME and M4MC carburetors

  1. Loosen the three retaining screws and remove the cover and coil assembly from the choke housing. On the 1978 and later models, also place the fast idle cam follower on the high step of the cam.
  2.  
  3. Push up on the thermostatic coil tang (counterclockwise) until the choke valve closes.
  4.  
  5. Insert the specified gauge into the hole in the choke housing.
  6.  
  7. The lower edge of the choke coil lever should just contact the side of the gauge.
  8.  
  9. Bend the choke rod to adjust.
  10.  

Choke Rod

See Figure 31



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Fig. Fig. 31: Choke rod fast idle cam adjustment - 1975-78 M4ME and M4MC carburetors

The choke coil lever adjustment must be made before making the following adjustment. The thermostatic cover and coil will remain off for this adjustment.

  1. Adjust the fast idle.
  2.  
  3. Place the cam follower on the second step of the fast idle cam firmly against the rise of the high step.
  4.  
  5. Close the choke valve by pushing up on the choke coil lever inside the choke housing.
  6.  
  7. Measure the clearance between the upper edge of the choke valve and the inside of the air horn wall.
  8.  
  9. Bend the tang on the fast idle cam to adjust the clearance. Be sure that the tang lies against the cam after bending it.
  10.  
  11. Recheck the fast idle.
  12.  

Air Valve Dashpot
  1. Seat the front vacuum diaphragm using an outside vacuum source. Plug the purge bleed hole with masking tape on models where used. Remove tape after making adjustment.
  2.  
  3. The air valves must be completely closed.
  4.  
  5. Measure the clearance between the air valve dashpot and the end of the slop in the air valve lever.
  6.  
  7. Bend the air valve dashpot rod to adjust the clearance.
  8.  

Front Vacuum Break

See Figure 32



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Fig. Fig. 32: Vacuum break adjustment information - M4ME and M4MC carburetors

  1. Remove the thermostatic cover and coil assembly from the choke housing.
  2.  
  3. Place the cam follower on the high step of the fast idle cam.
  4.  
  5. Seat the front vacuum diaphragm using an outside vacuum source.
  6.  
  7. Push up on the inside choke coil lever until the tang on the vacuum break lever contacts the tang on the vacuum break plunger.
  8.  
  9. Measure the clearance between the upper edge of the choke valve and the air horn wall.
  10.  
  11. Turn the adjusting screw on the vacuum break plunger lever to adjust.
  12.  
  13. Reconnect the vacuum hose after adjustment.
  14.  

Rear Vacuum Break

See Figure 33



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Fig. Fig. 33: M4MC and M4MCA rear vacuum break adjustment. Refer to the upper illustration for 1976-78 M4ME carburetors

  1. Remove the thermostatic cover and coil assembly from the choke housing.
  2.  
  3. Place the cam follower on the high step of the fast idle cam.
  4.  
  5. Plug the bleed hose in the vacuum break unit cover with tape.
  6.  
  7. Seat the rear vacuum diaphragm using an outside vacuum source.
  8.  
  9. Push up the choke coil lever inside the choke housing toward the closed position. Diaphragm should be pulled out until seated, bucking spring compressed (where used) on 1976 and later.
  10.  
  11. With the choke rod in the bottom slot of the choke lever, measure the clearance between the upper edge of the choke valve and air horn wall.
  12.  
  13. Bend the vacuum break rod if necessary to adjust.
  14.  
  15. After adjustment, remove the tape and install the vacuum hose.
  16.  

Automatic Choke Coil

See Figure 34



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Fig. Fig. 34: Choke coil lever adjustment - M4ME and M4MC carburetors

M4MC CARBURETOR
  1. Install the thermostatic coil and cover with a gasket between the choke cover and choke housing. On all models except the 1976 454 cu. in. V8, the thermostatic coil must be installed in the slot in the inside of the choke coil lever pick-up arm.
  2.  
  3. Place the fast idle cam follower on the high step of the fast idle cam.
  4.  
  5. Rotate the cover and coil assembly counterclockwise until the choke valve just closes.
  6.  
  7. Align the index point on the cover with the specified mark on the choke housing.
  8.  
  9. Tighten the retaining screws.
  10.  

M4ME CARBURETOR
  1. Install the electric choke assembly in choke housing, making sure coil tang contacts bottom side of inside choke coil lever pick-up arm.
  2.  
  3. Place fast idle cam follower on high step of cam.
  4.  
  5. Rotate cover and coil assembly counterclockwise until choke valve just closes.
  6.  
  7. Align index point on cover with specified mark on housing.
  8.  
  9. Install cover retainers and screws.
  10.  

Ground contact for the electric choke is provided by a metal plate located at the rear of the choke assembly. Do not install a choke cover gasket between the electric choke assembly and the choke housing. Do not immerse the electric choke assembly in any cleaning solution. Severe damage can result.

Unloader

See Figure 35



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Fig. Fig. 35: Unloader adjustment - M4ME and M4MC carburetors

  1. Perform Step 1 of the appropriate automatic choke coil procedure.
  2.  
  3. Hold the throttle valves wide open with the choke valve completely closed. On a warm engine, close the choke valve by pushing on the tang of the intermediate choke lever which contacts the fast idle cam. A rubber band will hold it in position.
  4.  
  5. Measure the distance between the upper edge of the choke valve and the air horn wall.
  6.  
  7. Bend the tang on the fast idle lever to adjust the clearance. Check to be sure that the tang on the fast idle cam lever is contacting the center of the fast idle cam after adjustment.
  8.  

Secondary Throttle Lockout

See Figure 36



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Fig. Fig. 36: Throttle secondary lockout adjustment - 1975-78 M4ME and M4MC carburetors

SECONDARY LEVER CLEARANCE
  1. Hold the choke valve and secondary throttle valves closed.
  2.  
  3. Measure the clearance between the lockout pin and the lockout lever.
  4.  
  5. If necessary, bend the lockout pin to adjust to 0.015 in.
  6.  

OPENING CLEARANCE
  1. Hold the choke valve wide open by pushing down on the fast idle cam.
  2.  
  3. Hold the secondary throttle valves slightly open.
  4.  
  5. Measure the clearance between the end of the lockout pin and the toe of the lockout lever.
  6.  
  7. If necessary, file off the end of the lockout pin to obtain 0.015 in. clearance.
  8.  

Secondary Closing

See Figure 37



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Fig. Fig. 37: Secondary closing adjustment - 1975-78 M4ME and M4MC carburetors

  1. Set the idle speed to specification.
  2.  
  3. Hold the choke valve wide open with the cam follower lever off the steps of the fast idle cam.
  4.  
  5. Measure the clearance between the slot in the secondary throttle valve pickup lever and the secondary actuating rod.
  6.  
  7. Bend the secondary closing tang on the primary throttle lever to adjust to 0.020 in. clearance.
  8.  

Secondary Opening

See Figure 38



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Fig. Fig. 38: Secondary opening adjustment - 1975-78 M4ME and M4MC carburetors

  1. Lightly, crack the primary throttle lever until the link just contacts the tang on the secondary lever.
  2.  
  3. With the link against the tang, the link should be in the center of the slot in the secondary lever.
  4.  
  5. Bend the tang on the secondary lever to adjust.
  6.  

Air Valve Spring Wind-Up

See Figure 39



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Fig. Fig. 39: Air valve spring wind-up adjustment - 1975-78 M4ME and M4MC carburetors

  1. Remove the front vacuum break diaphragm and the air valve dashpot rod.
  2.  
  3. Loosen the lockscrew.
  4.  
  5. Turn the tension adjusting screw counterclockwise until the air valve opens partway.
  6.  
  7. Turn the tension adjusting screw clockwise while tapping lightly on the casting with the handle of a screwdriver.
  8.  
  9. When the air valve just closes, turn the tension adjusting screw clockwise the specified number of turns after the spring contacts the pin.
  10.  
  11. Tighten the lockscrew and install the diaphragm and dashpot rod.
  12.  

Deceleration Throttle Stop

See Figure 40



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Fig. Fig. 40: Deceleration throttle stop adjustment - 1975-78 M4ME and M4MC carburetors

  1. Adjust the idle speed.
  2.  
  3. Push the hex end of the throttle stop plunger in (toward the throttle lever) until the plunger stem hits the stop inside the diaphragm.
  4.  
  5. With the plunger against the stop, turn the plunger adjusting screw in or out to obtain the specified deceleration rpm.
  6.  

1979 ROCHESTER M4ME/M4MC



See Figures 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51 and 52

For carburetor adjustments for the 1979 M4ME/M4MC Rochester carburetor, please refer to the following illustrations.



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Fig. Fig. 41: Float adjustment - 1979 M4ME and M4MC carburetors



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Fig. Fig. 42: Pump adjustment - 1979 M4ME and M4MC carburetors



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Fig. Fig. 43: Air valve spring adjustment - 1979 M4ME and M4MC carburetors



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Fig. Fig. 44: Choke coil lever adjustment - 1979 M4ME and M4MC carburetors



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Fig. Fig. 45: Choke valve angle gauge adjustment - 1979 M4ME and M4MC carburetors



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Fig. Fig. 46: Choke rod-fast idle cam adjustment - 1979 M4ME and M4MC carburetors



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Fig. Fig. 47: Vacuum break adjustment information - 1979 M4ME and M4MC carburetors



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Fig. Fig. 48: Front vacuum break adjustment - 1979 M4ME and M4MC carburetors



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Fig. Fig. 49: Rear vacuum break adjustment - 1979 M4ME and M4MC carburetors



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Fig. Fig. 50: Air valve rod adjustment (front) - 1979 M4ME and M4MC carburetors



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Fig. Fig. 51: Air valve rod adjustment (rear) - 1979 M4ME and M4MC carburetors



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Fig. Fig. 52: Unloader adjustment - 1979 M4ME and M4MC carburetors

 
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