GM Chevy Mid-Size Cars 1964-1988 Repair Guide

Rochester 4 MC, 4MV, M4MC 4-BBL Carburetors

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The Rochester Quadrajet carburetor is a two stage, 4-barrel downdraft carburetor. The designation MC or MV refers to the type of choke system the carburetor is designed for. The MV model is equipped with a manifold thermostatic choke coil. The MC model has a choke housing and coil mounted on the side of the float bowl.

The primary side of the carburetor is equipped with 1 3 / 8 in. diameter bores and a triple venturi with plain tube nozzles. During off idle and part throttle operation, the fuel is metered through tapered metering rods operating in specially designed jets positioned by a manifold vacuum responsive piston.

The secondary side of the carburetor contains two 2 1 / 4 in. bores. An air valve is used on the secondary side for metering control and supplements the primary bores.

The secondary air valve operates tapered metering rods which regulate the fuel in constant proportion to the air being supplied.

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.
  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 while supporting it with a screwdriver.
  8.  
  9. After adjusting, reset the secondary throttle tang and the slow idle screw.
  10.  

IDLE VENT ADJUSTMENT



This adjustment is not required on 1977-79 carburetors.

After adjusting the accelerator pump rod as specified above, open the primary throttle valve enough to just close the idle vent. Measure from the top of the choke valve wall to the top of the pump plunger stem. If adjustment is necessary, bend the wire tang on the pump lever.

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. Measure at a point 3 / 16 in. back from the top of the float on 1977-79 carburetors.

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

FAST IDLE



  1. Position the fast idle lever on the high step of the fast idle cam. Disconnect and plug the vacuum hose at the EGR valve.
  2.  
  3. Be sure that the choke is wide open and the engine is warm.
  4.  
  5. Turn the fast idle screw to gain the proper fast idle rpm.
  6.  

CHOKE ROD ADJUSTMENT



Position the cam follower on the second step of the fast idle cam, touching the high step. Close the choke valve directly on models up to 1976. On 1977 models, remove the choke thermostatic cover, and then hold the choke closed by pushing upward on the choke coil lever. Gauge the clearance between the lower edge of the choke valve and the carburetor body on models to 1975, and between the upper edge of the choke valve and the carburetor body on 1976 and 1977 models. Bend the choke rod to obtain the specified clearance. On 1977 models, install the choke thermostatic cover and adjust it to specification when the adjustment is complete. This adjustment requires highly specialized tools on 1978 and later models, and so is not included here.

AIR VALVE DASHPOT ADJUSTMENT



See Figure 1

Set the vacuum break diaphragm. On 1977-79 models, this requires plugging the bleed purge hole on the back of the diaphragm with tape, and the use of an external vacuum source. Hold the air valve tightly closed on all models. Gauge the clearance between the dashpot rod and the end of the slot in the air valve lever. Bend the rod to adjust. Remove the tape from the bleed purge hole.



Click image to see an enlarged view

Fig. Fig. 1: Adjusting the air valve dashpot-Rochester 4MV

CHOKE COIL ROD



1967-76
  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 carefully bent at the curved portion just below the upper lever.
  10.  

CHOKE COIL LEVER AND CHOKE THERMOSTATIC COIL



1977-79
  1. Remove the three mounting screws and retainers, then pull the thermostatic coil cover assembly off the choke housing and set it aside.
  2.  
  3. Place the fast idle cam follower on the high step of the cam and then push up on the thermostatic coil tang in the choke housing until the choke is closed.
  4.  
  5. Insert a 0.120 in. (3.048mm) plug gauge or rod of that diameter into the hole in the housing located just below the lever. With the choke closed, the lever should just touch the gauge.
  6.  
  7. If necessary, adjust the choke rod by changing the angle of the bend it makes just below the choke itself.
  8.  
  9. Then, install the coil cover back on the choke housing, making sure that the thermostatic coil engages the tang. Install the three retainers and screws, but do not tighten. With the fast idle cam follower still on the high step of the cam, rotate the cover assembly counterclockwise until the choke closes. Set all models 2 notches lean except 1977 models with manual transmission; set these three notches lean. Hold the position of the housing while tightening screws evenly.
  10.  

VACUUM BREAK



1967-76
  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.
  8.  
  9. If the clearance is not correct, bend the vacuum break link.
  10.  

1977

Adjustment procedures for 1978 and later models require the use of an expensive and sophisticated special tool, so procedures are not included here.

  1. Remove the choke thermostatic cover. Place the fast idle cam follower on the high step of the cam.
  2.  
  3. Where there is a purge bleed hole on the back of the choke vacuum break, put tape over the hole. Then, set the diaphragm using an outside vacuum source.
  4.  
  5. Push the inside choke coil lever counterclockwise until the tang on the vacuum break lever touches the tang on the vacuum break plunger stem.
  6.  
  7. Place a gauge of the proper diameter between the upper edge of the choke butterfly and the inside wall of the air horn:

    California Engines: 0.165 in. (4.191mm)
     
    All Other Engines: 0.160 in. (4.064mm)
     

  8.  
  9. If the dimension is incorrect, adjust the screw on the vacuum break plunger stem until all play is taken up and choke butterfly just touches the gauge when it's held vertically.
  10.  
  11. Reconnect the vacuum line to vacuum break port of carburetor, remove tape from purge hole (if applied), then reinstall and adjust choke thermostat (see above).
  12.  

CHOKE UNLOADER ADJUSTMENT



Performing this adjustment on 1978 and later models requires the use of sophisticated special tools, so the procedure is not included here.

On 1977 models, make sure the choke thermostatic spring is properly adjusted (see above). Close the choke valve and secure it with a rubber band hooked to the vacuum break lever. Open the primary throttles all the way. Then measure the distance between the air horn and edge of the choke butterfly. On models up to and including 1976, use the bottom side of the butterfly for this measurement; on 1977 models, use the top side. Bend the fast idle lever tang to achieve the proper opening of the choke.

SECONDARY LOCKOUT



1967-76

Completely open the choke valve and rotate the vacuum break lever clockwise. Bend the lever if the measurement between the lever and the secondary throttle exceeds specifications. Close the choke and gauge the distance between the lever and the secondary throttle shaft pin. Bend the lever to adjust.

1977-79

See Figure 2

With the throttle valves and choke valve closed use a gauge pin to check clearance between the lockout lever and the choke arm. Clearance should be no more than 0.015 in. (0.381mm). If clearance is more than specification, carefully bend the lockout lever to achieve proper clearance.

Hold the choke valve wide open by pushing down on the tail of the fast idle cam, then check lockout lever for opening clearance using the gauge pin. If necessary, file the end of the pin if additional clearance is necessary.



Click image to see an enlarged view

Fig. Fig. 2: Adjusting the secondary lockout-1977-79

AIR VALVE SPRING ADJUSTMENT



Loosening and tightening the locking screw to adjust the air valve spring requires a hex wrench on 1977 and later models carburetors.

Remove all spring tension by loosening the locking screw and backing out the adjusting screw. Close the air valve and turn the adjusting screw in until the torsion spring touches the pin on the shaft, then turn it the additional number of turns specified. Secure the locking screw.

SECONDARY CLOSING ADJUSTMENT



This adjustment assures proper closing of the secondary throttle plates.

  1. Set the idle as per instructions in the appropriate section. Make sure that the fast idle cam follower is not resting on the fast idle cam.
  2.  
  3. There should be 0.020 in. (0.508mm) clearance between the secondary throttle actuating rod and the front of the slot on the secondary throttle lever when the throttle lever closing tang is resting against the actuating lever.
  4.  
  5. Bend the tang on the primary throttle actuating rod to adjust.
  6.  

SECONDARY OPENING ADJUSTMENT



  1. Open the primary throttle valves until the actuating link contacts the upper tang on the secondary lever.
  2.  
  3. With 2-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 0.070 in. (1.778mm) clearance between the link and the middle tang.
  6.  
  7. Bend the upper tang on the secondary lever to adjust as necessary.
  8.  

 
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