Chevrolet Full Size Cars 1979-1989

Rochester M4MC 4-bbl Carburetor

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The Rochester Quadrajet carburetor is a two stage, 4-barrel downdraft carburetor. The designation MC refers to the type of choke system the for which the carburetor is designed. Some earlier models were designated MV and were equipped with a manifold thermostatic choke coil. The MC model was used on some vehicles covered by this guide and has a choke housing with 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 1979 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 1979 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



This adjustment requires highly specialized tools on all 1979 and later models and so is not included here.

AIR VALVE DASHPOT ADJUSTMENT



Set the vacuum break diaphragm. On 1979 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.

CHOKE COIL LEVER AND CHOKE THERMOSTATIC COIL



1979

  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. Hold the position of the housing while tightening screws evenly.
  10.  

VACUUM BREAK



Adjustment procedures for 1979 and later models require the use of an expensive and sophisticated special tool, so procedures are not included here.

CHOKE UNLOADER ADJUSTMENT



Performing this adjustment on 1979 and later models requires sophisticated special tools, so the procedure is not included here.

SECONDARY LOCKOUT ADJUSTMENT



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. For more information please refer to the secondary lockout adjustment illustration for the M4ME and E4ME carburetors, found earlier in this section.

AIR VALVE SPRING ADJUSTMENT



Loosening and tightening the locking screw to adjust the air valve spring requires a hex wrench.

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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