Chevrolet Full-size Cars 1968-1978 Repair Guide

Rochester 4GC 4-Bbl. Carburetor

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ADJUSTMENTS



Automatic Choke Rod

Set the cover index mark on the one notch lean mark on the housing.

Intermediate Choke Rod

The intermediate choke rod is adjusted with the choke cover and baffle removed.

  1. Hold the choke valve closed, exert a light pressure on the choke piston to take up any lash, and then note if the choke piston is at the end of its sleeve.
  2.  
  3. If necessary, bend the intermediate choke rod for correct piston positioning.
  4.  
  5. Install the choke baffle and cover.
  6.  

Choke Rod
  1. Turn the idle speed screw in until it just touches the second step of the fast idle cam.
  2.  
  3. Ensure that the choke trip lever is touching the choke counterweight lever.
  4.  
  5. While holding the idle speed screw on the second cam step and against the shoulder of the high step, there should be 0.043" clearance between the choke valve edge and the air horn dividing wall.
  6.  
  7. Bend the choke rod at a lower angle, if necessary.
  8.  

Choke Unloader
  1. Hold the throttle valve wide open, while the choke trip lever touches the choke counterweight.
  2.  
  3. Clearance between the top of the choke valve and the dividing wall of the air horn should now be 0.235". Bend the fast idle cam tang, if necessary.
  4.  

Secondary Throttle Lockout
  1. Close the choke valve so that the secondary lockout tang is in the fast idle cam slot. Clearance between the fast idle cam and the tang should be 0.015".
  2.  
  3. Bend the tang horizontally as necessary to obtain the correct clearance.
  4.  

Float Level and Drop


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Fig. Fig. 1 Float level adjustment on the Rochester 4GC carburetor

  1. Remove the bowl cover.
  2.  
  3. Install a new gasket on the bowl cover surface.
  4.  
  5. Invert the cover and install the float lever gauges supplied with the carburetor rebuilding kit over the primary and secondary floats. The floats should just touch the gauges. The height from the bottom of the float to the bowl cover gasket is 1 33 / 64 " for the primaries and 1 37 / 64 " for the secondaries. Bend the float arms as necessary to obtain the correct level.
  6.  
  7. Center the floats in the level gauge, bending them to the left or right as necessary.
  8.  
  9. While holding the bowl cover in an upright position, measure the distance from the bowl cover gasket to the bottom of the float. This distance, float drop, should be 2 1 / 4 ". Bend the float tang on the end of the hinge arm to correct the drop.
  10.  
  11. Install the bowl cover as outlined under overhaul.
  12.  

 
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