Bulletin 9D-95  February 1953

division or GENERAL MOTORS. Rochester, n. y.
The following design changes have been made on Models B, BG. and 4GC Rochester Carburetors for 1953.
Model B and BC (Chevrolet Manual and Automatic Choke)
1.    The vacuum power piston on all new 1953 Model B and BC Rochester Carburetors has been redesigned. This new design is referred to as an "Air Bleed Power Piston."
Refer to figure 1
The new power piston differs from the previous design in that it has a hollow stem which bleeds air into the power valve of the Main Well Support Assembly whenever the power valve ball check is off its seat. Air is taken into the stem through a cali­brated hole drilled through the side of the power piston above the stem. This calibrated air bleed hole is located above the float bowl fuel level at all times.
In addition to the hollow stem and air bleed hole, the tip of the power piston stem, which contacts the power valve ball check, is slotted. This slot permits air to bleed through the hollow stem past the ball check valve at low manifold vacuums.
The purpose of the "Air Bleed Power Piston" is simply to decrease the effect of the metering force upon the power restriction, thereby supplying correct mixtures at high air flows.
2.    The pump link on new 1953 Model B and BC Rochester Carburetors is curved to accommodate a Power Glide transmission function. This new pump link is grooved to distinguish it from the curved link used on the Model BC Pontiac "6" carburetors. They are not interchangeable.
Since the new curved link is the same length as the straight link used on earlier model Chevrolet Carburetors it may be used as a replacement part on these earlier models. However, the straight link must not be used for servicing 1953 carburetors.
3.    All new 1953 Model B and BC carburetors are calibrated using a square approach main metering jet. This change serves to provide the correct mixture in the upper portion of the part throttle range.
4.    It will be noted that the new 1953 model B and BC passenger car carburetors incorporate a slotted vacuum spark takeoff hole rather than the two drilled vacuum holes used on earlier models.
5.    No Throttle Return Check Assembly will be used on 1953 Power Glide cars. However, both the carburetor throttle lever and throttle body are de­signed to accommodate the return check if so desired.
NOTE: Since the 1953 Model B and BC caburetors have been completely recalibrated for use on 1953 engines, they must not be used to replace previous model carburetors. Always consult the ap­plication index and parts list for correct service parts and adjustment information.
Figure I
Model 4GC (Cadillac and Oldsmobile)
1.    The 1953 Model 4GC Rochester Carburetor incorporates a 3/16" higher fuel level in the Float Bowl than 1952 models. The purpose of raising this fuel level is to increase the amount of fuel in the bowl so that when a tight turn in either direction is being made, the surge of fuel to the side of the bowl will not uncover or expose the main metering jets.
The following changes were made as a result of having raised the liquid fuel level in the float bowl:
a.    The two fuel level sight plugs, located in the side of the float bowl, were also raised 3/16".
b.    The main discharge nozzle and venturi cluster assemblies were raised 3/16". This was accomplish­ed by raising the height of the cluster mounting pads, located in the float bowl assembly.
c.    In order to maintain the proper venturi rela­tionship, the length of the secondary venturii located on the venturi cluster assemblies, was increased 3/16". Thus the lower or trailing edges of the sec­ondary venturii remain well within the metering force area of the large or primary venturii, located in the float bowl assembly.
d.    A newly designed air horn casting on 1953 model 4GC carburetors provides relief for the raised venturi cluster assemblies which protrude a slight distance above the upper surface of the float bowl casting.
2.    The float balance spring and clips assembly has been removed from all 1953 Model 4GC car­buretors. Accordingly the float hanger support on 1953 air horns will be left undrilled to prevent con­fusion in the field.
3.    The change in fuel level and removal of the float balance springs require new float adjustment specifications. Using the same method, gauge, and dimensions for both 1953 Oldsmobile and Cadillac, adjust both sets of floats as follows:







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