ROCHESTER CARBURETORS

Bulletin 9D-95  February 1953

FLOAT LEVEL ADJUSTMENT
With the air horn gasket in position and the air horn inverted on a flat surface:
a.   Carefully bend float arms vertically until floats appear level in relation to each other.
b.    Place float gauge BT-87 in position so that the gauge is located against the curvature in the bore of the carburetor air horn, and is centered above each float.
c.   Adjust both floats together by bending the
arm at the rear of the float assembly. Bend the arm until each float just touches the top portion of the gauge between the gauge legs. (The scale dimen­sion from the gasket to the bottom of each float is 1 9/16".)
d. Then bend arms horizontally until each float is centered between the gauge legs. Tilt the air horn assembly 90" each way and check that floats do not touch gauge legs. This insures that floats will not rub sides of float bowl.
FLOAT DROP ADJUSTMENT
Bend the float tang at the rear of the float as­sembly, against the needle seat to lessen the drop and away from the needle seat to increase the drop. The tension is correct when the scale dimension
from the bottom of the air horn gasket to the bottom of the floats, with the air horn held in an upright position, is 2-1/2".
4. The Accelerator Pump System of the 1953 Model 4GC Carburetors is basically the same in theory of operation as the 1952 models. However, a few changes in design and calibration have been made for ease of service and improved performance.
tmp107-1.jpg
CHECK VALVE SPRING SPRING GUIDE
Refer to figure 2
The brass pump discharge needle used on 1952 models has been replaced by a ball check valve spring and retainer. This type of outlet check serves to hold the ball check on its seat regardless of road conditions or severe brake applications.
Refer to figure 3
The brass pump discharge jets used on 1952 models have been replaced by two drilled and cali­brated discharge holes. As shown, two "U" shaped cups have been added to the venturi cluster adjacent to these discharge holes. As the air passing through the carburetor under high velocity strikes these cups the air pressure at the pump discharge holes is in a state of balance, thus eliminating the possibility of pump pull-over. With this improved design, the use of a pump discharge vent valve is no longer required.
NOTE: From the above discussion, it can readily be seen that many parts for the 1953 model 4GC carburetors have been redesigned and recalibrated and are, therefore, not interchangeable with those used on 1952 models. Always consult the applica­tion index and parts lists for correct service parts and adjustment information.
BALL CHECK VALVE
Figure 2
tmp107-2.jpg
V SHAPED CUPS
PUMP DISCHARGE HOLES
RP Form 1105
Printed in U.S.A.
Figure 3

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