1953 Oldsmobile Rochester 4GC Carburetor

This 1953 Rochester Model 4GC carburetor section, filed with Oldsmobile, comes from the Carburetion chapter of a factory shop manual (pages 8-46 to 8-60, Figs. 8-93 to 8-121). It explains the 4GC as two dual carburetors in one casting: a primary or pump side that meters fuel through idle and part throttle, and a secondary or fuel inlet side that adds fuel at power and wide open throttle.

Cutaway drawings show the float, idle, part throttle, power, accelerating pump and choke circuits, including the atmospheric idle vent valve, the power piston that opens below about 7 inches Hg, and the secondary throttle lockout. Numbered steps cover disassembly of the air horn, bowl and throttle body, cleaning and inspection, and reassembly. Adjustment specifications include float level 1-9/16 inch, float drop 2-1/4 inch, pump rod 1-1/16 inch, choke rod .053 inch, unloader .092 inch, and fast idle of 1400 to 1500 rpm on the car, using gauges BT-66, BT-67 and BT-68. It suits owners, restorers and mechanics rebuilding a 4GC.

This 15-page copy is free to read online. Use the page box and thumbnails to move through the circuit descriptions and the rebuild and adjustment steps.