Rochester 4G, 4GC Technical

The model 4G and 4GC carburetors are a 4-bore downdraft type which provide the advantages of two 2-bore carburetors in one unit. The model 4G carburetor has a manually operated choke while the model 4GC has a completely automatic choke. Some model 4G carburetors are equipped with a vacuum controlled governor to control engine speed on heavy duty truck applications.

The 4-bore Rochester carburetor has a concentric float bowl which allows the fuel in the float bowl to completely sturround the bore and venturi. This has advantages because the correct correct fuel level can be maintained for efficient metering during cornering or up and down hill operation of the vehicle.

The Rochester air bled mainw ell system is used with a removable venturi cluster. The venturi cluster containst he main metering parts of the carburetor and can easily be removed for cleaning and inspection. The cluster is insulated from the float bowl by a mounting gasket. The main well and idle tubes are suspended int eh fuel in the main fuel well. Both the above tend to insulate the main metering parts from engine heat. This insulation helps prevent fuel vapors caused by engine heat from distrupting carburetor metering.

To aid in description and the proper identification of parts, the carburetor is divided into a primary and secondary side. 

The primary side covers the forward half of the carburetor assembly. This section is essentially a complete 2-bore carburetor containing a float system, adjustable idle system, main metering system, pump system, power system and choke system.

The secondary side is a supplementary 2-bore carburetor which feeds extra air and fuel to the engine when needed for power requirements. This section contians a float system, main metering system and, on some applications, a fixed idle system. It has a set of throttle valves and separate auxiliary valves which are located in the bores above the throttle valves.

The primary throttle valves are operated by the accelerator pedal which is connected by linkage to the primary throttle lever. The secondary throttle valves are operated by linkage which is connected to another lever on the primary throttle shaft. The secondary throttle linkage is designed so that the secondary valves do not begin to open until sufficient air velocity can be maintained through the carburetor for good metering. Although the secondary valves do not begin to open until after the primary throttle valves are partially open, they both reach the wide open position at the same time. This is accomplished by lever ratios between the primary and secondary throttle levers.

There are six basic operating systems used in the model 4G and 4GC carburetors. They are float, idle, main metering, power, pump, and choke systems.

IDENTIFYING YOUR ROCHESTER 4 JET CARBURETOR
Where to find the carburetor number: