Rochester 4G, 4GC Technical Help

Rochester 4G, 4GC, 4 Jet 4-barrel carburetor technical information. Find rebuilding instructions, technical info, and troubleshooting help here.

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:

Application	Carburetor Numbers
1956 Oldsmobile	7009775
1956 Buick Change in Primary Jets	7009200
1956-58 Chevrolet	7008737, 7009846, 7012126, 7012128, 7011108
1956 Pontiac	7010600
1962 Buick Adjustment Specifications	7020040, 7020041
1962-65 Chevrolet V8 Truck w/Governor	7020025, 7020029
1963 Buick Special V8 215"	7023044, 7023045
1963-65 Buick V8 401"	7023040, 7023143, 7024040, 7025040,7025140
1964 Buick V8 300"	7024044, 7024045
1965 Studebaker	7025186
1963-64 Oldsmobile	7023050, 7024154, 7024050
1965 Olds F-85 & Jetstar 88	7025054, 7025157, 7025158, 7025055
1965-67 Chevrolet V8 Truck 327	7025125, 7036125, 7027125, 7037125

Rochester 4G Marine Carburetor

Application	Carburetor Numbers
Chris Craft Adjustments & Specifications	7013071, 7015090
Kiekhaefer	7020995, 7023183
Kiekhaefer	7023180
Owens Adjustments & Specifications	7023184

Needle & Seat

The 4 Jet uses 2 sets of needle & seats. Some 4 jets have different size needle & seat for the primary & secondary. When this happens use the bigger needle & seat on the primary side.

Rochester 4G & 4GC Carburetor FAQ

Q: What is the difference between the Rochester 4G and 4GC carburetors?
A: The 4G is a 4-barrel downdraft carburetor with a manual choke, while the 4GC features a fully automatic choke. Some 4G models are equipped with a vacuum-controlled governor for engine speed control, especially in heavy-duty truck applications.

Q: What vehicles typically used the 4G and 4GC carburetors?
A: These carburetors were widely used on General Motors vehicles (Chevrolet, Pontiac, Oldsmobile, Buick, and GMC trucks) and some other brands from the mid-1950s through the 1960s.

Q: How does the float system work in the 4G/4GC?
A: Each side of the carburetor has its own float system, consisting of a float chamber, dual pontoon float, needle valve, and valve seat. The float maintains the proper fuel level and shuts off fuel flow when full. Some models use assist springs to help keep the needle closed under varying fuel pressures.

Q: What are the primary and secondary sides of the carburetor?
A: The primary side is essentially a complete two-barrel carburetor handling most driving conditions, including idle and cruise. The secondary side provides extra air and fuel during high-power demands. Secondary throttles only begin to open after enough air velocity is present, ensuring proper fuel metering.

Q: How does the idle system operate?
A: At small throttle openings, the idle system delivers the correct air-fuel mixture for smooth engine operation. Adjustable idle screws fine-tune the mixture, while some models use an idle air bypass system for extra stability. Fixed idle systems may be found on the secondary side for specific applications.

Q: What is the purpose of the main metering system?
A: The main metering system controls fuel flow between idle and full-power ranges. It ensures efficient fuel metering during cruising and transitions to richer mixtures when more power is needed. The system uses removable venturi clusters for easy cleaning and maintenance.

Q: How does the accelerator pump system work?
A: The accelerator pump is located on the primary side and supplies a quick squirt of fuel for smooth acceleration. It uses a double-spring plunger for a steady fuel charge and check balls to prevent air from entering the system.

Q: How does the automatic choke function on the 4GC?
A: The 4GC’s automatic choke uses a thermostatic coil, vacuum piston, and sometimes a hot water or split linkage system. The choke remains closed during cold starts, then gradually opens as the engine warms up, regulating the air-fuel mixture for optimal performance.

Q: What is the governor feature found on some 4G models?
A: The vacuum-controlled governor is designed for trucks to limit engine speed under light loads. It uses a vacuum diaphragm and linkage to partially close the throttle at high RPMs, preventing engine overspeed and wear.

Q: Are there any special maintenance tips for the 4G/4GC?
A: Yes. Regular cleaning and inspection of floats, needles, jets, and linkages are important. Use only recommended cleaners, avoid harsh solvents on rubber or leather parts, and always refer to the service manual for float settings, adjustments, and reassembly procedures. Never force jets or passages with drills.