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How to Adjust Rochester 2G Carbs

by Michelle Schaefer

The Rochester 2G carburetor has two bores, two venturis and two separate but identical metering systems. The two-bore carburetor is normally used on V-8 engines where each bore supplies the air/fuel mixture to four cylinders through a divide intake manifold. The model 2G was equipped with a manual choke and was primarily used on trucks and marine engines. The 2G carburetor was a popular choice for the "Tri-Power" triple carburetor muscle cars of the 1960s. Routine adjustment of the 2G carburetor can be done without removing the carburetor and will keep it operating at peak efficiency.

Shift the transmission into "park" if it is an automatic transmission or "neutral" if it is manual. Chock one tire and set the parking brake so the vehicle will not roll. Remove the air cleaner assembly and set it aside.

Loosen the idle stop screw until the throttle plates are completely closed in the carburetor body. Set a machinist's double-ended square to 1 1/8 inches and place it at the top of the air horn, with the measuring rod resting on the pump rod cam. Bend the pump rod with a pair of needle-nose pliers until the cam rests on the end of the square.

Set the double-ended square to 1 inch and place it on the air horn with the measuring rod resting on the idle vent cam. Open the throttle valve until the vent valve just closes. Bend the tang on the pump lever until the cam just touches the measuring rod on the double square.

Open the throttle plates wide open and close the choke. Insert a .055-inch wire feeler gauge between the choke plate and the wall of the air horn on the carburetor body. Bend the tang on the throttle lever to adjust the clearance.

Turn both idle-mixture screws clockwise and count the number of turns it takes until they gently seat. Return the screws to their original positions by turning them counter-clockwise the same number of turns that it took to seat them. If both idle-mixture screws were not adjusted to the same amount initially, set them both to two turns out at this time.

Connect the tachometer to the engine by following the manufacturer's directions. Connect the vacuum gauge to a "manifold" or constant vacuum source. Start the engine and take it to normal operating temperature. Set the idle speed to 850 rpm with the idle-speed screw. Note the reading on the vacuum gauge and turn one idle-mixture screw clockwise a half-turn and note the change on the vacuum gauge. If the vacuum reading increases, turn the other idle-mixture screw clockwise a half-turn. Note the vacuum gauge reading. If the gauge continues to climb, adjust each idle-mixture screw in another quarter-turn. If the engine stumbles or the vacuum drops when turning the idle-mixture screws clockwise, turn both screws out a half-turn and note the gauge reading.

Adjust the idle speed screw to maintain a constant engine speed of 850 rpm as the idle-mixture screws are being adjusted. Repeat steps 5 and 6 until the idle speed remains constant and the maximum vacuum reading is achieved by adjusting the idle-mixture screws. Ensure that each idle-mixture screw is turned in or out exactly the same amount.

Remove the tachometer and vacuum gauges. Replace the air cleaner and remove the wheel chock.


  • A fender cover or old blanket will protect the finish of your car when you are working under the hood.


  • Carbon monoxide gas is odorless and highly poisonous. Never work on a running engine inside a closed garage or other work space unless the exhaust is properly routed outside with approved exhaust hoses.
  • Use caution when working on a running engine. The rotating parts of the engine can cause personal injury and may damage tools and test equipment.

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About the Author

Michelle Schaefer began writing in 1998 for "The Pennsylvania Homeschooler" with advice for parents educating their handicapped children at home. She earned a bachelor's degree in education from Kutztown University in 1991.

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