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How to Adjust the Rochester Monojet Carb Air Fuel

by Max Stout

The "Monojet" is a single-barrel carburetor, and it is this design feature that inspired the name. Rochester Monojet carburetors were used on many six-cylinder automotive engines and provided sufficient power to the engine while maintaining good gas mileage. Proper adjustment of the air-to-fuel ratio of a Rochester Monojet is important for maintaining the efficient operation of the carburetor. You can adjust the carburetor in 30 minutes or less with the right tools and equipment.

Step 1

Place wheel chocks at the rear wheels to prevent the vehicle from rolling and place the shift lever in the "Park" position. If the vehicle is equipped with a standard transmission, place the shift lever in "First" gear. Set the emergency brake

Step 2

Open the hood and place a fender cover over the fender to protect the paint from scratches. The engine must not be running at this time.

Step 3

Remove the engine air cleaner from the Monojet carburetor. Turn the wing nut at the top of the cleaner shroud counterclockwise to remove.

Step 4

Connect the red lead clip of the tachometer to the negative (-) post of the ignition coil and connect the black lead to the ground post of the battery.

Step 5

Connect the vacuum gauge to a vacuum test nipple on the intake manifold. Remove the cap from the nipple and push the tester hose onto the nipple until it is fully seated.

Step 6

Locate the two adjustment screws on the Rochester Monojet carburetor. One is for the idle speed and contacts the throttle arm. The other adjustment screw is for the idle mixture and is below and left of the fuel line.

Step 7

Start the engine and allow it to warm up for five minutes.

Step 8

Adjust the idle speed first by turning the idle screw with a flat head screwdriver. Turn the screw in a clockwise direction slowly until the tachometer reads 600 to 700 rpm.

Step 9

Turn the engine off.

Step 10

Turn the idle mixture screw slowly in a clockwise direction with a screwdriver counting the number of turns until it is fully seated. Do not tighten the adjustment screw.

Step 11

Turn the screw counterclockwise 2-1/2 turns and restart the engine.

Step 12

Continue to turn the screw counterclockwise in small increments until the indicated vacuum and idle speed no longer increase.

Step 13

Turn the adjustment screw clockwise 1/8-turn until the vacuum begins to drop or the engine idle becomes erratic or shaky. When that point is reached, back the screw out 1/4-turn, and recheck the idle speed.

Step 14

Repeat Steps 8 through 13 if needed.

Step 15

Turn the engine off.

Step 16

Remove the tachometer connections from the engine and the vacuum hose from the test nipple. Replace the nipple cap.

Step 17

Replace the air cleaner onto the Monojet and tighten the wing nut by turning clockwise.

Remove the fender cover and account for all tools and equipment before closing the hood.

Items you will need

  • Safety glasses
  • Wheel chocks
  • Fender cover
  • Vacuum gauge
  • Tachometer
  • Flat head screwdriver

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