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  5. Adjusting Idle Mixture
How to Adjust Your Idle Mixture

There is no exact adjustment of the idle mixture screws while the carburetor is on the bench. A good starting point is to seat the screws, then back them out 1 1/2 turns. Be careful not to force them in as you will damage the screw.
  • Warm the engine to operating temperature.
  • Set the idle to specifications - If the engine is idling too fast, then adjusting the idle mixture will not do you any good.
  • Do you have the 3rd idle mixture screw between the 2 regular idle mixture screws? Some of the older AFB carburetor had these.
    • Adjust this middle larger screw clockwise until the rpm starts to drop. Turn it back 1/8 of a turn.
  • Hook up a vacuum gauge to a vacuum port off the carburetor, or intake manifold. You can also use a tach gauge for RPM. Some of us simply listen to the engine.
  • Alternate between each screw and start by turning them counter clockwise 1/2 a turn - they may already be in too far.
  • Now turn the screw in 1/4 turn each time, alternating and watch for RPM, or vacuum start to drop.
  • Turn the screw back out 1/4 turn.
  • You can also watch the vacuum gauge to get the best steady vacuum reading.


Getting a rough idle?

Pull the idle mixture screws and blow out the passage. Make sure you get air through to the port inside the carburetor bore.
Clean out any idle vent passage.

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