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.