Always make air fuel mixture screw adjustments with a warm engine and the engine running, so you can listen to how the adjustments affect the engine’s idle speed. Note that this process is the same for any type of engine with an air fuel mixture screw. It could be a car, motorcycle, scooter, ATV, or anything else with a carburetor.

The air filter on a car might be large and round. It usually sits on top of the carburetor. On a motorcycle, the air filter usually comes off the side of the engine and faces the rear of the bike.

Most air fuel mixture screws are located on the side of the carburetor, but it depends on the specific engine.

Tightening the screw weakens the air and fuel mixture and decreases the amount of fuel flowing to the engine. Tightening the screw is also called making the fuel mixture leaner, which lowers the RPMs at which the engine idles. Running a lean fuel mixture makes the engine run with less fuel than it needs to run efficiently. This can damage an engine because there is more friction between moving parts and the engine operates at a higher temperature.

Loosening the screw strengthens the air and fuel mixture and increases the amount of fuel flowing to the engine. Loosening the screw is also called making the fuel mixture richer, which increases the RPMs at which the engine idles. Running a rich fuel mixture causes the engine to run with more fuel than it needs to operate efficiently. This means that it will burn fuel much faster than necessary, although the engine may run with more power and at a cooler temperature.

For example, if you turned the screw 2 full turns counterclockwise from the spot where the engine’s idle started to sound rough, now turn the screw clockwise 1 full turn.

You might notice that turning the screw 1/2 a turn in either direction makes the engine start to sound rougher or more irregular, in which case you can just set the screw back to the middle position. This adjustment process is also known as balancing the idle mixture. Most engines have an ideal air fuel ratio, or AFR, around 14. 7:1. You could find your engine’s exact AFR using a special meter, but this isn’t really necessary unless you want to be ultra-precise, such as if you’re tuning a high-performance race car or motorcycle. [10] X Research source