* Altitude: Higher altitudes require leaner mixtures.
* Temperature: Colder temperatures may require richer mixtures.
* Air filter condition: A dirty air filter will necessitate a richer mixture.
* Engine condition: Wear and tear on the engine can subtly alter the ideal mixture.
The fuel mixture screw is used for fine-tuning the carburetor's idle mixture. The best approach is to find the factory service manual for your specific model. This manual will give you the initial starting point and instructions for properly adjusting the screw using a method like the "plug chop" method (checking the spark plug's color after a short run to determine the richness of the mixture).
Without the manual, attempting to set the screw blindly could lead to engine damage from running too lean (too little fuel) or too rich (too much fuel). A poorly adjusted mixture can cause poor performance, hard starting, and even engine damage.
In short: Consult your service manual. If you can't find the manual, a qualified motorcycle mechanic will have the knowledge and tools to properly adjust the carburetor for optimal performance.
