How the weather affects tuning your carb

(From http://www.eric-gorr.com/techarticles/techarticles.html)

The weather can have a profound affect on the carb jetting because of the changes in air density. When the air density increases, you will need to richen the air-fuel mixture to compensate. When the air density decreases, you will need lean-out the air-fuel mixture leaner to compensate. Use the following as a guide to correcting your jetting when the weather changes:

  • Air temperature: When the air temperature increases, the air density becomes lower. This will make the air-fuel mixture richer. You must select jet sizes with a lower number to compensate for the lower air density. When the barometric pressure decreases, the opposite effect occurs.
  • Humidity: When the percentage of humidity in the air increases, the engine draws in a lower percentage of oxygen during each revolution because the water molecules (humidity) take the place of oxygen molecules in a given volume of air. High humidity will make the air-fuel mixture richer, so you should change to smaller jets.
  • Altitude: In general, the higher the altitude the lower the air density. When riding at racetracks that are at high altitude, you should change to smaller jets and increase the engine's compression ratio to compensate for the lower air density.