How Temperature, Altitude & Humidity Affect Carburetor Jetting
The carburetor’s sole purpose is to deliver the correct mixture of fuel and air to the engine. Sounds simple enough, right? Unfortunately, one of these variables, air, changes depending on the weather. As temperatures rise during the heat of the day the air molecules spread further apart and the air becomes less dense with fewer oxygen molecules in a given volume of air. Similarly, as altitude increases the air density decreases. Humidity, water molecules in the air, also displace oxygen molecules. These changes in air density can have a profound affect on carburetor jetting. Lets examine these factors in more detail assuming we started with a perfect air-fuel mixture.
Temperature
As the air temperature increases, the air density falls. This will make the air-fuel mixture richer. Why? Less oxygen content in the air but no change in the amount of fuel being supplied by the carburetor. In order to regain the perfect air-fuel mixture its necessary to "jet down" (chose smaller jets) to compensate for the lower air density. Conversely, cooler air temperatures will necessitate more fuel i.e. larger jets.
Altitude
Generally speaking as altitude increases the air density decreases. This will require smaller jets to compensate for the lack of oxygen content at higher altitudes. Less oxygen and less fuel means less power. Having less power is not something racers warmly embrace. High altitude race engines often employ much higher compression ratios to make the most of the "thin air".
Humidity
Humidity, water molecules in the air, also displace oxygen molecules making the air less dense. This will make the air-fuel mixture richer and will require smaller jets to compensate for the lack of oxygen content in the humid air.
