The Effect of Temperature, Altitude and Humidity on Jetting
Once your jetting is set it’s not necessarily set for life. Changes in air temperature, altitude and humidity can have an effect on how your engine runs.
If you captured a measured volume of air on a humid 90° F day at sea level or a cool dry 40° F day at 10,000 feet both would contain about 22% oxygen. The density and therefore the total number of oxygen molecules however would differ enough to effect the performance of your engine.
Temperature- For most of us changes in air temperature will have the greatest effect on our jetting. As the air temperature gets colder the air density increases. The air molecules become less active ( move around less ) and therefore take up less space. Because they take up less space more air, and therefore more oxygen, can fit into a measured volume of air as the temperature decreases. As the temperature drops the engine will begin to run leaner and more gasoline will need to be added to compensate. As the temperature increase the engine will begin to run richer and less gasoline will be needed.
Altitude- Again this is an issue of air density. At sea level atmospheric pressure is around 15 psi and as the altitude increased the atmospheric pressure decreases. Because less pressure is exerted on a measured volume of air as the altitude increases the air molecules are able to relax and they take up more space leaving less space for additional molecules. The higher the altitude the less air in a measured volume and therefore less oxygen present so jetting will have to be leaned to compensate.
Humidity- Humidity is a measure of how much water vapor is in the air. The higher the humidity the less space there is for additional molecules of air and therefore oxygen. As the humidity increases there is less oxygen and therefore the engine runs richer. Jetting that may have been spot on in the cool dry morning air may start to run rich as the temperature and humidity increase over the course of the day.
Rash22 (from The Effects of Temperature, Altitude and Humidity on Jetting By Canadian Dave)