Carb Tuning - Page 5, By Eric Gorr

Eric Gorr

Engine Builder
Jun 29, 1999
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Jetting for Riding Techniques

Certain types of riders require jetting to compliment their technique. For example beginner minibike riders will need slightly richer jetting on the pilot/slow jet and the needle clip position to mellow the powerband and make it easier to ride. Conversely desert racers who hold the throttle wide open for long periods of time need rich main jets to compensate for the high load.

The Weather Makes The Biggest Difference!

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 temperatureÑWhen 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.

HumidityÑWhen 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.

AltitudeÑIn 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.

Track Conditions and Load

The conditions of the terrain and the soil have a great affect on jetting because of the load on the engine. Obstacles like big hills, sand, and mud place a greater load on the engine that requires more fuel and typically richer jetting. In motocross, track conditions tend to change over the course of the day. Typically in the morning the air temperature is cooler and the soil wetter requiring richer jetting. In the afternoon when the temperature rises and the track dries out, leaner jetting is needed in order to keep the engine running at peak performance. Other changes for mud and sand riding might include changing to a lower final-drive ratio (rear sprocket with more teeth) to reduce the load on the engine and help prevent it from overheating. Advancing the ignition timing will make the engine more responsive at low to middle rpm.

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