Whether you are racing a two-stroke dirt bike, tuning a vintage motorcycle, or adjusting a small engine for high-altitude work, getting your jetting right is the difference between peak performance and a seized piston. Use our calculator below to determine your required main jet size based on changes in altitude and temperature.
Note: This is an estimate based on air density changes. Always check your spark plug color.
Understanding Engine Jetting and Air Density
Carburetors work on the principle of the Bernoulli effect, mixing air and fuel in a specific ratio. However, the "weight" of air changes based on your environment. Since the carburetor measures air by volume, but the engine needs oxygen by mass, you must adjust your jets to maintain the stoichiometric ratio.
The Altitude Factor
As you climb in elevation, the atmospheric pressure drops. This means there are fewer oxygen molecules in every cubic inch of air. If you keep the same jetting you used at sea level while riding at 8,000 feet, your engine will run "rich" (too much fuel for the available oxygen). This leads to:
- Sputtering at high RPMs.
- Fouled spark plugs.
- Reduced power and "bogging" feel.
The Temperature Factor
Cold air is denser than warm air. On a crisp 40°F morning, your engine is getting significantly more oxygen than on a 95°F afternoon. If you tuned your bike in the summer and ride it in the winter without adjusting, you will run "lean" (too little fuel), which can cause the engine to overheat or even melt a hole in the piston.
How to Use the Jetting Calculator
To get the most accurate results from this tool, follow these steps:
- Identify your "Baseline": Use the jet size that currently makes the engine run perfectly. Note the altitude and temperature of that location.
- Input Target Conditions: Enter the elevation and expected temperature of your riding destination.
- Read the Result: The calculator provides a theoretical main jet size based on the Relative Air Density (RAD) change.
Signs Your Jetting is Off
Even with a calculator, you should always listen to your machine. Here is what to look for:
Rich Condition (Too much fuel)
The engine sounds "blubbery" or muffled. It may produce excessive smoke (in 2-strokes) and will struggle to reach its top RPM. The spark plug will appear black and oily.
Lean Condition (Too much air)
The engine sounds "pingy" or hollow. When you let off the throttle, the RPMs might hang high before settling. This is dangerous; a lean engine runs very hot. The spark plug will appear white or blistered.