Carburetor Jetting Calculator
Optimize your air-fuel mixture for altitude and temperature
Tuning Recommendations
Understanding Carburetor Jetting & Air Density
Carburetor jetting is the process of adjusting the fuel flow to match the amount of oxygen available in the air. Because internal combustion engines require a specific air-fuel ratio (usually around 14.7:1 for gasoline), changes in air density require changes in jet sizes.
How Altitude Affects Jetting
As you increase altitude, atmospheric pressure drops and air becomes "thinner" (less dense). Since there is less oxygen in a given volume of air, you must reduce the amount of fuel (use a smaller/leaner jet) to maintain the correct ratio. Conversely, dropping in altitude requires a larger (richer) jet.
How Temperature Affects Jetting
Cold air is denser than warm air. When the temperature drops, more oxygen molecules are packed into the same space, requiring more fuel (richer jetting). When it gets hot, the air expands and becomes less dense, requiring less fuel (leaner jetting).
Example Calculation
- Baseline: #160 Main Jet at Sea Level (0 ft) and 70°F.
- New Scenario: Riding at 5,000 ft and 50°F.
- Result: The air is thinner due to altitude but thicker due to the cold. The calculator determines a Correction Factor (e.g., 0.94).
- New Jet: 160 x 0.94 = 150.4. You would likely install a #150 or #152 jet.
Jetting Symptoms
| Condition | Symptoms |
|---|---|
| Too Rich (Too much fuel) | Engine stutters at high RPM, "blubbery" sound, black smoke, fouled spark plugs. |
| Too Lean (Too much air) | Engine runs hot, "bogs" or dies when opening throttle, popping on deceleration, white spark plugs. |