Carb Jet Size Calculator
Your expert tool for precision motorcycle and ATV carburetor tuning.
Calculate Your Main Jet Size
Enter the main jet size that is correct for your baseline conditions.
The altitude where your current jetting is correct.
The temperature where your current jetting is correct.
The new altitude where you will be riding.
The new temperature where you will be riding.
Recommended New Main Jet Size:
Baseline Correction
—
New Conditions Correction
—
Baseline Standard Jet
—
Formula: New Jet Size = (Current Jet / Base Correction) * New Correction
Jet Size vs. Altitude Chart
| Altitude (ft) \ Temp (°F) | 20°F | 40°F | 60°F | 80°F | 100°F |
|---|---|---|---|---|---|
| 0 | 1.056 | 1.033 | 1.011 | 0.989 | 0.967 |
| 2000 | 1.035 | 1.013 | 0.991 | 0.969 | 0.947 |
| 4000 | 1.018 | 0.995 | 0.973 | 0.951 | 0.929 |
| 6000 | 0.998 | 0.975 | 0.953 | 0.931 | 0.909 |
| 8000 | 0.985 | 0.962 | 0.940 | 0.918 | 0.896 |
| 10000 | 0.971 | 0.948 | 0.926 | 0.904 | 0.882 |
What is a Carb Jet Size Calculator?
A carb jet size calculator is a specialized tool designed to help motorcycle and ATV enthusiasts determine the optimal main jet size for their carburetor based on changing atmospheric conditions. The density of air changes with altitude and temperature, which directly affects the air-fuel ratio your engine needs to perform efficiently. A less dense air mixture (found at high altitudes or high temperatures) requires less fuel, meaning a smaller jet is necessary. This calculator removes the guesswork from this critical tuning process.
Anyone who rides a carbureted vehicle in varying elevations or climates should use a carb jet size calculator. It’s particularly useful for dirt bike riders, trail riders, and racers who transport their vehicles to different locations. Common misconceptions are that you only need to re-jet for huge altitude changes, but even a 20°F temperature swing can be enough to warrant a jet change for peak performance.
Carb Jet Size Calculator Formula and Mathematical Explanation
The core principle behind a carb jet size calculator is correcting for air density. The calculator first establishes a “standard” jet size for your engine based on your current setup, then applies a correction factor for the new conditions. This is a more reliable method than simply guessing.
The process is as follows:
- Calculate Baseline Correction Factor: First, a correction factor is determined for your current (baseline) altitude and temperature. This tells us how your current conditions compare to a sea level standard (0ft, ~68°F). [1, 3]
- Calculate Standard Jet Size: Your current jet size is divided by the baseline correction factor. This gives a theoretical “Sea Level Standard Jet Size” — the jet you would use in ideal, standard conditions.
Standard Jet = Current Jet Size / Baseline Correction Factor - Calculate New Correction Factor: Next, a new correction factor is calculated for your new riding altitude and temperature.
- Calculate Final Jet Size: The “Standard Jet Size” is then multiplied by the new correction factor to recommend the new jet size.
New Jet Size = Standard Jet Size * New Correction Factor
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Current Jet Size | The size of the main jet currently in the carburetor. | Varies (e.g., Mikuni, Keihin sizes) | 80 – 500 |
| Altitude | Elevation above sea level. | feet (ft) or meters (m) | 0 – 14,000 ft |
| Temperature | Ambient air temperature. | Fahrenheit (°F) or Celsius (°C) | 0°F – 110°F |
| Correction Factor | A multiplier to adjust for air density. | Dimensionless | 0.85 – 1.10 |
Practical Examples (Real-World Use Cases)
Example 1: From Sea Level to the Mountains
A rider has their dirt bike perfectly tuned with a 180 main jet at their home near sea level (500 ft) on a cool 60°F day. They plan a trip to a mountain trail system at 7,000 feet, where the forecast is 75°F.
- Inputs: Current Jet: 180, Base Alt: 500, Base Temp: 60, New Alt: 7000, New Temp: 75.
- Calculation: The carb jet size calculator determines the air at 7,000 ft is much less dense.
- Output: The calculator suggests a new main jet size of approximately 168. This prevents the engine from running too rich, which would cause bogging and poor throttle response at elevation.
Example 2: Seasonal Temperature Change
A 4-wheeler is jetted with a 155 main jet and runs perfectly during the spring at 1,500 feet and 70°F. Now it’s the middle of summer, and the temperature is consistently 95°F.
- Inputs: Current Jet: 155, Base Alt: 1500, Base Temp: 70, New Alt: 1500, New Temp: 95.
- Calculation: The carb jet size calculator accounts for the less dense hot air.
- Output: The recommendation would be a smaller jet, around 150. This simple change can restore crisp throttle response that was lost due to the richer condition caused by the hot weather. Check out this Keihin jetting chart for more info.
How to Use This Carb Jet Size Calculator
Using this calculator is a straightforward process to ensure your engine runs at its best. Follow these steps:
- Enter Baseline Data: Input your current main jet size, along with the altitude and temperature where that jetting works perfectly. This is your “known good” setup.
- Enter New Conditions: Input the altitude and temperature of the new location where you plan to ride.
- Analyze the Results: The calculator will instantly provide a “Recommended New Main Jet Size.” This is your primary result. It will also show intermediate values like the correction factors used, which helps in understanding the adjustment.
- Make the Change: Purchase a jet of the recommended size (it’s wise to get one size up and one size down as well for fine-tuning). Installing the new jet should get your air-fuel mixture very close to optimal for the new conditions. For a detailed walkthrough, see our Mikuni carburetor adjustment guide.
Key Factors That Affect Carb Jetting Results
Several factors beyond this carb jet size calculator can influence your final jetting selection. Precision tuning requires considering the whole system.
- Altitude: This is the most significant factor. As altitude increases, air pressure drops and air becomes less dense. This requires a smaller main jet to lean out the mixture. [5]
- Temperature: Warmer air is less dense than cold air. Therefore, an increase in temperature requires a smaller jet, and a decrease requires a larger jet. [6]
- Humidity: Humid air has less oxygen than dry air. High humidity can require leaning the mixture slightly (a smaller jet), though its effect is less pronounced than altitude or temperature.
- Engine Modifications: Changes to the exhaust system (e.g., a performance pipe) or air intake (e.g., high-flow filter) will almost always require rejetting. These mods typically increase airflow and demand a larger jet. Our motorcycle jetting guide can help.
- Fuel Type: Different fuel formulations, especially oxygenated race fuels, can change the ideal air-fuel ratio and necessitate jetting adjustments.
- 2-Stroke vs. 4-Stroke: Two-stroke engines are generally more sensitive to jetting changes than four-strokes. A detailed guide on two-stroke engine tuning is a must-read for 2T owners.
Frequently Asked Questions (FAQ)
Symptoms include bogging on acceleration, poor fuel economy, black sooty smoke from the exhaust, and a fouled, black spark plug. The engine may feel “lazy” or slow to respond. [12, 13]
Symptoms include engine hesitation or surging at steady speeds, backfiring or popping on deceleration, running hot, and a white or very light-colored spark plug. A severe lean condition can cause engine damage. [12, 20]
The pilot jet and air/fuel screw control the mixture from idle to 1/4 throttle. The needle position controls the mid-range (1/4 to 3/4 throttle), and the main jet controls the top end (3/4 to full throttle). This carb jet size calculator focuses on the main jet. [16]
A plug chop is a method to test main jet settings. You run the engine at full throttle for a short distance, then hit the kill switch and pull in the clutch simultaneously. You then inspect the spark plug color to determine the mixture. A tan or coffee color is ideal. For more on this, learn how to read a spark plug. [4, 7]
No. EFI systems have sensors (like O2 and MAP sensors) that automatically adjust the fuel delivery for changes in air density. This calculator is only for carbureted engines.
It’s very accurate for providing a strong baseline. It uses industry-standard air density correction formulas. [1, 2] However, it should be considered a starting point. Final tuning may require minor adjustments based on your specific engine and conditions.
Always start with the slightly richer (larger) size. It’s generally safer to be slightly rich than slightly lean, as a lean condition can cause the engine to run hot and potentially lead to damage.
Yes, higher humidity means less oxygen in the air, which can create a slightly rich condition. For very precise tuning, you might go down one jet size on extremely humid days, but the effect is minor compared to altitude and temperature. [6]