Updated: October 2023 | Expert verified for carburetor tuning accuracy.
Tuning your engine requires precision. This carb jet size calculator helps you determine the ideal jetting based on altitude changes, displacement, and volumetric efficiency to ensure your fuel mixture is perfect.
Carb Jet Size Calculator
Enter any 3 values to calculate the 4th.
carb jet size calculator Formula
*Density is approximated using Altitude Lapse Rate formulas.
Formula Source: Holley Performance Engineering | Mikuni Tuning Manual
Variables:
- Original Jet Size: The diameter (usually in mm) of your current main jet that runs well.
- Current Altitude: The elevation where your current tuning is optimized (0 = Sea Level).
- Target Altitude: The destination elevation where you plan to ride/drive.
- New Target Jet: The compensated jet size required for thinner or thicker air.
What is carb jet size calculator?
A carb jet size calculator is a specialized tool used by mechanics and performance enthusiasts to determine the correct fuel metering orifice size for a carburetor. As air density changes—either due to altitude or temperature—the ratio of fuel to air shifts.
If you move to a higher altitude, the air becomes less dense. Without adjusting your jetting, your engine will run “rich” (too much fuel), leading to poor performance, fouled spark plugs, and wasted fuel. This calculator automates the complex physics of air density to give you a baseline tuning starting point.
How to Calculate carb jet size calculator (Example)
- Determine your current baseline: e.g., a 1.20mm main jet at sea level (0 ft).
- Identify your target altitude: e.g., 6,000 ft in the mountains.
- Calculate the air density ratio (typically around 0.82 for 6,000 ft).
- Apply the square root rule: New Jet = 1.20 × √0.82 ≈ 1.08mm.
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Frequently Asked Questions (FAQ)
Should I go larger or smaller when going up in altitude?
You should go smaller. Higher altitude means thinner air, so you need less fuel (a smaller jet) to maintain the correct Air-Fuel Ratio.
Does temperature affect jet size?
Yes. Colder air is denser and requires more fuel (larger jet), while hot air is less dense and requires less fuel (smaller jet).
Can I use this for pilot jets too?
Yes, the density compensation principle applies to all fuel circuits, though main jets are the most common adjustment point.
Is this 100% accurate?
It provides a mathematical baseline. Always verify tuning with a spark plug reading or a Wideband O2 sensor.