Motorcycle Jet Size Calculator

Carburetor jetting is the process of selecting the right fuel-metering components so your motorcycle's engine runs at the correct air-to-fuel ratio — and getting it wrong causes poor performance, overheating, or engine damage. Enter your engine displacement, number of cylinders, altitude, temperature, air filter type, and exhaust system into the Motorcycle Jet Size Calculator to get your recommended main jet and pilot jet sizes. You'll also see the percentage jetting change from your current setup and a target air/fuel ratio.

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Results

Recommended Main Jet

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Recommended Pilot Jet

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Jetting Change

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Target A/F Ratio

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Frequently Asked Questions

How do I know if my motorcycle needs re-jetting?

Signs include poor throttle response, black smoke from exhaust, excessive fuel consumption, difficulty starting, or if you've modified the air intake or exhaust system.

What's the difference between main jet and pilot jet?

The main jet controls fuel flow at wide open throttle (3/4 to full throttle), while the pilot jet controls fuel flow at idle and low throttle positions (up to 1/4 throttle).

How does altitude affect carburetor jetting?

Higher altitude has thinner air, requiring smaller jets to maintain proper air-fuel mixture. For every 2000 feet of elevation gain, you typically need to reduce jet size by 5-10%.

Can I use this calculator for any motorcycle carburetor?

This calculator provides general recommendations for most motorcycle carburetors. However, specific carburetors may have unique characteristics requiring fine-tuning through testing.

What happens if my jets are too big or too small?

Jets too large cause rich running (black exhaust, fouled plugs, poor fuel economy). Jets too small cause lean running (overheating, poor performance, potential engine damage).

Should I change both jets at the same time?

It's best to change one at a time and test. Start with the circuit that shows the most problems - main jet for high RPM issues, pilot jet for idle and low-speed problems.

How do temperature changes affect jetting?

Cold air is denser and requires larger jets, while hot air is thinner and needs smaller jets. A 20°F temperature change can require a 2-5% jet size adjustment.

What tools do I need to change carburetor jets?

You'll need basic hand tools to remove the carburetor, a jet removal tool or small screwdriver, and replacement jets. Always have a selection of jet sizes for tuning.