Carb Jet Calculator

Carburetor Main Jet Size Calculator

Recommended Main Jet

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Note: Always start rich and plug-chop to verify. Lean conditions can cause engine damage.

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Understanding Carburetor Jetting and Air Density

Correct carburetor jetting is essential for maintaining the ideal air-fuel ratio (AFR). Engines require a specific ratio of air to fuel (typically around 12.5:1 to 14.7:1 for gasoline) to run efficiently. Because air density changes based on your environment, the amount of fuel delivered by the main jet must also change.

The Role of Altitude

As you increase in altitude, the atmospheric pressure drops, making the air "thinner" (less oxygen per cubic foot). If you keep the same large jet used at sea level, the engine will receive too much fuel for the available oxygen, causing a "rich" condition. This leads to sluggish performance, plug fouling, and excessive smoke. As a general rule, you must reduce (lean out) your jet size as you go higher.

Temperature Effects

Cold air is denser than hot air. On a cold morning, your engine needs more fuel (larger jet) because there is more oxygen in the air. On a hot afternoon, the air expands and becomes less dense, requiring a smaller jet to prevent the engine from running rich.

How This Calculator Works

Our calculator utilizes standard atmospheric correction factors. It assumes a baseline of Sea Level (0 ft) and a standard temperature of 70°F (21°C). The math follows these physics principles:

• Altitude Correction: We apply a reduction of approximately 1.5% in fuel flow for every 1,000 feet of elevation gain.
• Temperature Correction: We adjust fuel flow by approximately 1% for every 10-degree variance from the 70°F baseline.
• Increment Rounding: Since jets come in specific sizes (like Keihin 152.5, 155), the tool rounds to the nearest commercially available size based on the selected brand.

Example Calculation

Suppose you have a Keihin 160 main jet installed and you are tuned perfectly for sea level at 70°F. You plan a trip to a mountain trail at 6,000 feet where the temp is 80°F.

1. Altitude Factor: 6,000 feet = ~9% reduction in density (Factor: 0.91).
2. Temp Factor: 10°F increase = ~1% reduction in density (Factor: 0.99).
3. Math: 160 x 0.91 x 0.99 = 144.14.
4. Rounding: The tool would suggest a 145 or 142.5 main jet depending on the brand increments.

Pro Tip: When tuning for a new environment, it is safer to be slightly "rich" (larger number) than "lean" (smaller number). A lean engine runs hot and can melt pistons or seize.