Carburetor Jet Size Calculator

Air becomes thinner as altitude increases — there are fewer oxygen molecules per cubic foot. Since your carburetor delivers a fixed volume of fuel relative to airflow, the air-fuel mixture becomes richer (too much fuel for the available oxygen) at higher altitudes. This causes poor throttle response, black exhaust smoke, fouled spark plugs, and reduced power. Re-jetting with a smaller main jet restores the optimal 14.7:1 stoichiometric ratio for gasoline engines.

Temperature has a significant impact on air density. For every 20°F increase in temperature, air density drops by approximately 3-4%, requiring a correspondingly smaller jet to maintain proper mixture. A motorcycle tuned perfectly on a 50°F morning may run noticeably rich by afternoon when temperatures reach 90°F. This is why racers re-jet between morning practice and afternoon races even at the same track.

Density altitude is the theoretical altitude in the International Standard Atmosphere where the current air density would exist. It combines the effects of actual altitude, temperature, and humidity into a single number. A sea-level location on a 100°F day with high humidity might have a density altitude of 3500 feet — meaning the engine breathes as if it were at 3500 feet elevation on a standard day. Density altitude is the single best predictor of how your carburetor needs to be jetted.

The pilot (or slow) jet controls the fuel mixture at idle and low throttle openings (0-25%), while the main jet governs mid-to-full throttle. When making large main jet changes (5 or more sizes), the pilot jet usually needs a proportional adjustment. A general rule: if you drop the main jet by 2 or more standard sizes, go down one pilot jet size. The pilot circuit and main circuit overlap in the 1/4 to 1/2 throttle range, so mismatched jetting creates a flat spot in that transition zone.

A less restrictive exhaust allows the engine to expel spent gases more efficiently, which draws in more fresh air-fuel mixture on the intake stroke — a phenomenon called scavenging. This effectively leans out the mixture because the carburetor delivers the same fuel volume into a larger air charge. Most aftermarket slip-on exhausts require 3-5% richer jetting (1-2 main jet sizes up), while full exhaust systems with header changes may need 5-8% enrichment depending on the cam timing and port design.

Running rich (too much fuel): black or sooty spark plugs, black smoke from exhaust, sluggish throttle response, strong fuel smell, poor fuel economy, and engine loading up or hesitating at steady throttle. Running lean (too little fuel): white or light gray spark plugs, popping on deceleration, engine runs hot, surging at steady cruise, hesitation when opening throttle quickly, and in severe cases pinging or detonation that can cause engine damage.

Yes, the air density correction principles apply to any carbureted engine — motorcycles, ATVs, cars, karts, snowmobiles, and small engines. The jet size numbering system varies between manufacturers (Keihin, Mikuni, Weber, Holley, etc.), but the proportional relationship between air density and required jet area is universal. Enter your current jet size number regardless of manufacturer, and the calculator provides the correct proportional change.

Humidity has a smaller effect than altitude or temperature, but it is measurable. Water vapor molecules displace nitrogen and oxygen molecules in the air. Since water vapor does not participate in combustion, humid air contains fewer combustible oxygen molecules per unit volume than dry air. At high humidity levels (above 80%), the air density reduction can be equivalent to a 500-1000 foot increase in density altitude, potentially requiring a half to one jet size smaller.

The jet needle is a tapered rod that sits inside the needle jet (emulsion tube) and controls fuel delivery in the 1/4 to 3/4 throttle range. It has grooves near the top where a small clip can be placed. Moving the clip UP on the needle lowers the needle further into the jet, reducing fuel flow (leaner). Moving the clip DOWN raises the needle out of the jet, increasing fuel flow (richer). Most needles have 5 clip positions, and each position change affects the mixture by roughly 3-5% in the midrange.

This calculator provides an excellent starting point that gets you within 1-2 jet sizes of optimal in most cases. However, it cannot account for variables like engine wear, carburetor condition, specific cam profiles, compression ratio changes, or fuel quality. For competitive racing or maximum performance, use this calculator to establish baseline jetting, then fine-tune on a dynamometer with wideband oxygen sensor feedback. For recreational riding and seasonal altitude changes, the calculated jetting is typically accurate enough for safe, enjoyable operation.

Most carburetor manufacturers use increments of 2.5 (e.g., 155, 157.5, 160, 162.5, 165) for main jets, though some use increments of 2 or 5. Keihin and Mikuni carburetors commonly found on Japanese motorcycles use 2.5 increments. Holley carburetors for American V8s use different numbering. This calculator rounds to the nearest 2.5 increment. When the calculated jet falls between available sizes, it is generally safer to choose the richer (larger) option to protect against lean-condition engine damage.