Carburetor Jet Size Calculator
Calculate the correct carburetor jet size based on altitude, temperature, and engine modifications for optimal air-fuel ratio tuning.
Frequently Asked Questions
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.
What Is a Carburetor Jet Size Calculator?
A carburetor jet size calculator takes your current jetting setup and environmental conditions — altitude, temperature, and humidity — and tells you exactly what main jet to install at your destination. Without it, you're guessing. With it, you arrive with the right jet already in your carb.
Air density is what actually matters to your engine. A carburetor meters fuel based on how fast air rushes through the venturi. Change the air density — by climbing 4,000 ft or riding in 100°F heat — and that same jet suddenly delivers too much fuel. The bike bogs, fouls plugs, and loses power.
This tool applies the same air density physics used by Keihin and Mikuni engineers when they publish jetting charts. It's built for motorcycle riders, ATV and UTV owners, kart racers, snowmobile riders, and anyone running a carbureted small engine who wants an accurate starting point without paying for a dyno session. If you're heading to a high-altitude ride, our altitude rejetting guide walks through the full process start to finish.
Enter your current jet size and conditions, set your target location, and you'll get a recommended main jet, pilot jet guidance, needle clip suggestion, and the underlying density altitude so you understand why the number changed. See our methodology for a full breakdown of the formulas used.
How to Use & How We Calculate
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Carburetor Jetting Guide: What Every Rider Should Know
Understanding Altitude's Effect on Air-Fuel Ratio
Your engine burns fuel at a specific air-fuel ratio — roughly 12:1 to 14:1 for a carbureted four-stroke at full throttle. The carburetor's jet size is calibrated to deliver that ratio at a specific air density. Gain elevation and the air gets thinner. The same jet now flows too much fuel relative to the air available, pushing the mixture rich. You'll notice a bog off idle, black smoke from the exhaust, and a loss of top-end power.
A rough rule of thumb: drop one jet size per 2,500 ft of elevation gain. So if a 168 main jet works at sea level, you'd try a 162 or 160 at 5,000 ft. The calculator applies the actual physics rather than this rule of thumb, which doesn't account for temperature or humidity variations. For the full science, see our density altitude explainer.
Temperature and Humidity: The Hidden Tuning Factors
Most riders account for altitude but forget temperature. Riding the same trail in July at 95°F versus October at 45°F is effectively a 2,000+ ft density altitude swing. Cold air is denser — your engine wants more fuel in fall than in summer, even at the same elevation.
Humidity is the sneaky one. Humid air is actually less dense than dry air because water vapor (molecular weight 18) displaces oxygen (32) and nitrogen (28). At 80% relative humidity on a hot summer day, air density drops enough to require a 1-2 jet size reduction versus a dry winter day at the same altitude. This is why bikes can feel rich in summer — high temperature and high humidity combine to thin the air substantially. See our seasonal jetting guide for specific recommendations by season.
Reading Your Spark Plug: The Tuner's Report Card
After any jetting change, pull the spark plug and read the color. The porcelain insulator around the center electrode tells you the whole story. A correct mixture leaves a light tan or coffee-brown deposit. Rich running leaves a black, sooty, or oily deposit. Lean running leaves the plug white, pale gray, or blistered — this is the one that causes engine damage.
To get a meaningful reading, do a "plug chop": ride at full throttle for 30-60 seconds, then kill the engine by flipping the kill switch without touching the throttle. Coast to a stop and pull the plug before the engine cools. This captures the jetting at full load. Learn more in our complete spark plug reading guide.
When to Rejet vs. Adjust the Needle
The main jet controls fueling from about 3/4 throttle to full throttle. The needle and needle clip position control the 1/4 to 3/4 throttle range. If your problem is at full throttle (bog, flat power curve, rich plug), start with the main jet. If the issue is mid-throttle hesitation or hanging revs, adjust the needle clip first. Moving the clip down raises the needle (richer), moving it up lowers the needle (leaner). The pilot jet handles idle to 1/4 throttle — if the bike is hard to start or has an off-idle stumble after a large altitude change, go up or down one pilot jet size. Our pilot vs. main jet guide walks through this in detail.
Who Should Use This Calculator?
Anyone running a carbureted engine who moves between elevations or rides in varying temperatures. Specifically:
- Enduro and trail riders who travel to mountain events and need to rejet before the first moto
- Motocross racers tuning for specific tracks at elevation (Washougal at 300 ft vs. Glen Helen at 1,300 ft vs. a Colorado national at 5,000+ ft)
- ATV and UTV owners taking their machines into the mountains for hunting season — a significant altitude jump that most carbed rigs aren't jetted for
- Kart racers competing at circuits that range from coastal flatlands to high-altitude venues
- Vintage motorcycle restorers bringing older carbureted bikes back to life and dialing them in for modern riding locations
- Snowmobile riders who run at altitude and need to compensate for thin, cold mountain air
- DIY mechanics who want a solid starting point before spending time on the dyno — or who want to avoid the dyno cost entirely for a straightforward altitude correction
You don't need to be a professional tuner to use this. If you know your current jet size and can read a thermometer, you have everything you need. The calculator handles the physics. Always verify with a plug chop after installing the new jet — the tool gives you the right neighborhood, real-world conditions confirm the address.
ToolSite Team
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