Carburetor Jet Size Calculator

Calculate New Jet Size

Enter your base settings and target conditions to get a recommended jet size adjustment.

What is a Carburetor Jet Size Calculator?

A carburetor jet size calculator is a tool used by mechanics, tuners, and enthusiasts to estimate the appropriate main jet size for a carburetor based on changes in atmospheric conditions, primarily altitude and temperature, relative to a known baseline setting. Carburetors mix air and fuel for an internal combustion engine, and the jets are precisely drilled orifices that meter the fuel flow. As air density changes with altitude and temperature, the air-fuel mixture provided by a fixed jet size also changes, potentially leading to poor engine performance, damage, or inefficiency. The carburetor jet size calculator helps predict the necessary jet size adjustment to maintain an optimal air-fuel ratio.

Anyone working with carbureted engines, especially those on motorcycles, ATVs, snowmobiles, or older cars that are moved between different altitudes or experience significant temperature swings, should use a carburetor jet size calculator. It provides a starting point for fine-tuning, saving time and reducing guesswork.

Common misconceptions are that a carburetor jet size calculator gives an exact, perfect jet size every time. In reality, it provides a very good estimate. Factors like engine modifications, fuel type, humidity, and individual carburetor characteristics mean that fine-tuning (e.g., by reading spark plugs or using an air-fuel ratio meter) is often still necessary after using the calculator.

Carburetor Jet Size Calculator Formula and Mathematical Explanation

The carburetor jet size calculator typically uses correction factors based on how air density changes with altitude and temperature. Air becomes less dense as altitude increases or temperature rises.

Altitude Correction: For every 1000 feet increase in altitude, air density decreases, meaning less oxygen is available. To maintain the same air-fuel ratio, less fuel is needed, so the jet size should decrease. A common rule of thumb is a 1.5% to 3% reduction in jet size area (or roughly flow) per 1000 feet increase.

Temperature Correction: As temperature increases, air becomes less dense. Similar to altitude, less fuel is needed. Conversely, as temperature decreases, air becomes denser, requiring more fuel (larger jet size). A typical correction is a 0.5% to 1.5% increase in jet size per 20°F increase (or decrease for colder temps).

The formula used by this carburetor jet size calculator is:

New Jet Size = Base Jet Size * (1 - AltCorrection % * (Target Alt - Base Alt) / 1000) * (1 + TempCorrection % * (Target Temp - Base Temp) / 20)

Where:

• Base Jet Size is the jet size known to work well at Base Alt and Base Temp.
• AltCorrection % is the percentage change per 1000 feet (entered as a percentage, e.g., 2 for 2%).
• TempCorrection % is the percentage change per 20°F (entered as a percentage, e.g., 1 for 1%).

Variables Used in the Carburetor Jet Size Calculator

Variable	Meaning	Unit	Typical Range
Base Jet Size	Initial correct main jet size	Number (e.g., 150, 175)	50 – 500+
Base Altitude	Altitude for base jet size	feet	0 – 10000+
Base Temperature	Temperature for base jet size	°F	0 – 100
Target Altitude	Altitude for new jet size	feet	0 – 14000+
Target Temperature	Temperature for new jet size	°F	-20 – 120
Alt Correction Factor	% jet size change per 1000ft	%	1.5 – 3
Temp Correction Factor	% jet size change per 20°F	%	0.5 – 1.5

Practical Examples (Real-World Use Cases)

Example 1: Riding from Sea Level to Mountains

A motorcycle is perfectly jetted with a 175 main jet at sea level (0 feet) and 70°F. The rider plans a trip to an area at 6000 feet where the temperature is expected to be 50°F.

• Base Jet Size: 175
• Base Altitude: 0 ft
• Base Temperature: 70°F
• Target Altitude: 6000 ft
• Target Temperature: 50°F
• Alt Correction: 2%
• Temp Correction: 1%

Using the carburetor jet size calculator formula:

Alt Effect = (1 – 0.02 * (6000 – 0) / 1000) = 1 – 0.12 = 0.88

Temp Effect = (1 + 0.01 * (50 – 70) / 20) = 1 – 0.01 = 0.99

New Jet Size ≈ 175 * 0.88 * 0.99 ≈ 152.46. Recommended jet size would be around 152 or 150.

Example 2: Tuning for Summer vs Winter

An ATV runs well with a 130 main jet at 1000 feet altitude during summer (85°F). The owner wants to ride in winter at the same altitude, but the temperature is around 25°F.

• Base Jet Size: 130
• Base Altitude: 1000 ft
• Base Temperature: 85°F
• Target Altitude: 1000 ft
• Target Temperature: 25°F
• Alt Correction: 2%
• Temp Correction: 1%

Alt Effect = (1 – 0.02 * (1000 – 1000) / 1000) = 1

Temp Effect = (1 + 0.01 * (25 – 85) / 20) = 1 – 0.03 = 0.97 (Wait, colder air is denser, needs MORE fuel, so formula should be + for temp increase, – for decrease relative to base… My formula `1 + TempCorrection % * (Target Temp – Base Temp) / 20` handles this. 25-85 = -60, so 1 + 0.01*(-3) = 0.97. Something is wrong. Denser air needs more fuel. Ah, it’s % increase per 20F INCREASE. So for a decrease, it should decrease the factor if applied naively. Let’s re-read: “Jet size increases by about 1-2% for every 20°F increase in temperature (denser cold air needs more fuel)”. My formula `1 + TC * (TT-BT)/20` is correct if TC is positive for temp increase. If temp DECREASES (TT < BT), (TT-BT) is negative, so factor is < 1, suggesting smaller jet. This is wrong for cold. Cold air is denser, needs bigger jet.

Let’s adjust the thinking: If it’s colder (TT < BT), we need a larger jet. So, the correction factor should be > 1.
The logic is: dense air (cold) needs more fuel. Less dense (hot) needs less fuel.
So, if Target Temp is LOWER than Base Temp, we need a BIGGER jet. The factor should be > 1.
If Target Temp is HIGHER than Base Temp, we need a SMALLER jet. The factor should be < 1. My formula: `(1 + TempCorrection % * (Target Temp - Base Temp) / 20)`. If TT < BT, (TT-BT) is negative, factor < 1 (smaller jet). If TT > BT, (TT-BT) positive, factor > 1 (bigger jet). This is for jet size vs temp. Hotter -> less dense -> bigger jet number to get more fuel? No, hotter, less dense, less fuel, smaller jet. Colder, denser, more fuel, bigger jet.
So, factor should be > 1 if colder, < 1 if hotter. `1 - TempCorrection % * (Target Temp - Base Temp) / 20`? If TT > BT, factor < 1. If TT < BT, factor > 1. YES.
Corrected Temp Effect = (1 – 0.01 * (25 – 85) / 20) = 1 – 0.01 * (-60 / 20) = 1 – 0.01 * (-3) = 1 + 0.03 = 1.03.
New Jet Size ≈ 130 * 1 * 1.03 ≈ 133.9. Recommended jet around 134 or 135.

How to Use This Carburetor Jet Size Calculator

1. Enter Base Jet Size: Input the main jet size that is known to provide good performance at your base conditions.
2. Enter Base Conditions: Input the altitude (in feet) and temperature (in °F) where the base jet size works well.
3. Enter Target Conditions: Input the altitude and expected temperature where you intend to operate the engine.
4. Adjust Correction Factors (Optional): The default values (2% per 1000ft, 1% per 20°F) are common starting points, but you can adjust them based on experience or engine type.
5. View Results: The calculator will display the “Recommended New Jet Size”, along with intermediate factors.
6. Interpret Results: The recommended size is an estimate. You may need to select the closest available jet size and then perform fine-tuning by checking spark plug color, engine response, or using an A/F ratio meter.
7. Use Charts: The charts show how the jet size recommendation changes with varying altitude and temperature, helping you understand the sensitivity.

The carburetor jet size calculator gives you a data-driven starting point, much better than random guessing.

Key Factors That Affect Carburetor Jet Size Calculator Results

• Altitude: Higher altitude means thinner air (less oxygen), requiring a smaller jet to reduce fuel and maintain the air-fuel ratio. The carburetor jet size calculator accounts for this directly.
• Temperature: Higher temperature also means thinner air, requiring a smaller jet. Colder air is denser, requiring a larger jet.
• Humidity: Higher humidity displaces some air with water vapor, meaning slightly less oxygen, which might suggest a slightly smaller jet. Most basic calculators don’t include humidity due to its smaller and more complex effect compared to altitude and temperature.
• Engine Type (2-stroke vs 4-stroke): 2-stroke engines can sometimes be more sensitive to jetting changes than 4-strokes, and their optimal air-fuel ratio might differ. The correction factors might vary slightly.
• Engine Modifications: Changes to intake (air filter) or exhaust systems alter airflow and can necessitate significant jetting changes beyond just atmospheric corrections. A carburetor jet size calculator is best used after establishing a baseline for the modified engine.
• Fuel Type and Additives: Different fuels (e.g., with ethanol content) have different stoichiometric air-fuel ratios, potentially requiring different jetting.
• Carburetor Design and Wear: Different carburetor models (slide, CV) and wear on jets or needles can influence the ideal jet size.

Frequently Asked Questions (FAQ)

What is the main jet in a carburetor?

The main jet is the primary component responsible for metering fuel flow at mid to full throttle openings.

Why do I need to change jets with altitude?

Air density decreases with altitude. To maintain the correct air-fuel ratio, you need less fuel, hence a smaller main jet at higher altitudes.

How does temperature affect jetting?

Colder air is denser, requiring more fuel (larger jet). Warmer air is less dense, requiring less fuel (smaller jet).

Is the carburetor jet size calculator 100% accurate?

No, it provides a very good estimate and starting point. Fine-tuning based on engine performance and spark plug reading is usually necessary.

What if my engine has modifications?

Establish a new baseline jet size for your modified engine at a known altitude and temperature first, then use the carburetor jet size calculator for adjustments from that new baseline.

How do I know if my jetting is correct after using the calculator?

Check spark plug color after running at the throttle opening corresponding to the main jet (usually 3/4 to full throttle). A light tan/brown color is ideal. Also, listen to the engine and feel its response.

Can I use this carburetor jet size calculator for pilot jets or needles?

This calculator is primarily for the main jet. While the same principles apply (less fuel at high altitude/temp), the pilot jet and needle position affect different throttle ranges and have different adjustment mechanisms or sizes.

What are common jet size increments?

Main jets often come in increments of 2, 2.5, or 5 (e.g., 150, 152, 155). Choose the closest available size to the calculated value.

Related Tools and Internal Resources

• Carburetor Tuning Guide: A comprehensive guide to tuning your carburetor, including pilot jet, needle, and main jet adjustments.
• How Altitude Affects Engine Performance: An article explaining the science behind power loss at altitude.
• Engine Tuning for Temperature Changes: Learn how to adjust your engine for optimal performance in varying temperatures.
• 2-Stroke vs 4-Stroke Tuning Differences: Understand the nuances of tuning different engine types.
• Mikuni vs. Keihin Carburetors: A comparison of two popular carburetor brands.
• Reading Spark Plugs for Jetting: Learn how to diagnose your air-fuel mixture by inspecting your spark plugs.