Carburetor Jetting Calculator

Carburetor Jetting Calculator

Adjust your carburetor’s main jet size based on changes in air temperature and altitude to maintain optimal engine performance.

Jet Size vs. New Temperature/Altitude

Chart showing recommended jet size at different new temperatures (blue) and altitudes (green), keeping other factors constant.

Correction Factor Examples

Temp Change (°F)	Altitude Change (ft)	Approx. Correction Factor	Recommended Jet (if 150)
-20	0
+20	0
0	+3000
0	-3000
+20	+3000

Table showing approximate correction factors and resulting jet sizes for common temperature and altitude changes from a baseline (e.g., 68°F, 0 ft, jet 150).

What is Carburetor Jetting (and a Carburetor Jetting Calculator)?

Carburetor jetting refers to the process of selecting and installing the correct size fuel and air jets in a carburetor to achieve the optimal air-fuel mixture for an engine under specific operating conditions. The “jets” are small, precisely drilled orifices that control the flow of fuel or air. A carburetor jetting calculator is a tool designed to help you estimate the correct jet size when ambient conditions like air temperature or altitude change.

Proper jetting is crucial for engine performance, fuel efficiency, and engine longevity. If the mixture is too rich (too much fuel), the engine may run rough, foul spark plugs, and waste fuel. If it’s too lean (too little fuel), the engine can overheat, detonate, and suffer serious damage.

Anyone who operates a carbureted engine (motorcycles, ATVs, older cars, small engines) and experiences changes in altitude or significant temperature swings should understand and potentially adjust jetting. A carburetor jetting calculator is particularly useful for racers, riders who travel to different elevations, or those tuning an engine after modifications.

Common misconceptions include thinking that jetting is a “set and forget” adjustment or that the factory jetting is always optimal everywhere. In reality, jetting needs to be checked and potentially adjusted for significant changes in air density.

Carburetor Jetting Formula and Mathematical Explanation

The core principle behind jetting adjustments is maintaining the correct air-fuel ratio by mass. As air density changes (due to temperature and altitude/pressure), the mass of air entering the engine changes, even if the volume is the same. To compensate, the mass of fuel delivered by the jets needs to be adjusted proportionally.

The flow rate through a jet is related to the square root of the pressure difference, but for practical purposes related to air density changes, the required jet area (and thus diameter, or size) is often adjusted proportionally to the square root of the air density ratio:

New Jet Size ≈ Old Jet Size × √(New Air Density / Old Air Density)

Air density (ρ) is proportional to Pressure (P) and inversely proportional to Absolute Temperature (T): ρ ∝ P/T.

So, Air Density Ratio = (P_new / T_new_abs) / (P_old / T_old_abs) = (P_new / P_old) × (T_old_abs / T_new_abs)

Where:

• P_new and P_old are the air pressures at the new and old altitudes.
• T_new_abs and T_old_abs are the absolute air temperatures (Kelvin or Rankine) at the new and old conditions.

The pressure at a given altitude can be estimated using the barometric formula. A simplified form for pressure ratio based on altitude in feet (h) relative to sea level is (1 – 6.875e-6 × h)^5.25588 compared to sea level pressure.

The carburetor jetting calculator uses these relationships to estimate the new jet size.

Variables Used

Variable	Meaning	Unit	Typical Range
Old Jet Size	Size of the current main jet	–	50 – 500+
Old/New Temp	Air temperature	°F or °C	-20 to 120°F (-29 to 49°C)
T_abs	Absolute Temperature	K or R	244 to 322 K
Old/New Altitude	Altitude above sea level	feet or meters	0 to 14000 ft (0 to 4300 m)
P	Air Pressure	atm, Pa, etc.	0.6 to 1 atm
New Jet Size	Recommended new main jet size	–	Calculated

Table explaining the variables involved in the carburetor jetting calculation.

Practical Examples (Real-World Use Cases)

Example 1: Riding to Higher Altitude

You have a motorcycle tuned with a 175 main jet at sea level (0 ft) and 70°F. You plan to ride in the mountains at 6000 ft where the temperature is expected to be 50°F.

• Current Jet: 175
• Current Temp: 70°F
• Current Altitude: 0 ft
• New Temp: 50°F
• New Altitude: 6000 ft

Using the carburetor jetting calculator, you’d find the air is less dense at 6000 ft and 50°F. The calculator would recommend a smaller main jet, perhaps around 160-165, because less fuel is needed to match the less dense air.

Example 2: Seasonal Temperature Change

Your ATV runs perfectly with a 120 main jet in the summer at 90°F near sea level (500 ft). You want to ride in the winter when the temperature drops to 30°F at the same altitude.

• Current Jet: 120
• Current Temp: 90°F
• Current Altitude: 500 ft
• New Temp: 30°F
• New Altitude: 500 ft

The colder air is denser. The carburetor jetting calculator will suggest a larger main jet, maybe around 125-130, to provide more fuel for the denser air charge.

How to Use This Carburetor Jetting Calculator

1. Enter Current Jet Size: Input the size of the main jet currently installed and known to work well under the “Current” conditions.
2. Enter Current Conditions: Input the air temperature and altitude where your current jetting is correct. Select the correct units (°F/°C, ft/m).
3. Enter New Conditions: Input the new air temperature and altitude for which you want to calculate the jet size. Select the units.
4. Calculate: Click “Calculate” or observe the real-time update.
5. Read Results: The “Recommended New Main Jet Size” is the primary result. Intermediate values show the correction factors.
6. Interpret: Jet sizes come in steps (e.g., 150, 152.5, 155). Choose the available jet size closest to the calculated value. Fine-tuning (checking spark plug color, engine response) is always recommended after a jet change based on a carburetor jetting calculator.
7. Chart & Table: The chart and table provide visual aids to understand how jetting changes with conditions.

Key Factors That Affect Carburetor Jetting Results

• Air Temperature: Colder air is denser, requiring more fuel (larger jet). Hotter air is less dense, requiring less fuel (smaller jet).
• Altitude: Higher altitude means lower air pressure and less dense air, requiring less fuel (smaller jet). Lower altitude means denser air, requiring more fuel (larger jet).
• Humidity: Higher humidity displaces some air with water vapor, making the air effectively less dense in terms of oxygen content. This might require leaning the mixture slightly (smaller jet), although our calculator focuses on temp and altitude.
• Fuel Type: Different fuels (e.g., gasoline with ethanol, race gas) have different stoichiometric ratios and densities, potentially requiring jetting adjustments.
• Engine Modifications: Changes like high-flow air filters, performance exhausts, or engine internal work often require significant re-jetting, beyond what a simple air density carburetor jetting calculator predicts.
• Carburetor Type and Condition: Different carburetor designs (slide, CV) and their condition (wear, cleanliness) can influence jetting requirements.

Frequently Asked Questions (FAQ)

What is the main jet in a carburetor?

The main jet primarily controls the fuel mixture from about 3/4 to full throttle.

Does this calculator work for the pilot jet or needle?

This calculator is primarily for the main jet based on air density changes. Pilot jet and needle adjustments are also affected but have different ranges of operation and might require different correction factors or methods.

How accurate is a carburetor jetting calculator?

It provides a good starting point based on air density changes. However, it’s an estimation. Always perform final tuning by checking spark plug color, engine response, and feel.

Why does my engine run worse at high altitude?

At high altitude, the air is less dense. If the jetting isn’t leaned out (smaller jets), the engine will run too rich, leading to poor performance, bogging, and fouled plugs.

Why does my engine run worse in cold weather?

Cold air is denser. If the jetting isn’t richened (larger jets), the engine may run too lean, which can cause hesitation, backfiring, and potentially engine damage from overheating.

How do I know if my jetting is rich or lean?

Rich mixture signs: black sooty spark plug, bogging on acceleration, poor fuel economy, black smoke. Lean mixture signs: white or very light tan spark plug, engine runs hot, surging, backfiring on deceleration, hesitation.

What if the calculated jet size is between available sizes?

Generally, it’s safer to err on the slightly richer side (larger jet) initially, then fine-tune. For example, if the carburetor jetting calculator suggests 153.5, and you have 152.5 and 155, start with 155 or try 152.5 and check carefully.

Do 2-stroke and 4-stroke engines require different jetting approaches?

The basic principles of air density correction apply to both, but 2-strokes can be more sensitive to jetting changes and running lean can be more damaging quickly. The target air-fuel ratio might also differ slightly.