Carburetor setup and atmospheric correction
Carburetor Jetting Calculator
Translate a known baseline main and pilot jet to new altitude, temperature, humidity, and pressure conditions using dry-air density—then compare exact, nearest, higher, and lower available jet numbers without hiding the assumptions.
Calculated output
EstimatedJetting correction summary
Condition Preview
Hover pointsAir-density comparison
Focus or point to a chart marker for atmospheric details.
Calculated Dimensions
relative scaleEquivalent metering-area change
Audit the estimate
no hidden constantsAtmosphere and jet breakdown
| Item | Baseline | Target / result |
|---|---|---|
| Atmospheric pressure | — | — |
| Pressure source | — | — |
| Saturation vapor pressure | — | — |
| Actual water-vapor pressure | — | — |
| Dry-air partial pressure | — | — |
| Dry-air density | — | — |
| Total moist-air density | — | — |
| Fuel-demand ratio | Known baseline = 1.0000 | — |
| Equivalent diameter ratio | Known baseline = 1.0000 | — |
| Selected number model | — | — |
| Main available series | — | — |
| Pilot available series | — | — |
Calculation method
Atmospheric and jet-number formulas
The calculator corrects a known baseline rather than pretending engine displacement alone can determine a carburetor jet. Water vapor displaces dry, oxygen-bearing air, so the correction uses dry-air density rather than total moist-air density.
1. Standard pressure
P = 101325 × (1 − 2.25577×10⁻⁵ × altitude)⁵·²⁵⁵⁸⁸
This troposphere approximation is used only when measured station pressure is zero. A valid local station-pressure input overrides it.
2. Water-vapor pressure
Pv = RH × 610.94 × exp(17.625T ÷ (T + 243.04))
Relative humidity is applied as a fraction. The saturation approximation uses degrees Celsius and returns pascals.
3. Dry-air density
ρdry = (P − Pv) ÷ (287.058 × Tkelvin)
The dry component controls the oxygen-bearing air-mass comparison. Moist density additionally includes vapor density using its own gas constant.
4. Fuel-demand ratio
F = ρdry target ÷ ρdry baseline
This is a first-order steady-state air-mass correction. It does not model carb pressure depression, fuel viscosity, atomization, signal strength, or engine volumetric efficiency.
5. Flow-index model
target jet number = baseline number × F
Use only when numbers are reasonably proportional to rated fuel flow within the same genuine jet family.
6. Diameter-code model
target jet number = baseline number × √F
Use only when numbers are proportional to metering diameter. Required area changes by F, so equivalent diameter changes by its square root.
Tuning and safety
Carburetor jetting FAQ
Can this calculate jetting from engine size alone?
No. Absolute jetting depends on carburetor family and size, needle jet, needle, slide, air correction, fuel, intake, exhaust, porting, compression, ignition, load, and engine condition. The tool only corrects a known physical baseline between atmospheres.
Which jet-number model should I select?
Use the manufacturer’s documentation for the exact genuine jet family. Some numbers approximate rated flow, some resemble bore diameter, and others are calibration identifiers. Do not mix Mikuni, Keihin, Dell’Orto, pattern, drilled, or unknown jets by number.
Why does humidity reduce the correction basis?
Water vapor replaces part of the dry air at the same total pressure. Because oxygen is carried by the dry-air component, increased vapor pressure reduces oxygen-bearing air mass even though total moist density changes differently.
Should weather-app barometric pressure be entered?
Only if it is local station pressure. Many weather services report pressure corrected to sea level; combining that value with actual altitude would be misleading. Leave pressure at zero when uncertain so altitude-derived standard pressure is used consistently.
Is the nearest calculated main jet automatically safe?
No. A lower-number result can create a damaging lean condition if any assumption is wrong. Begin from a known conservative rich setup and verify under controlled load using the carburetor and engine manufacturer’s tuning procedure.
Why is pilot-jet correction labelled first-order?
The pilot circuit also depends strongly on air screw or fuel screw position, bypass holes, slide cutaway, needle straight diameter, throttle opening, and engine signal. Atmospheric scaling is only a starting comparison.
Can the calculator choose a needle clip position?
No. Clip direction and effect depend on needle orientation and carburetor design, while needle taper and needle-jet interaction are not represented by one atmospheric factor. Tune each throttle range separately.
What must remain unchanged from the baseline?
Carburetor and genuine jet family, fuel and oil ratio, float height, fuel level, reeds or intake, filter, exhaust, porting, compression, ignition, cooling, engine condition, and test method should remain equivalent for a meaningful atmospheric correction.
How should target jetting be verified?
Follow the manufacturer’s staged procedure, beginning rich and changing one circuit at a time. Use controlled plug chops or appropriate instrumentation, inspect piston crown and plug evidence correctly, monitor temperature or detonation where applicable, and stop immediately when symptoms are unsafe.
Is this a tuning instruction or warranty of engine safety?
No. It is a transparent mathematical comparison. Incorrect jetting can seize or damage an engine and can cause loss of control. Final setup and testing remain the responsibility of a competent tuner using the exact machine.