Dirt Bike Jetting Calculator

Dirt Bike Jetting Calculator

A dirt bike jetting calculator typically uses formulas based on altitude, temperature, humidity, and other environmental factors to adjust the main jet, pilot jet, needle position, and air screw settings. While jetting calculators are often software-based, the fundamental formulas involve corrections to air density and fuel mixture.

Basic Jetting Formula

A common jetting correction formula is based on air density changes: New Jet Size=Current Jet Size×(New Air DensityOld Air Density)\text{New Jet Size} = \text{Current Jet Size} \times \left( \frac{\text{New Air Density}}{\text{Old Air Density}} \right)New Jet Size=Current Jet Size×(Old Air DensityNew Air Density​)

Where air density is calculated as: Air Density=PR×T\text{Air Density} = \frac{P}{R \times T}Air Density=R×TP​

  • PPP = Atmospheric Pressure (hPa)
  • RRR = Specific Gas Constant for Air (287.05 J/kg·K)
  • TTT = Absolute Temperature (Kelvin)

Temperature Correction for Jetting

A simple rule of thumb for temperature adjustment: New Main Jet=Current Main Jet×(1+ΔT100×0.015)\text{New Main Jet} = \text{Current Main Jet} \times \left( 1 + \frac{\Delta T}{100} \times 0.015 \right)New Main Jet=Current Main Jet×(1+100ΔT​×0.015)

Where:

  • ΔT\Delta TΔT = Temperature change in °C (from the baseline tuning temperature)

Altitude Correction

A common correction factor for altitude: New Main Jet=Current Main Jet×(1−Altitude (ft)1000×0.03)\text{New Main Jet} = \text{Current Main Jet} \times \left( 1 – \frac{\text{Altitude (ft)}}{1000} \times 0.03 \right)New Main Jet=Current Main Jet×(1−1000Altitude (ft)​×0.03)

For every 1,000 feet (300 meters) of elevation gain, you reduce the main jet size by 2-3%.

Other Adjustments

  • Pilot Jet & Air Screw: At higher altitudes or warmer temperatures, you may need to lean out the pilot circuit (smaller pilot jet or open air screw slightly).
  • Needle Position: Raising the clip on the needle (lowering the needle) leans out the midrange, useful for high-altitude tuning.

How It Works:

  • The user enters the altitude, temperature, and current jet size.
  • When the “Calculate New Jet Size” button is clicked, the JavaScript function calculates the new jet size using the formulas and displays it.