Dirt Bike Jetting Calculator

How The Jetting Calculator Works

A carburetor meters fuel by the speed of the air going past the slide, not by its mass. When the air gets thinner, from elevation, heat or humidity, the same jets still pull nearly the same fuel but there is less oxygen to burn it. The mixture goes rich. The usual engineering relation, from Heywood's Internal Combustion Engine Fundamentals (chapter 7), is that a carburetor's fuel to air ratio scales with about one over the square root of air density.

The calculator compares the air where you are riding with the air your bike was jetted for. If your air is 10 percent thinner than the baseline, the carb runs about 5 percent rich, and you jet leaner. If your air is denser, for example a cold morning lower down, it runs lean and you jet richer.

It uses four steps.

1. Pressure from elevation. p = 101,325 × (1 − 2.25577×10⁻⁵ × h)^5.25588 pascals, with h in meters. This is the troposphere formula of the U.S. Standard Atmosphere 1976.
2. Water vapor. Saturation vapor pressure from the Tetens equation, e_s = 610.78 × 10^{7.5T / (T + 237.3)} pascals with T in °C, times relative humidity.
3. Density. ρ = (p − p_v) / (287.058 × T_K) + p_v / (461.495 × T_K), the dry air plus water vapor parts with their gas constants.
4. Compare. Density ratio σ = ρ / 1.225 kg/m³. Density altitude is the standard-atmosphere elevation with the same σ, from σ = (1 − 2.25577×10⁻⁵ × h)^4.25588. Mixture change versus baseline is √(σ_baseline / σ) − 1.

As a check, standard air at 3,000 m (−4.5 °C, dry) comes out at 0.909 kg/m³, 74 percent of sea level, which matches the published standard atmosphere table.

What To Change, And In What Order

Match the change to where the bike misbehaves. On the carbureted KTM 250 SX, KTM gives the idling jet and air screw the most influence from closed to about 1/4 throttle, the needle position the most at 1/2, and the main jet from 3/4 to full. Mikuni's HS40 tuning manual draws the same picture: the needle from 1/4 to 3/4 and the main jet from about 3/4.

• Running rich (you went higher or hotter). A smaller main jet first. If the midrange is still soggy, lower the needle by moving the clip one groove toward the top. If the idle is lazy and the air screw does nothing, a smaller idling jet.
• Running lean (colder, denser air than your baseline). The reverse. Larger main jet, raise the needle, larger idling jet if it pings or hangs off idle.
• Within about 2 to 3 percent. Leave the jets. Reset the idle with the air screw, which KTM says to do after any big change in temperature or elevation.

Change one thing at a time and read a new plug after a short full-throttle run. KTM's rule is that a very light or white insulator, or knocking, needs a bigger main jet and a dark brown or sooty one needs a smaller main jet.

Example: KTM's Factory Jetting Chart

KTM printed a full chart for the carbureted 2022 250 SX with a Mikuni TMX 38. At 16 to 24 °C (61 to 75 °F) it steps the main jet from 450 near sea level to 440 at 751 to 1,500 m and 430 above 1,500 m, and above 2,300 m the needle goes from the 3rd to the 2nd clip position. Temperature matters as much: at 0 to 300 m the main runs from 470 below −7 °C to 430 above 37 °C.

This chart is for one bike and one carburetor. Jet numbers are not linear, and Mikuni and Keihin number jets differently, so use your own manual's chart as the starting point. The full chart with idling jet, needle and air screw is in our dirt bike jetting guide.

Honest Limits Of Any Jetting Calculator

• It uses standard-atmosphere pressure for your elevation. A passing storm front can move real pressure a few percent either way.
• The square-root rule describes the main circuit well. The idle circuit, two-stroke pipe tuning and load (deep sand, a big hill) all shift what a given engine wants.
• A calculator cannot see a dirty air filter, an air leak, a varnished pilot jet or a clogged spark arrestor, all of which feel like jetting problems.
• Fuel injected bikes measure air pressure and temperature themselves. KTM's 2024 two-stroke TBI system has dedicated ambient pressure and intake air temperature sensors. You do not jet them.

Keep A Jetting Log

Every time you change a jet, write down the elevation, temperature, humidity, the setting and how it ran. After a season you have a chart for your own bike. MotoMind's mod log keeps those notes per bike on your iPhone, next to service history tracked by engine hours.