2-Stroke Jetting Calculator

How the 2-stroke jetting calculator works

A carburettor meters fuel by volume, but an engine makes power from the mass of air it breathes. When the air gets thinner — hotter, higher, or more humid — the same jets deliver too much fuel for the available oxygen and the bike runs rich. When the air gets denser — colder, lower, drier — the mixture goes lean. This calculator estimates the air-density change between the conditions your bike is jetted for now and the conditions you are about to ride in, then tells you whether to go richer or leaner and by roughly how many main and pilot jet sizes.

It works out air density from temperature (in Kelvin), barometric pressure derived from altitude, and an optional humidity correction, then scales your current main and pilot jets by the density ratio. The result is a sensible starting point you can confirm at the track — not a guaranteed final number.

Rich or lean — which way do you go?

• Hotter or higher than before: air is thinner, the mixture goes rich, so you jet leaner (smaller numbers). A bike jetted for a cold coastal morning will blubber and foul plugs at a hot mountain track.
• Colder or lower than before: air is denser, the mixture goes lean, so you jet richer (bigger numbers). Lean is the dangerous direction on an air-cooled or hard-ridden 2-stroke — it raises combustion temperature and is the classic cause of a cold seizure.
• Humidity: water vapour displaces oxygen, so very humid air behaves a little like thinner air and pushes the mixture slightly rich. It is a small effect next to temperature and altitude.

Main jet vs pilot jet — what each one controls

• Main jet: dominates from about half throttle to wide open. It is the jet most affected by air density, so it is the first thing to change for a big altitude or temperature swing.
• Pilot (slow) jet: controls idle to roughly a quarter throttle and the off-idle response. It changes in coarser steps and usually only needs a move on larger density changes. The fuel screw fine-tunes around it.
• Needle: the clip position handles the midrange between the pilot and main circuits. This calculator focuses on the jets; raise or lower the needle clip if the midrange is off after you set the jets.

Confirm it at the track

Treat the suggested jets as a baseline, then read the bike: a fat, four-stroking blubber high in the revs and a sooty black plug point to rich; a flat, hanging, hard-to-clear note, a pale or grey plug, and a bike that pings or runs hot point to lean. Make one change at a time, ride, and re-read. Jet types and circuits vary between Keihin, Mikuni and TM carbs, so always cross-check with your bike's jetting chart and a fresh plug.