Two-stroke jetting is the most air-density-sensitive tuning problem in motorsport. The same bike that was perfect at home can be dangerously lean — or hopelessly soggy — three hours up the road.
Read this before you touch anything. On a two-stroke, lean is what melts pistons. Rich costs you a fouled plug and ten minutes; lean costs you a top end and possibly a crank. When you are not sure, stay one size fat. Nothing on this page is worth a seizure.
Riders talk about altitude, but a carburettor cannot measure altitude. It responds to air density — how much oxygen is actually in a given volume of air — and density is set by three things: pressure, temperature, and humidity.
That is why a cold morning at sea level and a hot afternoon at sea level are not the same jetting problem. Hot air is thinner. Humid air is thinner still, because water vapour is lighter than the nitrogen and oxygen it displaces. A muggy 95°F day at the beach can be worth thousands of feet of altitude to your engine.
Density altitude is the single number that folds all three together: the altitude at which standard atmosphere would have the density you are actually breathing. It is what you should be jetting against. There is a fuller explanation here.
This is where people get hurt, so here it is in one line:
Thinner air → less oxygen → the engine needs less fuel → smaller main jet.
Thicker air → more oxygen → more fuel → bigger main jet.
Thinner air means higher density altitude — a hot, humid, high day. So as the density-altitude number goes up, your jet numbers come down.
Fuel flow through a fixed orifice scales with the square root of the pressure drop across it, while the signal pulling that fuel through — the depression in the venturi — scales with air density. Work it through and you get:
New main ≈ old main × √(today's density ÷ the density it was jetted in)
The practical consequence: roughly one jet number per 350 feet of density altitude, or about three numbers per thousand feet. Because of the square root it flattens out slightly as you climb, but across the range anyone actually rides, the rule of thumb holds.
A bike known to be right on a 175 main at 1,500 ft density altitude, on a Keihin, snapped to jet sizes that actually exist:
| Density altitude | Main jet | Change |
|---|---|---|
| 0 ft (thick, cold, sea level) | 178 | Richer |
| 1,500 ft | 175 | Baseline |
| 3,000 ft | 172 | Leaner |
| 5,000 ft | 165 | Leaner |
| 7,000 ft | 162 | Leaner |
| 9,000 ft | 155 | Leaner |
These match published jetting charts, which is the sanity check that matters — the physics and the paper agree.
Tell MXDialed the main jet you know is right and the density altitude it was right at. It pulls live observed conditions for your track and gives you today's main, pilot, and clip — snapped to real Keihin or Mikuni sizes.
Open the jetting calculator →Free, no signup. Live air from real station observations — no key, no AI.
The main jet owns wide-open throttle. The needle owns the midrange, which is where you actually spend most of a moto.
Clip positions are counted from the TOP groove. Position 1 is the leanest.
Moving the clip up (toward position 1) drops the needle further into the jet, restricting fuel — leaner.
Moving the clip down lifts the needle out, letting more fuel through — richer.
So thinner air means a lower position number. This trips up a lot of riders, because "clip up" intuitively sounds like "more", and it is the opposite.
Rule of thumb: about one clip position per 4% change in air density.
A needle clip has authority over roughly two positions in either direction. Beyond that you are not fixing the mixture, you are distorting the whole midrange to chase one part of it.
If the air has moved far enough to want more than two positions, the honest answer is that the needle taper itself is wrong for those conditions, and you need a different needle rather than a heroic clip position. A tool that just keeps counting grooves is lying to you.
The pilot controls idle and the first crack of throttle. It is far less sensitive to air density than the main, and it is generally not worth touching for small swings.
A reasonable threshold is around 7% density change before the pilot is worth re-sizing. Below that, leave it alone — you will chase a hard-starting bike for no gain.
Calculations get you close. The plug tells you the truth.
After a hard third-gear pull, kill the engine and coast to a stop — do not idle back to the pits, or you will read the pilot circuit instead of the main.
| Plug reads | Verdict | Do this |
|---|---|---|
| Tan to light brown | Correct | Nothing. Write it down. |
| Chalky white or blistered | Lean — dangerous | Stop riding. Go richer before the next pull. |
| Wet, black, sooty | Rich | Go one size down and re-read. |
A blistered or chalky-white plug is not a tuning observation, it is a warning. Do not take one more lap on it.
Different problem entirely. Your ECU reads air pressure and intake temperature and already trims fuel for density — you are not going to fix thin air with a fuel map, and you do not need to.
What altitude actually changes for an EFI bike is how much power is on the table, and therefore:
Every calculation on this page is relative. It needs one anchor: a jetting setup you know was right, and the air it was right in.
Most riders have the first half and none of the second. They remember the 175 main; they have no idea it was a 55°F morning at 900 feet. Without the air, the jet number is just trivia.
Next time the bike is genuinely dialled, record the main, the pilot, the clip position, and the density altitude together. That single line makes every future session arithmetic instead of guesswork.
Leaner. Thinner air holds less oxygen, so the engine needs less fuel to keep the same ratio — a smaller main jet as density altitude rises.
About three jet numbers, or one number per 350 feet of density altitude. It follows a square root rather than a straight line, so it flattens slightly as you go up, but the rule of thumb is sound across normal riding altitudes.
Up, toward the top groove — positions are counted from the top and position 1 is leanest. Raising the clip drops the needle into the jet and leans the midrange.
No. They control different parts of the throttle range — the needle owns the midrange, the main owns wide open. A big density change needs both, and the clip only has about two positions of real authority before the needle taper itself is the problem.
Yes. Water vapour is lighter than the air it displaces, so humid air is genuinely less dense. A hot humid day can be worth thousands of feet of density altitude compared to a cool dry one at the same place.
Start from a known-good setting on a day you can measure, and treat that as your new anchor. Guessing the baseline just moves the guesswork one step back — and err rich while you establish it.