Jetting for Altitude & Temperature
The main jet that keeps your mixture right when the air thins — altitude and heat together, with a grid across both.
Where it is jetted now
Where you are riding
Your jet across altitude and temperature
Every cell is the main jet that keeps the same mixture as your current setup, rounded to the nearest even size. Your own conditions are highlighted.
Why the correction is linear
Air density is pressure divided by absolute temperature. Altitude thins the air through the pressure term and heat thins it through the temperature term, and the two multiply — a hot afternoon at altitude is the thinnest air an engine will ever meet, and the two effects do not partly cancel the way people sometimes hope.
A main jet number is essentially a flow rating, and the fuel you need is proportional to the mass of air the engine swallows. So the jet scales linearly with density: 17% less air wants 17% less fuel. Reference conditions here are sea level at 68°F, which is what a factory spec normally assumes.
Which mistake is expensive
Jetted for sea level and ridden at altitude, the engine runs rich — it fouls plugs, feels soggy and blubbers, and it is annoying rather than dangerous. The reverse is the one that costs money: jetted lean for thin air and then ridden in dense air at sea level, the engine runs hot and lean at full throttle, which is how pistons get holed. If you are going to be wrong, be wrong rich.
Things worth knowing
- Altitude and heat push the same way. Both thin the air, so a hot afternoon at altitude is the leanest condition an engine ever sees — and the one that holes pistons.
- Lean runs hot. An engine jetted for sea level and ridden at 8,000 feet is running rich and will foul plugs and feel soggy; the reverse — jetted for altitude and ridden at sea level — is the one that does damage.
- Main jet governs roughly the top quarter throttle. The needle handles the middle, and the pilot jet and air screw handle idle to about an eighth — a plan that only changes the main fixes only part of the range.
- Change one thing at a time and ride the same loop between changes. Two changes at once means you learn nothing when it improves.
- The plug tells you the truth. Read it after a hard pull with the engine cut before you close the throttle, not after idling back to the pits, which colours it uselessly.
- Humidity leans the mixture slightly, since water vapour displaces air. It is a small effect next to altitude and temperature, and it is not in these numbers.
This is a starting point, not a tune. Every engine, pipe and filter combination behaves a little differently, and the plug and the way it runs are the authority — not the arithmetic.
How to use
- Enter the main jet you run now and the altitude and temperature it was set at.
- Enter where you are actually riding.
- Read the suggested jet, the needle clip shift, and the grid across conditions.
Frequently asked questions
How much does altitude affect jetting?
Roughly three to four per cent of air density per thousand feet at first. At 5,000 feet you have 83 per cent of sea-level density, so a 180 main becomes about a 150 to keep the same mixture.
Does temperature matter as much as altitude?
Not as much, but it works the same way and the two multiply rather than partly cancelling. Going from 68°F to 95°F costs about five per cent of density on its own, and a hot afternoon at altitude is the thinnest air an engine ever sees.
Is the jet correction linear or a square root?
Linear. A main jet number is essentially a flow rating, and the fuel required is proportional to the mass of air the engine swallows — so 17 per cent less air wants 17 per cent less fuel.
Which way is the dangerous mistake?
Running lean. Jetted for sea level and ridden at altitude the engine is rich — it fouls plugs and feels soggy, which is annoying. Jetted for thin air and then ridden at sea level it runs hot and lean at full throttle, which is how pistons get holed.
Does the main jet fix everything?
No. The main governs roughly the top quarter of throttle. The needle handles the middle and the pilot jet and air screw handle idle to about an eighth, so a change that only touches the main fixes only part of the range.
Does humidity matter?
A little — water vapour displaces air, so humid air is slightly less dense and leans the mixture. It is a small effect next to altitude and temperature and it is not included in these numbers.
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