Jet Size Calculator
Enter the altitude and temperature where your engine runs well, the place you are going, the jet fitted and how the jets are numbered. The calculator works out the air density change and the new jet.
- New main jet
- Density change
- Direction
Change one step at a time and read the plug.
Air density ratio σ = (P ÷ P0) × (T0 ÷ T); diameter jets scale by √σ. A 150 jet set up at sea level and 15 °C, ridden at 2,000 m and 10 °C, sees air about 20% thinner. A diameter numbered jet goes down to about 135.
Jet by altitude chart
Diameter jets from 150 at sea level. Search for an altitude.
| Altitude | Air density | New jet |
|---|---|---|
| 0 m | 100% | 150 |
| 500 m | 95% | 146 |
| 1,000 m | 91% | 143 |
| 1,500 m | 86% | 139 |
| 2,000 m | 82% | 136 |
| 2,500 m | 78% | 133 |
| 3,000 m | 74% | 129 |
| 4,000 m | 67% | 123 |
Jet size.
Altitude and jet
Standard air, 150 jet. Slide the altitude.
Old and new jets side by side.
How to rejet for altitude
Air to jet. Step through it.
Base
Jet that runs well.
Going richer is safer than going leaner.
Leaner, same or richer
Jet changes. Compare them.
Smaller jet.
No change.
Bigger jet.
Hot, high air needs a leaner jet.
Before you rejet
Jet checks. Tick each item.
Tick each item as you go.
Check the needle and pilot jet too.
What you get
Jet sizing. Each tile explains one result.
New size.
Change.
Both.
Both.
Lean or rich.
Air is about 10% thinner each 1,000 m.
Frequently Asked Questions
No question matches that search.
How do I change jet size for altitude?
Scale the jet by the change in air density, smaller as you go higher.
How much should I jet down for altitude?
About one to two percent in jet size for every three hundred metres.
Does temperature affect jetting?
Yes. Hot air is thinner and needs a slightly leaner jet.
What happens if the jet is too lean?
The engine runs hot and can seize.
How do I read a spark plug?
Tan or light brown is right, white is lean and black is rich.
Do fuel injected engines need rejetting?
No. Their computer adjusts for air density.
Runs right up high
Rejet for the air you ride in and the engine pulls clean at any altitude.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.