Carburettor jetting calculator

A carburettor meters fuel by air volume, but the engine needs fuel by air mass. In warm, humid air and at altitude the air is thinner and the mixture gets richer. The calculator works out the air density and suggests a new main jet size.

Result

Suggested main jet	(nearest common size )
Change	%
Air density	→ kg/m³
Density altitude	→ m
Needle	No needle change needed.

Results are calculated estimates. Always check your vehicle manufacturer's instructions.

Matching products: Nozzles

How to use

Enter your current main jet and the conditions the engine is set up well for (altitude, temperature, humidity).
Enter the conditions you are going to ride in.
Check the suggested jet and needle change. Change the jet one size at a time and check the result by plug colour or riding.

Frequently asked questions

Why do I need a smaller jet in warm weather?

Warm air is thinner – the same volume holds less oxygen, but the carburettor delivers the same amount of fuel. The mixture gets rich, so the jet is reduced. In cold weather it is the other way round.

How much does altitude matter?

Every 1000 m of altitude reduces air density by about 10%, which means a roughly 5% smaller main jet. In flat countries the effect is small, but it matters when riding in the mountains.

Is the result exact?

It is a calculated starting point. Confirm the final setup by plug colour, riding or on a dyno, because engines and carburettors differ.

What does the needle clip do?

The needle affects the mixture at mid throttle (about 1/4–3/4). Raising the clip to a higher groove lowers the needle and leans the mixture, lowering the clip makes it richer.

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