A carburettor meters fuel by the flow of air, not by its weight. High up, and on hot days, the air is thinner, so the same jets give a richer mixture. Low down, and on cold days, the air is denser, so the mixture goes weaker. A bike jetted on the coast can run rich, smoky and slow in the hills, and a bike jetted in the mountains can run weak and hot at sea level.
Fuel-injected bikes measure the air and correct themselves. This page is for carburettor bikes.
Safety
- Petrol spills when you open the float bowl. Tap off, engine cold, no flames.
- Going weaker is the risky direction. A mixture that is too weak can hole a piston, most of all on a two-stroke. When unsure, stay on the rich side.
The rule
- Higher altitude: smaller main jet.
- Hotter air: smaller main jet.
- Lower altitude or colder air: larger main jet.
- The change is in small steps. Use the chart or slide rule from your carburettor maker (the course links a Mikuni pocket tuner and the Amal tuning guide). Do not guess a size.
Steps
- Find the altitude and the usual air temperature where the bike was jetted, and where it will now be ridden.
- Read the size stamped on the main jet now fitted: Change the main jet.
- On the maker's chart, apply the temperature change first, then the altitude change, as the Mikuni tuner instructs. Each gives a plus or minus step from the jet you have.
- Fit the new main jet. Check the jet is the same type (thread and head shape) as the old one.
- If the change is large, the needle and pilot may need a matching change. The Amal guide warns that a weak or rich condition at half throttle is a needle job, not a main jet job: Set the jet needle clip.
- Reset the idle mixture with the engine warm: Set the idle screws.
- Ride and read the plug: Read a spark plug and choose the jet by throttle position.
Good to know
- Jet numbers are not the same scale on every make or jet style. On the Mikuni tuner, round head and hex head main jets use two different number series. Compare sizes only within the same jet type.
- A bike that is ridden up and down a big climb each day should be jetted for the middle, or for the lower end to stay safe from a weak mixture.
- Changing the air filter or exhaust changes jetting too. See Bike runs wrong after an air filter or exhaust change.
Check your work
- Plug colour light brown at the new altitude.
- No black smoke, and the engine pulls cleanly to full throttle.
If it did not fix the problem
- Check for a dirty air filter, which also makes the mixture rich: Service the air filter.
- Check the choke is fully off: Check the choke.
- Tune the whole range in order: Tune a slide carburettor in the right order.