Check the mixture after changing the air filter or airbox

The airbox and air filter on a modern bike are often restrictive on purpose. The carburettor jets are chosen with them in place. If the filter type or the airbox changes, or the filter is left off, the bike can run too rich or too lean. Some bikes do not notice a change, but others run badly without the box.

Safety

• Test on a quiet road. Hot exhaust and plug: wait for them to cool.
• Never run the bike without an air filter for more than a very short check.

Steps

1. Before changing anything, note how the engine runs now.
2. If a replacement filter or a home-made foam filter is fitted, make sure it seals and is oiled if it is a foam or fabric type.
3. Run the engine warm, then check idle and throttle response. A bike that barely runs with the filter off, but idles well with it on, was jetted for it.
4. Take a plug reading as in Read a spark plug and choose the jet by throttle position.
5. If the bike runs rich, a smaller jet may be needed. If it runs lean, a larger jet may be needed. Use sizes from your bike's manual.
6. A cracked airbox can cause the same trouble. A bike with a split airbox may miss and pop, and run well after the box is replaced.

Check your work

• Smooth idle, clean throttle response and a normal plug colour.

If it did not fix the problem

• Engine runs hot and lean.
• Engine runs rich.
• Runs rich or lean after an air filter or airbox change.