Air filter

The air filter sits in the airbox, usually under the seat or behind a side cover, and cleans the air before it reaches the carburettor or throttle body. Elements are paper, foam (sponge) or oiled gauze. The K-Light 125 uses a foam element.

What it does

An engine draws in a large volume of air. Dust in that air acts like sandpaper on the cylinder, rings and valves. The filter traps it. As the filter fills, the engine gets less air, the mixture goes rich, and power drops. A torn or badly seated filter lets dirt straight in.

Signs it is worn or failing

• Black smoke and runs rich.
• Low power and high fuel use.
• Black, sooty plug.
• Dust inside the intake past the filter, or a soaked filter after rain or washing.

When to replace it

K-Light 125: clean every 3000 km and replace every 12000 km, more often on dusty or wet roads. Replace any element that is torn, hard, swollen or has a loose seal. Paper elements cannot be washed.

What to match when buying

• Match make, model and year. Fit and the sealing edge matter more than the material.
• Foam elements must be the right thickness and density; oil them with filter oil if the maker says so.
• Fakes and poor copies often have thin paper, glue gaps or a seal that does not fill the airbox. Hold a paper element to the light: you should see an even, dense pleat with no pinholes.

Fitting notes

• Rider job: Clean or replace the air filter.
• Never run the engine with the filter out. Do not wash it in petrol.
• Seat the edge all round and check the airbox clips. See Check air cooling if the engine runs hot.

Reference (K-Light 125)