How To Clean And Oil A Dirt Bike Air Filter

How A Foam Air Filter Actually Stops Dust

A dirt bike filter is open-cell foam soaked in tacky oil. The foam is a deep maze, and the oil film on the cell walls is what grabs fine dust as the air winds through. Dry foam is only a coarse screen. That is why KTM has you oil the filter every time you clean it, and why the dirty outside of a used filter is a sign it is working.

The other half of the job is the seal. Every bit of air has to go through foam. KTM greases the filter's sealing area and locks it in place, and warns that a filter that is not mounted correctly lets dust and dirt into the engine. A cracked intake flange behind the filter bypasses it completely, so the flange gets checked every time too.

Fine dust is the enemy. A Machinery Lubrication article on oil analysis explains that particles about the size of the oil film between moving parts do the most damage, and that dust which gets past an air cleaner ends up between the rings, piston and cylinder.

Service The Filter Step By Step

1. Lift out the filter and plug the intake. Remove the filter and its cage straight up so loose dirt stays on the outside, then cover or plug the intake so nothing falls into the boot.
2. Wash it in foam filter cleaner. Take the foam off the cage and wash it thoroughly in filter cleaning liquid. Do not use fuel or petroleum solvents; KTM warns they attack the foam.
3. Press it out, never wring it. Squeeze the cleaner out with flat hands. Wringing tears the foam and its glued seams.
4. Let it dry completely. Leave the foam until it is fully dry, so the oil can soak in evenly.
5. Oil it evenly. Work high-grade foam filter oil into the dry foam until every cell is coated. Twin Air recommends oiling at least 24 hours before riding.
6. Refit the cage and grease the sealing rim. Mount the foam on its cage the right way up, then grease the sealing area with long-life grease.
7. Clean the airbox and install the filter. Clean the airbox and the intake flange, check the flange for damage, remove the plug, then seat the filter and lock it so it cannot leave a gap.

In the figure the window is a section through the foam wall and the rim where it sits on the airbox seat. The moving dots are an illustration of the mechanism, not test data. With the foam oiled and the rim greased they stop in the outer cells. Break the foam and many slip through; break the rim and some go under it, straight to the engine.

Mistakes That Let Dirt In

• Gasoline or solvent. KTM says fuel and petroleum attack the foam. Use a foam filter cleaner.
• Wringing. Twisting tears the foam and the glued seams. A torn seam is an open hole.
• Oiling wet foam. The oil cannot spread through foam that still holds cleaner or water.
• A dry or crooked rim. No grease, or a filter that is not on its pin and tab, leaves a path around the foam.
• Washing toward the airbox. Honda warns not to spray water at the air cleaner when you wash the bike.

How Often To Clean It

KTM's 2024 SX-F and SX motocross race schedules clean the air filter and airbox every 15 operating hours, and its 2026 XC-W enduro manual says the same 15 hours, so this job does not get longer on KTM's enduro bikes (PDF p. 55). KTM says to check it about every 30 minutes in sand, mud and dusty heat. Honda's CRF250R and CRF450R race schedules service it every race, about 2.5 hours, and after every moto when it is dusty. Honda's CRF450X, on its non-competition trail schedule, cleans it every 600 miles (1,000 km) or 6 months and more often when wet or dusty (PDF p. 45 and 47). That is a big spread, so start from the schedule for your own bike and let conditions decide. Many riders keep two or three oiled filters in sealed bags and swap them. Our air filter cleaning guide covers products, oil amounts and storage in more detail.

FAQ

Can I clean a dirt bike air filter with gasoline?

No. KTM's manual says fuel and petroleum solvents attack the foam. Use a dedicated foam filter cleaner, then oil the filter once it is dry.

Why do you grease the rim of a dirt bike air filter?

The grease fills the small gaps between the filter's sealing rim and the airbox seat, so air cannot go around the foam. KTM greases the sealing area every time the filter goes back in.

How long should a dirt bike air filter dry before oiling?

Until it is completely dry, so the oil can soak in evenly. After oiling, Twin Air recommends waiting at least 24 hours before riding so the oil spreads through the foam.

How often should I clean my dirt bike air filter?

KTM lists every 15 operating hours on its 2024 SX-F and SX motocross race bikes and on its XC-W enduro bikes, with a check about every 30 minutes in sand or dust. Honda's CRF250R race schedule services it every race, about 2.5 hours, and its CRF450X non-competition trail schedule every 600 miles or 6 months. Dusty days call for more.

Does a dirty air filter mean it failed?

No. A dirty outer layer means it is catching dust. What matters is a clean inner side, a clean intake boot and an intact, greased seal.