🌬️Air filter

How it works

The air filter is the engine's first line of defense on the intake side. All the air the engine breathes passes through it before reaching the carburetor or injectors: it traps dust, sand, insects, and abrasive particles that would otherwise prematurely wear the cylinders, rings, and valves. But its job isn't limited to filtration: it also plays a part in mixture metering, since the engine runs on a precise balance between air and fuel (on the order of 14.7 parts air to 1 part fuel by mass on a 4-stroke gasoline engine). If the filter clogs, it lets less air through: the mixture richens, power drops, consumption rises, the spark plug fouls, and black smoke can appear. There are three main families: the pleated dry paper filter (disposable, for replacement), the oiled foam filter (washable and re-oilable, common in off-road use and on DNA filters), and the oiled cotton-gauze panel filter on a frame (washable, K&N-type), known for being more free-flowing. Each has its own maintenance routine. Failure modes: gradual clogging (dust, insects), water or oil saturation on foam filters, a torn filter medium (which then lets abrasives straight through into the engine - very damaging), and over-oiling on washable filters, which chokes the intake and can foul an airflow sensor on an injected engine. Signs of a tired filter: loss of pep especially at high revs, excess fuel consumption, black smoke, a temperamental idle. It's one of the simplest, cheapest, and most worthwhile maintenance jobs on a motorcycle, and the interval depends heavily on the environment (dust, rain, off-road use).

Maintenance tips

• ✓Visually check the filter at every service, and more often in dusty, wet, or off-road conditions. A healthy filter lets daylight through; a grey-black, clogged filter needs cleaning or replacing. Check the exact interval in your manual.
• ✓Paper filter: replace it, never wash it and never blast it aggressively with air, or you'll destroy the filter medium. Use the original reference or an equivalent.
• ✓Foam or oiled cotton filter: wash it with the dedicated product (or soapy water for foam), let it dry completely in open air, then re-oil it evenly with the manufacturer's specific oil.
• ✓On washable filters, don't over-oil (excess chokes the intake and fouls any airflow sensor) or under-oil (the filter no longer traps anything). Aim for an even, lightly tinted coat of oil.
• ✓Never run the engine without the filter fitted, even for a few seconds: you'd be drawing abrasive dust straight into the cylinders.
• ✓Check the housing cover's seal and gasket condition after every service: a poorly sealed housing lets in unfiltered air that wears the engine.
• ✓Use the opportunity to inspect the intake duct: a forgotten rag, leaves, or a rodent's nest after winter storage can block it.