A dirty air filter in your car reduces air flow into the engine, resulting in less power and often lower gas mileage. The typical filter is made of pleated paper. It may have an outer band of plastic foam or fiber that traps large particles. Inspect the filter every 6 months or every 12,000 miles.
In most cars the filter is in a housing at the top of the carburetor. Remove the wing nut or disconnect the clips holding the housing’s cover and lift the cover off.
In other cars the filter housing is connected to the air intake by a duct. Unscrew and remove the cover, disconnecting the duct from the housing if necessary to remove a nut beneath.
Examine the filter, tapping it against a wooden surface to dislodge loose dirt. Alight oily film on the filter is normal as a car ages, but if the filter is very oily, have a mechanic check the engine. If the filter is dirty all around, replace it. Most replacement filters are marked Top. If not, either side can go up.
If the old filter has a plastic foam band, carefully remove the band and immerse it in an automotive solvent such as the carburetor cleaner you pour into a fuel tank. Wring it gently, let it dry, then refit it.
Many air-filter housings also have a small filter for the positive crankcase ventilation (PCV) system. Change this filter if it is dirty.
In some cars, primarily older ones, the filter housing is sealed, and the entire unit must be replaced. The manufacturer advises you how often to change it, but the filter may plug beforehand, especially if you drive in dusty areas. If the car stalls or is hard to start, road-test it briefly, but only briefly, without the filter unit. If the problem then disappears, the sealed filter is probably plugged. Replace it.