A lawn mower air filter should be cleaned and replaced regularly. The round air filter on a Yard Machines 21" High Wheel Push Mower is one that can't be cleaned as well as some other filters, so you may find yourself replacing it sooner than you would for a different-style filter. A new filter can be purchased online or from a mower parts or hardware store (part #798452). Amazon.com sells it for $10.50 and Sears.com has it listed at $11.99.
To remove the old air filter, first locate the air filter cover. It is a black plastic cover, on the right side of the engine as you're looking at the engine from the front of the mower. (It's on the opposite side of the engine from the muffler.) This cover can be easily removed by popping the tab on the top of the cover. Once the tab is released, you can pull the cover outward, away from the engine. (Don't worry if it falls off.)
With the cover off, the air filter itself will be exposed, an oval-shaped filter with pleated paper inside the caging that goes around its sides. Pull this filter straight out, away from the engine. If you have a rag or paper towel, you can wipe down the housing where the air filter sits.
Install the new air filter into the seat that you removed the old filter from. (The two holes in the bottom of the filter should face the engine.) Once the filter is in place, reattach the cover. If it came completely off, slide the two tabs on the bottom of the cover into their slots, and then push the top of the cover closed until it snaps in and locks.
If you bought your mower new, it more than likely did not come with a pre-cleaner -- a foam sponge-like layer that wraps around the air filter. (If your mower does have one of these pre-cleaners, be sure to remove it from the old filter and install it on the new one.) You can purchase a pre-cleaner to help prolong the life of the air filter. The pre-cleaner can be cleaned regularly, and it allows less dirt into the air filter itself, making the filter last longer.
(Originally published on Helium.com, July 2012)