Though it may not seem like it, the air filter on a lawn mower is very important to optimal engine performance. With a dirty, old, or clogged air filter, the engine won't get the proper mix of fuel and air, and the engine may run erratically -- or may not run at all.
For a Yard-Man 20" mulching mower, you can purchase a replacement air filter (part #497725S) from many different websites, as well as lawn mower parts stores and some hardware stores. Amazon sells a genuine Briggs & Stratton replacement air filter for $11.57, or a cheaper replacement Stens filter for $7.95. (Both filters are eligible for free shipping.)
To replace the air filter, follow the steps below.
1. Remove the air filter cover
The air filter is located on the front right side of the engine (if you're standing behind the mower) and is hidden beneath a plastic cover with two screws in the top of it. Loosen these screws with a flathead screwdriver. You don't have to completely remove them; once they are loose enough, the cover can be popped off. You may have to slide it sideways a little bit -- there is a small tab that fits under the main engine cover.
2. Remove the old air filter
The air filter can be removed by pulling it straight up. If it has a pre-cleaner on it (a blue foam oval piece that wraps around the filter; you can see a picture of it on Amazon) take the pre-cleaner off, wipe off any dirt or debris, and put it around the new air filter.
The old air filter can be thrown away, unless you intend to service it or save it. While the filter is removed, you can wipe down the base that the filter sat on with a dry paper towel or rag to remove any built-up dust or dirt.
3. Install the new filter
Place the new filter onto the base vacated by the old filter. It may be a tight fit, and you may have to push it hard to get it to fit into place. Be sure to install it with the hollow side facing down.
4. Reattach the cover
Place the cover on top of the new filter and line the screws up with the screw holes. The tab that sticks out of the cover will slide underneath the engine cover. Once everything is lined up, start each of the screws until both are somewhat tight, then tighten them one at a time.