"The Lawn Mower Repair Man's Front Page"
The typical rotory mower engine is governed to run at around 3200 rpm. That's 6400 times a minute that the piston goes up or down. Just how long do you imagine it takes for even a small amount of grit to do severe damage should it not be filtered out before entering your engine? Would you take the head off your engine and go to sanding on the cylinder wall with a piece of sand paper? It would take a while for you to apply 6400 strokes (one minutes worth of running) by hand but I think it's fairly easy to understand the result. Not a good thing.
Most of the newer engines now use an automotive type of paper filter. They aren't intended to be cleaned but if they aren't too bad you can lightly tap them to knock out some of the collected dirt. You need to look at them when they are new so you know what they should look like. If they become discolored from fuel or oil contamination--replace them. If you don't know what a new one looks like you may not know it's discolored.
Hold a flashlight or bright house light up behind a new filter and you will see a good amount of light coming through, much as you will with a lamp shade. If your filter is dirty it no longer allows the light through and it's time to replace it.
There are a number of "will-fit" filters on the market, some are pretty good, others are junk. Some won't even fit tight enough to form a proper seal. It simply isn't worth a damaged engine to save a buck or two buying a non original air filter.
An upgrade engine feature may be the use of a foam pre-filter. Used to be the engine manufacturers advised lightly oiling these pre-filters, most no longer do. The trouble is if not done properly, oil will transfer to the paper filter and destroy it. I still like to oil pre-filters and here's why. If oiled properly, a pre-filter will stop over 90% of the dirt before reaching the paper filter. The pre-filter can be cleaned, re-oiled and re-used, allowing much longer use of the main filter.
Cleaning and oiling a pre-filter; Note: This also applies to older engines using a foam only air filter;
Wash the filter with dish soap in as hot of water as you can stand. Depending on the size, spread out an ounce or two of standard 30 wt motor oil and work it through the filter completely. A flat pre-filter with a screen backing takes a little more effort as you can't just wad it up and squeeze the oil around. Once you have oil through-out the filter, wrap in a clean paper towel and squeeze out any excess. Do it twice if needed, all you want left in the filter is a film of oil covering all the pores of the foam. Again, the flat-screen backed filters need a little more effort. Lay a couple of layers of paper towels on a flat surface, then the filter and then a couple more layers of paper towel over the filter. Pat it down to remove the excess oil.
A common error when re-installing the filter is getting the filter cover properly attached. Most have a slip latch on one side and a screw on the opposite side. The slip latch side often is not properly inserted allowing a loose fitting filter cover which will not press tightly against the filter. Take the time to double check the installation before returning the engine to service.
LMRM; Bob :<=