Air Filters for Small Engines

Richard L. Parish  |  11/18/2004 9:39:52 PM

Figure 1. Primary (paper) filter removed from rotary tiller.

Figure 2. Primary (paper) filter and foam prefilter removed from rotary tiller.

The typical homeowner has several small engines in the carport or toolshed on equipment like a lawnmower and string trimmer. Some may also have a garden tiller, a hedge trimmer, an edger or a chainsaw.
 
Most of these small engines have an air filter. One of the most important things you can do to maintain your small engines is to service the air filters as needed.

Why an Air Filter?
Lawn and garden equipment often operates in dusty, dirty conditions. Every engine needs to breathe in fresh air to mix with the fuel for burning. Any dust or dirt drawn into the engine with the intake air will not only clog carburetors and air passages but will cause internal engine wear. Dirty air can ruin an engine rapidly. Air filters are designed to trap the dust and dirt in the incoming air before the dirt particles can reach the engine.
 
Since lawn and garden engines are operated in such dirty conditions, the filters can become filled with dirt fairly rapidly. Unless you drive a lot of unpaved roads, you probably don’t need to change the air filter in your car more often than every few years; your small engine air filters need servicing much more often, perhaps after 2-4 hours of operation.
 
If the filter fills with dirt, two things can happen; first, the filter may plug and not allow enough air into the engine, thus reducing power and performance, or second, the plugged filter may collapse and allow dirt to bypass into the engine. The way to avoid these problems is to service air filters often.

Servicing the Primary Filter
Start with the operator’s manual. It will tell you how to service the filter and suggest a frequency for service, often stated in hours of operation. It is a good idea to check your air filter even more often than suggested until you get enough experience with your machine to know how fast the filter loads up.
 
A tiller in a dry, dusty garden or a garden tractor mowing tall, dry weeds will load air filters faster than a lawnmower used on a lush, well-watered lawn. It is not at all uncommon for a small engine air filter to load up in 2-4 hours of use under dusty conditions.
 
To check your air cleaner, remove the cover and look at it. If is dirty, you need to clean or replace it. Most small-engines use a pleated paper primary filter (Figure 1). You can tap the filter lightly on a hard surface to knock most of the dust off and reuse it, if it is not oily. Do not try to blow it out with an air hose; there is a risk of puncturing it. If it is oily or light tapping doesn’t clean it, replace it. You should also replace it after one or two cleanings.

Servicing a Foam Prefilter
Many engines also have a foam prefilter that either wraps around a round pleated paper primary filter or fits above a rectangular primary filter (Figure 2). Foam prefilters can be washed (lots of hot water and soap), dried (squeeze it in paper towels or let it air-dry) and then re-oiled. It is essential that foam filters be oiled before use; just pour about 1-3 teaspoons of clean engine oil onto the foam (before installation) and massage the filter to work the oil into it uniformly. Rubber gloves are handy at this stage! Then, just reinstall the foam prefilter.
 
Don’t overdo the oil; if it runs off onto the filter housing, you have used too much. Since foam prefilters are generally inexpensive, you might want to just replace the foam element rather than cleaning it. Even new foam filters will need to be oiled before use.

General Precautions
Be careful when removing or installing filters to avoid knocking dirt or dust into the engine. You should clean the housing before removing the filter to minimize the chance of knocking dirt inside when you remove the filter.
 
Be sure to use the correct size and type of filter for your engine. It is not necessary to buy a same-brand filter from a dealer; you can get good filters at some auto parts stores. Just be sure you get a good-quality filter that fits properly. Improper fit can allow dirt to enter the engine.

One of the best things you can do to maintain and preserve the life of a small engine is to service the air filter regularly. Use the manufacturer’s recommendations as a general guide, but you may need to service your filters more often if your operating conditions are very dusty or dirty. Be sure to oil foam prefilters. Your small engine will reward you with lower repair costs and longer life if you give it clean air to breathe.

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