The incoming combustion air passes through a cyclone cleaning system. Thee intake cyclones cause the air to rotate at a high speed. The rotation of the air causes the heavier dirt particles to move to the outside of the air stream inside the cyclone. Attached to the cyclone is a low pressure or vacuum tube from the engine flywheel. This low pressure ensures that all the dust and dirt separated in the cyclone is removed from the cyclone and ejected back into the atmosphere. This pre-cleaning system actually removes the largest percentage of dirt from the air stream.
This step includes a pleated paper main filter. An air filter is made up of specially designed paper. This material must withstand the temperature at which the machine operates and withstand high volume of small dirt particles that are not removed by the pre-cleaner. This filter element is designed to become covered with dust and debris, and this is a good thing. When we look at a filter we say it is dirty; however, that dirt is part of the filtering system. Now you say hang on, the dirty filter is part of the filtering system. Think of the filter as fibers laid in a pattern to form a screen like mesh; kind of like a screen door. The air passes through the holes in the mesh.
Step three is an auxiliary air filter, This is a final filter before the air enters the engine to remove any debris passing both pre- cleaner and the main pleated paper filter. The second function of this filter is to ensure that the main filter does not become damp from the "spit back" from the engine
A couple of things happen when you clean the filter. One, if you use an air hose to clean filters you force debris through the pleated paper, opening up those "holes", allowing larger pieces of dust and debris through the filter. Secondly, you can damage the seal and distort the filter, and as I said earlier, that "filter cake" increases the effectiveness of the filter so why remove it. We employ a carburetion system that automatically adjusts the air- fuel ratio depending on how freely air moves through the air filter, ensuring that the machine will not suffer loss of fuel economy because of an air filter becoming dirtier.
STIHL recommends that you do not service or inspect the air filter until the machine begins to lose power. When it does lose power, change both the pleated paper main filter and the auxiliary filter. Under no circumstances should you clean or "blow out" your filters.