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Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
entering the carburetor. Some engines have a
gasket between the air filter housing and the
carburetor. Some air filter housings themselves
have a surface that will seal when it is bolted
onto the carburetor.
Some air filter housings have the choke
mounted in the housing. The choke restricts air
into the carburetor when starting the engine.
Figure 5-30 shows the choke closed over the
air passage from the clean air side of the filter
to the carburetor. The phrase “clean air side of
the filter” refers to the sealed area between the
air filter element and the carburetor. No dust
or debris should be on this side of the air filter.
(Note: not all air filter housings have a choke.
The other style of choke is integrated into the
body of the carburetor.)
Air Filter Cover
The air filter cover protects the air filter element
from damage and allows air to pass through to
the air filter element. On air filters with a choke
lever built into the housing, the air filter cover
will have icons indicating the choke lever posi-
tions for the choke being completely closed,
partly open, and fully open.
Air Filter Element
The type of air filter element is selected by the
manufacturer to match the performance of the
two-stroke engine and the environment where
the engine will be used. The displacement of the
engine and engine speed determine how much
air the engine will pass through the air filter.
A foam filter uses a sponge-type material as
a filtration structure and must be pre-oiled. See
Figure 5-31. Pre-oiling is done by using the
manufacturer’s recommended oil to saturate
the foam. Contaminants that enter the foam
element become trapped by the oil, preventing
them from entering the engine.
A paper filter uses a porous paper that allows
air to flow through it but not contaminants. The
paper is pleated to increase the surface area
used for filtering, Figure 5-32. A foam prefilter
Air filter housing Choke
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Figure 5-30. The choke closes off air between
the air filter and the carburetor. The choke is part
of the starting process on two-stroke engines.
Foam filter
Goodheart-Willcox Publisher
Figure 5-31. Foam filters have a foam, or sponge,
structure and are pre-oiled. As air passes through
the filter, it is the oil that traps the contaminants.
Goodheart-Willcox Publisher
Figure 5-32. A pleated paper filter is made of
porous paper to allow air to flow through it while
stopping contaminants.