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CHAPTER 9 Two-Stroke Engine Troubleshooting and Evaluation
Copyright Goodheart-Willcox Co., Inc.
Note the condition of the chain cutters and
the chain. Dull cutters place a severe load on the
two-stroke engine and can cause serious dam-
age to the engine itself.
Evaluate the condition of the equipment’s
deflector or shield. Is the deflector present? Is it
fastened tightly? Is the cutter still present? See
Figure 9-17. The cutter sets a maximum string
length on a string trimmer. If the cutter were
not present, excessive string length could slow
the engine speed at WOT, causing the engine to
overheat.
If the equipment uses an attachment, note the
condition of the attachment. An attachment is
a component that performs work for the engine
and can be mounted to and removed from the
engine. Also, note if the equipment came in with
the attachment. If the engine is to be run after
evaluation and repairs, the cutting attachment
needs to be mounted correctly to the engine.
The engine cannot be adjusted or perform cor-
rectly unless the attachment is in place. A blower
must have the blower tubes attached before the
blower engine can be evaluated.
The anti-vibration (AV) system consists of
the rubber and/or spring vibration-absorbing
components between the engine and the handle.
See Figure 9-18. The AV system reduces opera-
tor fatigue due to continual vibration from the
engine. Inspect the components of the AV sys-
tem for damage or excessive wear.
Air Intake System
During engine operation, air must be brought
into the engine to support the combustion pro-
cess. Because contaminants in the air can dam-
age the moving parts of the engine, the air must
be cleaned. The air filter element removes con-
taminants mechanically through a filtering pro-
cess. As the engine operates, debris particles
pile up on the filter element’s surface.
In normal operation of the engine, the air fil-
ter must be serviced periodically. This servic-
ing removes the accumulated debris, allowing
the correct volume of air to flow into the engine.
Each engine manufacturer explains in the own-
er’s manual the steps for servicing the air filter.
Unless the air filter is cleaned periodically or
replaced, the engine could become damaged.
There are two surfaces to the air filter ele-
ment: the debris side and the clean air side. The
debris side faces incoming air. The clean air side
faces the carburetor. When evaluating the air
intake system, it is okay to see an air filter ele-
ment that is dirty on the debris side. Note how
much debris has been collected to determine if
the filter was restricting air.
It is very important that there not be any
debris on the clean air side of the air filter ele-
ment. The surfaces behind the filter element
must be completely clean. The throat of the car-
buretor on the air-intake side should be clean.
Gently touch the surface to feel if there is any
dust-sized debris. Also, if the air intake system
has an air filter backing plate between the air
filter element and the carburetor, the entire sur-
face on the clean air side of the backing plate
should be completely clean. See Figure 9-19.
If there is debris on the clean air surfaces, try
to determine how the debris was getting past
the air filter element. Check to see if the air filter
String cutter missing
Goodheart-Willcox Publisher
Figure 9-17. The cutter on the guard sets a max-
imum string length. If the cutter is missing, the
string will extend too far. Excessive string length
will cause the engine to overheat.
Spring Spring Rubber spacer
Goodheart-Willcox Publisher
Figure 9-18. The AV system on this equipment
consists of two springs and a rubber spacer.