186
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
compression, install a compression gauge into
the spark plug hole, pull the starter rope several
times until the needle on the gauge stops advanc-
ing, and record the compression reading. See
Figure 9-9. Refer to the service manual for the
compression ratio specifications for the engine
you are testing. Determine if the compression
reading is low or acceptable. A low compression
reading indicates a mechanical problem with
the engine, such as excessively worn or sticking
piston rings.
Checking for Spark
Before checking for spark, reinstall the spark
plug. Then, attach a two-stroke inline spark
tester. One end of the spark tester is attached to
the high tension-lead of the coil, and the other
end is attached to the top of the spark plug. See
Figure 9-10. Make sure the engine stop switch
is in the RUN position to allow a spark. Pull on
the starter rope while looking in the spark tes-
ter’s window to see if a spark is present. A glow
in the spark tester window indicates the ignition
coil is producing a spark. If there is a spark, the
ignition coil and spark plug are probably good.
A no-spark condition indicates that additional
tests must be performed to evaluate the ignition
system.
Performing Pop-Off Tests
Two pop-off tests can be performed to quickly
determine if the engine can run: a lower-end
pop-off test and an upper-end pop-off test. The
pop-off tests put liquid fuel into the engine,
bypassing the engine’s fuel delivery system.
WARNING
When the spark test is performed, the engine
could start and continue to run. When performing
the spark test, use the same safety procedures
you would use to actually start and run the engine.
Liquid fuel
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Figure 9-8. The presence of liquid fuel on the
spark side of the spark plug means fuel was trans-
ferred to the combustion chamber.
Compression reading
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Figure 9-9. Install a compression gauge into the
spark plug hole and pull the rope several times.
When the needle quits rising from one pull to the
next, record the compression reading.
Connected to spark plug
Sight glass Connected to
spark plug wire
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Figure 9-10. A spark tester is connected
between the ignition coil’s high-tension lead and
the top of the spark plug. Make sure the stop
switch is in the RUN position. Pull the rope sev-
eral times while looking in the window of the spark
tester. A glow indicates a spark.