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CHAPTER 9 Two-Stroke Engine Troubleshooting and Evaluation
Copyright Goodheart-Willcox Co., Inc.
Performing the test for intermittent spark
during this part of the evaluation process brings
the engine up to operating temperature. In order
for the technician to continue the evaluation
process, the engine needs to cool down to room
temperature. The test for intermittent spark can
also be performed after repairs are made but
before the unit is returned to the customer.
An adjustable spark tester can be used to test
the ignition coil for spark and for reserve volt-
age. Adjust the handle of the adjustable spark
tester to a 4 mm gap and pull the starter rope,
Figure 9-20. If there is a spark and the engine
begins to run, increase the spark tester gap to
6 mm and observe if there is a spark. The engine
should be running at idle at this point. Increase
the engine speed to WOT and observe the tester
for spark.
The 4 mm gap simulates normal operating
conditions for the ignition coil and causes the
ignition coil to produce a normal level of volt-
age. However, the ignition coil should be capable
of producing voltage greater than needed under
normal operating conditions. The maximum
amount of voltage the coil can produce above
normal voltage is called the reserve voltage. The
reserve voltage is available to compensate for an
increase in combustion chamber temperature
and compression as well as for a deterioration
of the spark plug condition. The 6 mm gap tests
the ignition coil’s reserve voltage.
If spark is interrupted during either the 4 mm
gap test or the 6 mm gap test, a problem exists
within the ignition system.
“Does the stop switch work correctly?” If a ”
spark is generated during the spark test, the stop
switch is open. Turn the stop switch to OFF and
check for spark. There should be no spark. Turn
the stop switch ON again and retest for a spark.
If there is a spark, then the switch is good.
Before replacing the original plug with a new
one, the original spark plug has to be removed.
While the spark plug is out, install a compres-
sion gauge in the spark plug socket to check
secondary compression. Pull the rope until the
needle on the compression gauge quits rising.
Record the measurement. Compare the reading
with the manufacturer’s specifications for that
particular engine.
“Was the original spark plug bad?” If there ”
was no spark when the original spark plug was
used, install a brand new spark plug (correct for
this engine), ensure it is properly gapped, and
retest for spark. If the engine sparks with the
new plug installed, the old spark plug was bad.
If there is no spark, some disassembly will be
required to disconnect the ground wire from
the ignition coil in order to test the ignition coil.
Examine the original spark plug that came
in with the engine. Compare the spark plug
manufacturer and model to determine if this
is the correct spark plug for this engine. See
Figure 9-21. Note the condition of the firing end
of the plug. Record whether the spark plug has
a normal appearance, if it is sooty, oily, worn
4 mm gap 4 mm
Goodheart-Willcox Publisher
Figure 9-20. The 4 mm gap tests the voltage the
coil can produce during normal engine operating
conditions. The 6 mm gap tests the ignition coil’s
reserve voltage.
Goodheart-Willcox Publisher
Figure 9-21. A tag on the ignition wire designates
the correct spark plug.