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CHAPTER 9 Two-Stroke Engine Troubleshooting and Evaluation
Copyright Goodheart-Willcox Co., Inc.
sample until the troubleshooting is complete and
the engine runs correctly. If the fuel itself is part
of the reason the engine runs poorly or not at all,
it should be tested for staleness, amount of deg-
radation, percentage of two-cycle oil, and per-
centage of alcohol. Save the sample in a sealed
container and with a label identifying the unit
or the customer. The customer may need to be
informed about the condition of the fuel and
given information about proper fuel storage.
Pull out the fuel filter and remove it from the
fuel line. Attach a vacuum gauge to the output of
the filter and attempt to pull a vacuum. If the fil-
ter is not obstructed, the vacuum gauge will leak
down immediately. If the gauge holds a vacuum,
the fuel filter is completely obstructed and must
be replaced. See Figure 9-6. If the gauge bleeds
down slowly, the filter is partially obstructed
and should be replaced before testing the engine
further.
Before reinstalling the fuel filter, attach a
pressure gauge to the fuel line. Then, disconnect
the other end of the fuel line from the carbure-
tor and clamp it off. Pressurize the line to about
7 psi (.5 bar). The fuel line should hold pressure,
Figure 9-7. If the pressure gauge bleeds down,
there may be a leak in the fuel line.
If the pressure gauge holds pressure and the
engine has a purge priming system, depress
and release the bulb several times and observe
the pressure gauge needle. If the purge system
is working correctly, the gauge needle should
drop slightly each time the bulb is released. If
the gauge needle does not drop when the bulb is
released, the carburetor must be evaluated.
Add known good fuel to the engine’s fuel
tank. Known good fuel is fresh and premixed
with two-stroke certified oil at the correct fuel-
to-oil ratio. This step ensures that the fuel itself
will not be an issue when troubleshooting the
engine further. Attempt to start the engine using
the manufacturer’s recommended starting pro-
cedures. If the engine starts and runs correctly,
the customer complaint was the result of poor
fuel quality or a clogged fuel filter. Return the
equipment to the customer and explain the
importance of using clean, fresh, and properly
mixed fuel.
If the engine does not start, remove the spark
plug to see if it is wet. A wet plug will have liquid
fuel on its rim edge, the edge that the ground
electrode is welded to. See Figure 9-8. A wet
plug means that the air-fuel mixture in the
crankcase has been transferred to the combus-
tion chamber. The engine did not run because
there is a possible problem with the ignition sys-
tem, there is low secondary compression, or the
crankcase has flooded. A dry plug means no
transfer has taken place. This indicates a prob-
lem with primary compression. Make a note of
the amount of spark plug gap, the plug manu-
facturer, and the plug model number. Also, note
the condition of the plug. Then, continue with
the troubleshooting process.
Checking Secondary Compression
While the plug is out, check the engine’s sec-
ondary compression. To check secondary
Vacuum pump
Fuel filter Vacuum gauge
Goodheart-Willcox Publisher
Figure 9-6. If the vacuum gauge holds a vacuum
when the fuel filter is tested, the fuel filter is
obstructed and must be replaced.
Clamp Fuel line
Pressure
gauge
Goodheart-Willcox Publisher
Figure 9-7. Pressurize the fuel line to about 7 psi.
If it holds pressure, there is no leak in the fuel line.