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CHAPTER 9 Two-Stroke Engine Troubleshooting and Evaluation
Copyright Goodheart-Willcox Co., Inc.
start and run but not have power at wide-open
throttle. It is important that the repair techni-
cian identify the cause(s) of the deposits on the
spark arrester mesh.
Remove the muffler and inspect the intake
opening of the muffler for any buildup of car-
bon or abrasives. Carbon will be black in color.
Abrasives will be a dark gray color from the par-
ticles becoming hot in the combustion chamber
and then fusing onto exhaust passages.
With the muffler removed, inspect the
exhaust port. A thin coating of carbon is nor-
mal. A buildup of carbon or abrasive crust indi-
cates problems with the engine. A carbon crust
restricts exhaust gases, causing the engine to
experience reduced power. See Figure 9-48.
The carbon can be removed with a hardwood
stick scraper, but it is important to identify the
causes of the carbon buildup and make sure
these are corrected. Abrasive particles from the
air intake system are heated close to their melt-
ing point when they experience the heat in the
combustion chamber. These particles then touch
and fuse to the surfaces of the exhaust port and
muffler. This abrasive crust buildup can also
cause a restriction for the exhaust gases.
Carbon or abrasive buildup in the exhaust
system can be cleaned. If the buildup is severe,
the affected parts need to be replaced. On emis-
sion-certified engines, no aftermarket parts
can be used. Only the OEM emission-certified
parts can be used without violating emissions
regulations.
Next, visually inspect the rings and exhaust
side of the piston and note their condition. Use
a pick tool to gently push the ring. If they move
slightly, they are not stuck.
Roll the piston up to inspect the exhaust side
of the piston skirt. Even after many hours of
normal operation, the piston skirt should still
show the original machine marks. The machine
marks are very fine lines parallel to the rings
and left over from the machining process. When
examined closely, the surface of the piston skirt
should show the peaks and valleys of the machin-
ing process, Figure 9-49. Normal engine opera-
tion does not wear down this machined surface.
Very fine abrasive particles can wear down
the machine marks to a very smooth, non-shiny
surface, giving the appearance of a piston with
no faults. See Figure 9-50. Even though the sur-
face looks undamaged, the machine marks are
worn off. Abrasive particles mix with the lubri-
cation oil on the cylinder surface and act as lap-
ping compound on the piston surface.
Any process that causes excessive heat in a
two-stroke engine will cause damage to the pis-
ton. If the metal on the exhaust side of the pis-
ton shows signs of smearing, abnormal heat was
involved. Figure 9-51 shows piston material that
Carbon crust restriction
Goodheart-Willcox Publisher
Figure 9-48. The exhaust port can become
restricted with a buildup of carbon from excessive
or incorrect oil or a rich-running engine. An abra-
sive crust can build up from contaminants ingested
into the engine from a faulty air intake system.
Parallel horizontal
machine marks
Goodheart-Willcox Publisher
Figure 9-49. Machine marks should be visible on
the piston skirt.