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CHAPTER 9 Two-Stroke Engine Troubleshooting and Evaluation
Copyright Goodheart-Willcox Co., Inc.
Fine abrasive material can wear away the
machine marks, resulting in a piston surface
that is smooth and dull. The abrasive process
will also remove the plating from inside a cylin-
der, Figure 9-57.
Abrasives enter the crankcase first and can
leave a deposit on the surfaces. Feel for abrasives
on the crankshaft, connecting rod, and crank-
case surface, Figure 9-58. Abrasives will also
wear out the connecting rod journal bearings.
Stale fuel will leave brown deposits on the piston
skirt, Figure 9-59, and in the crankcase. Often,
the stale fuel burns at a slower-than-normal rate
and ignites the fuel coming up the transfer ports,
leaving a burned carbon layer, Figure 9-60.
A raw gas failure is caused by running just
gasoline, rather than premix, through a two-
stroke engine. The lack of lubrication results
in metal-to-metal contact in the engine and a
large increase of heat. The largest quantity of
heat is concentrated on the exhaust port side of
the piston. Initially, two score lines are formed
on the piston corresponding to the cylinder sur-
faces adjacent to the exhaust port. The scoring
continues on the piston skirt area which covers
and uncovers the exhaust port. As a result, the
Chrome plating Intake port
Plating worn off
Abrasive debris
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Figure 9-57. Abrasives will also wear down the
plating on the cylinder surface.
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Figure 9-58. Abrasives will also be deposited on
the internal surfaces of the crankcase.
Varnish
Carbon deposits
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Figure 9-59. Unburned gum in the gasoline bonds
to the side of a heated piston.
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Figure 9-60. Blackened transfer ports show evi-
dence of fuel burning in the passages.