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CHAPTER 9 Two-Stroke Engine Troubleshooting and Evaluation
Copyright Goodheart-Willcox Co., Inc.
Overheating r r causes lubrication break down
on the engine surfaces, which become over-
heated. Any restriction to air movement across
the cylinder’s cooling fins will cause excessive
heat. The excessive heat causes oil to burn onto
the surface of the piston, leaving black car-
bon deposits. These deposits gradually reduce
the clearance between the piston and cylin-
der. Excessive heat from the cylinder plus hot
exhaust gases cause the exhaust side of the
piston to score severely. Figure 9-69 shows a
piston with carbon buildup on the exhaust side
and scoring marking through the carbon. The
carbon built up first then excessive heat began
smearing the surface of the piston.
Uncertified two-cycle oil is not engineered to
run in a high-performance two-stroke engine.
Certified two-cycle oil is engineered to lubri-
cate the moving parts of a two-stroke engine
that may be running at 242 revolutions per sec-
ond. The oil is transferred to the combustion
chamber, where, for some engines, the oil is
Exhaust scoring
Goodheart-Willcox Publisher
Figure 9-64. Two score marks, adjacent to the
exhaust port, are the beginning evidence of lean
seizure.
Intake scoring
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Figure 9-65. Score marks on the piston skirt
adjacent to the intake port result from piston
expansion during lean running.
Intake scoring
Exhaust scoring
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Figure 9-66. The score marks from the initial
stages of a lean-running engine beginning to seize
form an “X” pattern.
Scoring
Goodheart-Willcox Publisher
Figure 9-67. Excessive heat can cause large and
heavy scoring on the exhaust side of the cylinder
during lean running.