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Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
supposed to burn up with the fuel in as little
as 1/1900th of a second. Any oil other than an
ISO L0EGD or JASO FD certified oil will cause
damage to the engine. Oil that does not burn
up begins to bond to the surface of the piston,
leaving a dark carbon deposit. Eventually, the
buildup can cause the rings to stick or the piston
to seize. Figure 9-70 shows an example of car-
bon buildup from incorrect oil.
Repai ring the Equipment
When the repairs are made to the engine, it is
very important to tighten the fasteners to the
correct torque. Emission control components
must be OEM-certified emission control parts.
The service manual explains the correct proce-
dures for reinstalling components and reassem-
bling the engine.
A rebuilt carburetor must be adjusted to EPA
specifications by following the step-by-step pro-
cedures published by the engine manufacturer.
After the carburetor is adjusted, tamper-resis-
tant devices (limiter caps) must be installed on
the mixture adjustment screws.
If the carburetor is replaced with a new one,
it will need to be adjusted to EPA specifications
following the engine manufacturer’s procedure.
Fresh and correctly premixed fuel should be
used to operate the engine during the adjust-
ment procedure. The correct air filter, exact
spark plug, correct muffler, and emission-certi-
fied components must also be used. The engine
must have an operational load, as specified by
the engine manufacturer, during the carburetor
adjustment.
Goodheart-Willcox Publisher
Figure 9-68. A lean fuel mixture reduces the lubri-
cation to the connecting rod crankpin bearing. A
lean-running engine will have a higher-than-normal
engine speed. The combination of both of these
conditions prematurely wears out the bearing.
Connecting rod leans
Heavy scoring
on exhaust side
Goodheart-Willcox Publisher
Figure 9-69. With lubrication present, overheat-
ing causes the lubrication to bake onto the side
of the piston. As the overheating increases, the
surface of the piston begins to melt, smearing
and scoring the surface.
Uncertified two-cycle
oil deposit
Goodheart-Willcox Publisher
Figure 9-70. Uncertified two-cycle oil does not
lubricate fast enough for high-performance two-
stroke engines and will not burn up fast enough in
the combustion chamber. The unburned oil begins
baking onto the hot piston, leaving a dark, heavy
carbon deposit.