208
Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
should be carbon-free. Burned carbon depos-
its indicate the engine overheated. Inspect the
condition of the wrist pin needle bearing. If it is
damaged, it must be replaced. Inspect the piston
pin surface between the connecting rod and the
piston. A band of brown, purple, or blue indi-
cates the engine overheated. Inspect the crank-
pin bearing. If it is worn, the entire crankshaft
and connecting rod will need to be replaced as
a unit.
On the outside of the piston, the ring lands
should have a .004″ to .006″ ″ clearance. This allows ″
combustion pressure to push the rings down and
outward, creating a greater ring-to-cylinder seal.
Mechanical up-and-down movement of the ring
causes mechanical erosion of the ring groove,
resulting in excessive clearance. Abrasives will
also wear out the ring groove.
A hole through the top of the piston indi-
cates detonation. When part of the air-fuel
charge ignites prematurely, the flame front col-
lides with the spark flame front. This results in
a shock wave that mechanically erodes a hole
through the top of the piston.
Failure analysis is the process of looking
at engine components that failed to determine
the root cause of the failure. Each type of two-
stroke engine failure has indicators that iden-
tify how the engine failed. Melted aluminum on
the piston indicates the temperature of the pis-
ton surface reached the melting point for alu-
minum. When aluminum is heated, it does not
discolor like iron-based metals do. A piston pin
turns brown, purple, blue, or black because of
excessive heat. Aluminum will melt and not
show a change in color. The colored deposits on
an aluminum surface indicate a substance was
burned onto the surface. Different substances
have different colors. Stale fuel leaves a brown
deposit. Fuel and oil leave a black deposit. Abra-
sives may leave a gray deposit. Fine abrasives
wear a surface. Coarse abrasives leave well-de-
fined scratches. A two-stroke engine may have
a single cause for failure or a combination of
root causes. Typical failures seen in two-stroke
engines often result from one or more of the
following:
• Abrasive ingestion.
• Stale fuel.
• Use of raw gas.
• Running lean.
• Overheating.
• Use of improper oil.
Abrasive ingestion is typically caused by con-
taminants entering through the air system. Even
with an obstructed air filter element, a running
engine will attempt to pull air and tiny abrasive
particles through the element. An incorrectly
installed element will allow very large abrasives
into the airstream. The first location abrasives
will be present is on the clean air side of the air
filter housing, Figure 9-55. Abrasives that fuse
onto the top of the piston form a gray crust,
Figure 9-56.
Abrasives on clean
side of filter housing
Goodheart-Willcox Publisher
Figure 9-55. The first evidence of abrasives is on
the clean air side of the air filter housing.
Gray crust
Goodheart-Willcox Publisher
Figure 9-56. Because of combustion tempera-
tures, abrasive particles can become fused to the
crown of the piston.