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CHAPTER 9 Two-Stroke Engine Troubleshooting and Evaluation
Copyright Goodheart-Willcox Co., Inc.
Next, visually inspect the insulator block or
intake manifold. Remove the carburetor if it has
not already been removed. The intake passage
through the insulator block should be clean and
debris-free. Remove the insulator block and
closely inspect the block for cracks, especially
around mounting screw holes. Some engines
use a rubber manifold to connect the carbure-
tor to the intake port. Remove the manifold to
inspect its condition. The manifold should be
pliable and not stiff or brittle. The inside of the
manifold should be clean with no debris. Check
the manifold material itself for cracks.
Visually inspect the intake side of the pis-
ton. The intake side of the piston skirt can be
seen through the intake port. Visible machine
marks on the skirt indicate an unworn piston.
A smooth but dull surface indicates wear from
small abrasive particles. Scratches running up
and down the piston indicate damage caused by
large abrasive particles. Scoring on the piston
skirt indicates abnormal heat from overheating
the engine or from a lack of lubrication.
Roll the piston upward to inspect the exhaust
side of the cylinder. The cylinder surface should
be smooth and shiny. A dull surface indicates
wear from fine abrasives. Well-defined scratches
come from large, hard abrasive particles. A
scored cylinder indicates an engine that over-
heated, ran lean, or ran on raw gas for fuel.
Internal Engine Component Evaluation
Check the crankshaft for end play and radial
movement. Move the crankshaft out and in to
check for excessive end play, Figure 9-54A.
There should be only a slight end play to allow
for crankshaft expansion as the engine heats up
to operating temperature. Move the crankshaft
right and left to check for excessive radial move-
ment, Figure 9-54B. If the crankshaft bearings
are in good condition, there should be no radial
movement.
A crankcase pressure-vacuum test was per-
formed earlier in the evaluation, after the car-
buretor was removed but before the muffler
was removed. This test checked for leaks within
the engine and crankcase. The crankcase pres-
sure-vacuum test checks for leaks in the crank-
shaft seals, the engine mating surfaces, and
gaskets.
If the visual inspection through the intake
and exhaust ports indicated internal damage,
the cylinder needs to be removed. With the cyl-
inder removed, inspect the piston, connecting
rod, and cylinder. Carefully examine any dam-
age on the piston and cylinder.
Inspect the piston and connecting rod. Look
for a “moist” surface on the underside of the pis-
ton skirt. This indicates the presence of lubri-
cation oil. The underside of the piston crown
Check for end play
Check for radial
movement
A B
Goodheart-Willcox Publisher
Figure 9-54. Check for movement in the crankshaft. The crankshaft should have no radial movement
and only a slight end play movement. A—Test for end play by attempting to move the shaft up-and-down.
B—Test for radial movement by trying to move the shaft side-to-side.