Two-Stroke Pressure Testing; Excessive Vibrations - Arctic Cat Cougar Service Manual

TROUBLESHOOTING
Piston Seizure
Piston seizure or galling is the transfer of
metal from the piston to the cylinder bore. Fric-
tion and the heat causes piston seizure. Some
causes may be pistons with incorrect bore clear-
ances, piston rings with an improper end gap or
a compression leak. Other causes are incorrect
type of oil, lack of oil or an incorrectly operating
oil injection pump. A spark plug with a wrong
heat range, incorrect ignition timing or overheat-
ing may result in piston seizure.
A noticeable reduction of speed may be your
first sign of seizure, while immediate stoppage
indicates a full lockup. A top end rattle is often
an early sign of seizure.
When diagnosing piston seizure, the pistons
themselves can be used to troubleshoot and de-
termine the failure cause. High cy Iinder tempera-
tures normally cause seizure above the piston
rings while seizure below the piston rings is
usually caused by a lack of proper lubrication.
See Figures 19 and 20 for examples of piston
seizure.
Excessive Vibrations
Excessive vibrations may be caused by loose
engine, suspension or steering mount bolts.
59
Engine Noises
A change in the sound of the engine is often
the first clue that an engine problem exists.
Noises are difficult to differentiate and even
harder to describe. Experience is needed to ac-
curately diagnose problems using engine sounds
(Figure 21).
TWO-STROKE PRESSURE TESTING
Hard to start and generally poor performing
two-stroke engines may be suffering from leak-
ing crankcase compression. Fuel delivery, either
carburetor or EFI, and ignition systems may be
good, and a compression test may indicate that
the engine's upper end is okay. A conventional
compression test does not show a lack of primary
(crankcase) compression. In a two-stroke en-
gine, the crankcase must be alternately under
pressure and vacuum. After the piston closes the
intake port, further downward movement of the
piston causes the trapped mixture to be pressur-
ized so that it can rush quickly into the cylinder
when the scavenging ports are opened. Upward
piston movement lowers the pressure (creates a
vacuum) in the crankcase, drawing the air-fuel
mixture in from the carburetor or EFI.
NOTE
The operational sequence of a two-
stroke engine is illustrated in Chapter
One under Engine Principles.
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DETONATION
Ignition spark
Very rapid combustion
Early expansion causes
knock and blowby
TDC
Little expansion
after TDC results
in power loss