Description; Compression - Johnson SKEE-HORSE 25-201R Service Manual

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DESCRIPTION
This section provides trouble shooting procedures for the snow ma-
chine. Steps to be followed in determining causes of unsatisfactory per-
formance are outlined. A Trouble Check Chart at the end of this section
lists causes of unsatisfactory performance.
Being able to locate the cause of trouble in an improperly operating
snow mach.ine is as important as being able to correct the trouble. A
systematic approach to trouble shooting is important if the trouble is to
be located and identified in minimum time.
Any service operation can be broken down into three steps:
1. Identifying the problem
2. Determining the cause of the problem, and
3. Correcting the problem.
Familiarity with the factors which affect two-cycle engine perform-
ance is important in making a correct service diagnosis. Factors which
affect engine performance include the quality of the fuel and fuel mix-
tures, compression, spark and spark plug operation, and proper drive
system adjustment. This section discusses compression and spark
plugs and their relation to performance. A complete discussion of fuel
mixtures is included in Section 12. Familiarity with factors which con-
tribute to abnormal performance of an engine are similarly helpful. The
skilled mechanic's experience is a great asset here. The Trouble
Check Chart at the end of this section will assist in tracing symptoms of
trouble to the source.
COMPRESSION
The pistons and piston rings perform two functions. They compress
the mixture of fuel and air in the cylinders before ignition, and receive
the force of the power after ignition. For maximum compression, the
cylinder must be round and the piston and piston rings correctly fitted
to it. The rings must be properly seated in the ring grooves and free
to expand against the walls of the cylinder. The rings will not retain
the force of combustion if the pistons and cylinder walls are excessively
worn, scored, or otherwise damaged, or if the rings become stuck in
grooves because of carbon accumulation. Escape of compression past
the piston rings is referred to as "blow-by" and is indicated by dis-
coloration or carbon formation on the piston skirt.
Cylinder bores normally wear with operation of the engine. The de-
gree of wear will vary with length of operation, efficiency of lubricatioh,
and general condition of the engine. Excessive cylinder wear results
in loose fitting pistons and rings, causing blow-by, loss of compression,
loss of power and inefficient performance .
Piston rings are formed · in such a manner that when installed on the
piston, they bear against the cylinder wall with a light, even pressure.
Excessive ring pressure against the cylinder wall increases friction,
causing high operating temperature, sluggish performance, and abnor-
mal wear or scoring. Insufficient pressure allows blow-by, which re-
duces power, and causes overheating and carbon formation on the piston
skirt.
Since the ring tends to flex as it follows the cylinder contour during
engine operation, clearance or gap must be provided between the ring
ends to prevent butting. The ring gap also allows the ring to expand
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