Description; Factors Affecting Performance; Fuel System; Ignition System - Johnson SKEE-HORSE 25-201R Service Manual

5-2
D
E
17141
A WORN RING GROOVES
B WARPED RING - INSUFFICIENT
GAP
C CARBONED RING AND GROOVE
D WORN RING - ROUNDED EDGE
E SERVICEABLE RING - SQUARE
EDGE
Figure
5-1
DESCRIPTION
The purpose of a tune-up is to restore power and performance which
have been lost through wear or deterioration of one or more parts of
the snow machine. The successful completion of a tune-up depends on
an understanding of principles of two-cycle engine operation, and a fa-
miliarity with factors affecting performance. This section gives com-
plete tune-up procedures. Refer to Section 3 for principles of operation,
and to Section 4 for trouble shooting procedures. Lubrication proce-
dures and instructions for storage are included in Section 12.
FACTORS AFFECTING PERFORMANCE
In the normal operation of an engine, the operator may not be fully
aware of the decrease in performance which takes place slowly over a
long period of time. EconOmical, trouble-free operation can best be
assured if a complete tune-up is performed at least once each year,
preferably at the start of the season.
It
is seldom advisable to attempt to improve performance by correct-
ing one or two items only. Time will be saved and more lasting results
obtained by following a definite and thorough procedure of analysis and
by correcting all items affecting power and performance.
FUEL SYSTEM
A fresh fuel mixture, with the correct ratio of lubricant and gasoline,
is necessary for peak engine performance. The tank should be removed,
emptied of old fuel, rinsed out, installed and refilled with a fresh supply
at the beginning of the season and at every tune-up. A stale fuel mix-
ture may cause hard starting, stalling, and faulty operation.
An
alter-
native to the removal of fuel at the end of the season is the use of OMC
2+4 FUEL CONDITIONER. This additive stabilizes the fuel and prevents
gumming and varnishing. Inadequate fuel delivery, as the result of a
faulty fuel pump or clogged filter, will affect high-speed performance.
Incorrect carburetor needle adjustments may cause operating difficulties
at any speed. Faulty choke operation or incorrect use of the manual
choke by the operator may cause hard starting, rough running, or poor
fuel economy.
IGNITION SYSTEM
Spark plugs having the proper heat range are very important for peak
performance of the engine. See Section 7 for a complete description of
spark plugs. A weak spark, which may be the result of faulty ignition
system components, will cause hard starting, misfiring, or poor high-
speed performance. The spark plugs and ignition system components
are frequently checked first in a tune-up because of their importance
to the operation of the motor.
COMPRESSION
Compression must be well sealed by the piston and piston rings in the
cylinder to realize maximum power and performance. See Figure 5-1.
A compression check is important because an engine with low or uneven
compression cannot be tuned successfully to give peak performance.
It is essential that compreSSion be checked before proceeding with an
engine tune-up.
An
automotive type compreSSion gage may be used as follows: Make
certain that choke is open, throttle is wide open, and both spark plugs
are removed. THE COMPRESSION RELEASE KNOB MUST BE COM-
PLETEL Y IN. Turn the engine over quickly three or four
times with
the manual starter. Compression should be
105
pounds per square
inch minimum.
•