Johnson SKEE-HORSE 25-201R User manual

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SKEE-HORSE
SERVICE MANUAL
o
30 HORSEPOWER
MODELS 25-201R
25-201RA 25-201RSA
11 · 70 PART NO. 261894 LITHO U.S.A. © OUTBOARD MARINE CORPORATION 1970 ALL RIGHTS RESERVED

AVAILABLE IN ONE QUART
CANS AND IN 6 PACKS
FROM YOUR JOHNSON
DEALER
170 2.0
FUEL RECOMMENDATIONS
The correct fuel mixture ratio is 24 parts of a good grade regular gaso-
line to one part lubricant. For ease of measurement, this is equivalent
to one quart of lubricant to six gallons of gasoline, one pint of lubricant
to three gallons of gasoline, or 1/3 pint of lubricant to each gallon of
gasoline.
Use only Johnson Lubricant or a reputable automotive engine oil,
SAE 30 SD or SB. Avoid use of low price third grade (SA light duty)
oils. DO NOT USE MULTIPLE VISCOSITY OILS, SUCH AS 10W30, OR
ANY OUTBOARD MOTOR OILS OTHER THAN JOHNSON OUTBOARD
LUBRICANT.*
*EVEN THOUGH JOHNSON OUTBOARD LUBRICANT IS ADVER-
TISED AS A 50:1 RATIO LUBRICANT, IT IS IMPERATIVE FOR
SNOWMOBILE USE THAT IT BE MIXED AT A 24:1 GAS-
LUBRICANT RATIO.
DO NOT POUR GASOLINE OR LUBRICANT DIRECTLY INTO VEHICLE
FUEL TANK. USE AN APPROPRIATE CONTAINER FOR MIXING AND
STORING THE FUEL .
To prepare the snowmobile fuel properly, pour into a SEPARATE,
clean container half the amount of gasoline required and add all the
required lubricant.
Thoroughly shake this partial mixture. Next, add the balance of gaso-
line necessary to bring the mixture to the required ratio of 24:l.
Again, thoroughly agitate the mixture. A clean funnel equipped with a
fine
screen should be used when pouring the fuel mixt~re into the vehicle
tanle
24 to 1 lubricant is prediluted to provide excellent mixability with gaso-
line at low temperatures.
The addition of this dilutent does not in any way affect the lubrication
qualities of the lubricant.
Whenever is is necessary to mix fuel and lubricant at temperatures
below 0° F, the lubricant should be prediluted with gasoline to improve
its mixability. The lubricant should be prediluted with approximately
one part gasoline to one part lubricant. Predilution of the lubricant
should take place with the lubricant temperature above O°F.
Do not use kerosene or fuel oils for pre-mixing.

SECTION 1
INTRODUCTION
INTRODUCTION
SPECI FI CA TID NS
GENERAL SNOWMOBI LE IN FO RMATION
TROUBLE SHOOTING
TUNE - UP PROCEDURES
FUEL ' SYSTEM
IGNITION AND
ELECTRICAL SYSTEM
MANUAL STARTER
ENGINE
DRIVE TRAIN
STEERING, TRACK AND SUSPENSION
LUBRICATION AND STORAGE

1-2
1
The snow machine has been designed and built for dependable, high )
performance. It is important to every snow machine owner to be ab le
to receive skilled and thorough service for his vehicle when necessary.
It is important to the service dealer to be able to offer the type of skilled
service which will maintain the customer's satisfaction.
This manual, together with the regularly issued service bulletins and
Parts Catalogs, provide the serviceman with all the literature necessary
to service the Skee-Horse· snowmobiles. An effort has been made to
produce a manual that will not only serve as a ready reference book for
the experienced serviceman, but will also provide more basic informa-
tion for the guidance of the less experienced man.
The Parts Catalogs contain complete listings of the parts required
for replacement. In addition, the exploded views illustrate the correct
sequence of all parts. This catalog can be of considerable help as a
reference during disassembly and reassembly.
The Section Index on page 1-1 enables the reader to locate quickly any
desired section. At the beginning of each Section is a Table of Contents
which gives the page number on which each topic begins. This arrange-
ment simplifies locating the desired information within this manual.
Section 2 lists complete specifications on the 1971 snowmobiles. All
general information, including 2 cycle engine theory, trouble shooting,
and tune up procedures, are given in Sections 3 through 5 of this manual.
17029
Figure 1-1

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Sections 6 through 11 provide fully " illustrated, detailed, step-by-step
disassembly and reassembly instructions and adjustment procedures.
Section 12 provides lubrication and storage information. In this way,
the texts treat each topic separately; theory and practice are not
intermixed. This makes it unnecessary for the experienced service-
man to reread discussions of theory along with specific service in-
formation. lliustrations placed in the margins provide unimpeded
reading of explanatory text, and permit close relationship between
illustration and text.
Read this manual carefully to become thoroughly familiar with the
procedures described, then keep it readily available in the service shop
for use as a reference. If properly used, it will enable the serviceman
to give better service to the snowmobile owner, and thereby build and
maintain a reputation for reliable service.
This service manual covers all phases of servicing the snowmobile,
however, new service situations sometimes arise. If a service question
does not appear to be answered in this manual, you are invited to write
to the Service Department for additional help. Always be sure to give
complete information, including model number and vehicle serial
number.
All information, illustrations, and specifications contained in this
literature are based on the product information available at the time of
publication. The right is reserved to make changes at any time without
notice.
1. Instrument Panel Door
10. Speedometer/Odometer
2. Neutral Lockout
(Accessory)
3. Reverse Control
11. Throttle
4. Headlight Retracting Lever
12. Primer
5. Hand Brake and Parking
Lock
6. Tachometer (Accessory)
7. Choke
8. Fuel Level Gauge
9. Compression Release
13. Ignition/Lights Switch
14. Manual Starter
Handle
15. Lighter (Electric Start
Models)
Figure 1-2
1-3
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TABLE OF CONTENTS
2-1
SECTION 2
SPECIFICATIONS
SPECIFICATIONS. . . . . . • . . • . • . . . . • . 2-2
TORQUE SPECIFICATIONS ..••...•.•. 2-3
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2-2
Before
repair
your
proceeding with
or mai n tenance,
own protection
SAFJ;' . TY PRECAUTIONS ",
any
for
see
on pages: 4-8, 5-6, 6-6, 7- 6,
7-10, 8-2, 9-2, 9-6, 10-2 and
11-4.
SPECIFICATION S
CAUTION: Snow Vehicles are not manufactured for highway use and the
manufacturE!r does not represent that they are equipped with all the de-
vices legally required for such use.
Length ....... .
Width
..... . .
Height ...... .
Engine
Rating . ....
Start er
. . . . . . . . . . . . . . .. 103 inches
. ....... ...... 37 inches
47-1 / 2 inches with windshield
37 inches without windshield
OMC 2-cycle opposed twin
Maximum 30 hp at 5800 rpm
Model 25-201R and 25-201RA ...... . Manual rewind
Model 25-201RS and 25-201RSA . . . Electric and manual rewind
. . . . . . . Centrifugal operated sheave
Engages V-belt
Variable speed drive ..
Overall ratio . . . . . . . . . . . . . . . . . .. 5.07 to 1
Final drive ........................ ASA 35 double chain
Sprocket ratio . . ...... ..... .... ..... .... . .... 16 to 42
Reverse transmission ... ...... .. . ... Dog clutch and bevel gears
Muffler .•..• . ••.....••• Single, Tuned muffler for quiet operation
Brake ...... . ........ Disc type, hand operated
Throttle. . . . . . . . . . . . . .... ......... . Thumb operated
Track ... . . . . . . . . . . Specially designed fully adjustable
Width . . . . . . . . . . . . . . . . . . . . . . . . . 20.5 inches
Skis ........... Formed steel, equipped with shock-absorbing leaf
springs and replaceable wear runners
Seating capacity ...... ........ Two adults. Vinyl coated cover,
molded urethane foam cushion
Hood . .... .. '. . . . . . . . . . . . . . . Molded polycarbonate
Lighting . . . . . . . Retractable sealed beam headlight and taillight
Fuel tank. . . . . . ... Capacity 5 Imperial gallons, 6 U.S. gallons
Lubrication ......... ... 24:1 using Johnson Skee-Horse Lubricant
Carburetor Needle Adjustment
High spe ed ......... .
Low
speed ........... .
RPM Ratings
or SAE 30 SB or SD oil
One turn off seat minimum
... 1 to 1-1/4 turn off seat
Idle .................. . ....... . . 1300 -1600
Approx. 2700
Transmissi on belt engaging speed ..•.......
Maximum RPM at which neutral lockout
will operate .................... . Approx. 2000
Ignition
Breaker point gap .......................... 020 to .022
Spark plug. . . . . . . . ........ Champion J7J or equivalent
Spark plug gap . . . . . . . . . . . . . . . . . . . . .. .028 -.033 inch
Condenser capacity ...................... " .18 -.22 mfd
Magneto drive coil resistance. . . . . . . . . . . . . . . . . . . . .. .8 ohm
Ig niti on coil primary resistance ................... 1.5 ohm
Ignition coil secondary resistance . . . . . . . . . . . . . .. 10,000 ohms
Lighting coil resistance . ... ............ .. .562 to .678 ohm
Battery. . .. 12 volt Pr estolite Type 9948X or equivalent with a mini-
mum 32 ampere hour rating, and with a minimum of 2.2
minutes cold starting capacity at 150 amperes discharge,
0° Fahrenheit, and a 5-second voltage reading of 7.8
volts. Dimensions in inches are approximately 7-3 / 4
long, 5-1/ 8 wide
and 7-1 / 4 high (to top of terminals).
Weight dry 17 lbs., wet 21.4 lbs. Electrolyte to fill 0.44
U.S. gallons. Specific gravity 1.265
Engine
Bore and stroke ................. .
Piston displacement ........ .
Compression ratio ......... .
Cylinder compression ... ... .... . .
Ring
diameter
Ring thickness ................. .
Clearances
Piston - wrist pin ..... .
2-3 / 4 x 2-1 / 4 inches
26.7 cubic inches (437 cc)
.
............ 6.8 to i
Minimum 105 PSI
. . . .. 2-3 / 4 inches
. . . . . . . 1/ 16 inch
. . . . . . . . press fit
Pist on ring gap ..
. ........... , .007-.017
Piston ring - ring groove .
Cylinder - piston ..... . . . . . . . . . . . .
.. .002-.004
. .. Top of piston to cylinder .012- .015
Bottom of piston to cylinder .006- .008
Specifications and features may be changed at any time without notice
and without ob ligation towards vehicles previously manufactured.

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TORQUE SPECIFICATIONS
PART APPLICATION
SiZE TORQUE
IN. /
FT./
LBS. LBS.
*Nut Ball Joint to Steering Arm
and Steering Column 3 /8 -24 18-20
Nut Cable
to Solenoid 36-60
Screw Coil Clamp to Main Frame 35-45
*Screw Connecting Rod 29-31
*Screw Crankcase
60-80 5-7
*Nut
cYlinder to Crankcase
16 - 20
*Screw
Engine to Engine Frame
Assembly 3/ 8-16 18-20
*Nut
Exhaust Manifold to Cylinder 5/16 - 24
10-12
*Screw
FlangeUes to Frame
3/ 8-16 20-25
*Nut
Flywheel 40-45
*Screw
Engine Frame to Main
Frame
3/ 8-16
18 - 20
*Nut
Front and Rear Truck Axles
5/8 - 18
50-60
*Screw
Idler Axle to Frame
3/ 8-16 20-25
Setscrew Locking Collar
#10-32 25 - 35
Bolt and Rear Axle Pivot
Nut 5/ 16-24 12-15
Setscrew Rear Sprocket 3/ 8- 16 18-20
Nut
Rear Suspension to Frame
5/ 16-24 12-15
*Nut
Runner to Ski 5/ 16-18 90-100
*Nut U
Bolt to Saddle
10-12
Spark Plug 20 - 20-1/2
*Nut
Throttle Cable Adjusting
Screw 5/ 16 - 18
60-80
* Nut
Tie Rod 3/ 8- 24
18-20
. *Screw
Truck to Frame 3 /8 -16
25 - 30
*Screw Truck to Frame 7 /1 6-14 25-30
*Screw
Secondary End Cap to
Shaft
3/ 8- 16
22-25
Screw Drive Sprocket
1/ 4-20 15-17
*Screw Shifter Clevis to Pinion
Shaft
1/4-28
160-180
* Screw
Primary Sliding Sheave
to Hub 1/4-20 7-10
* Bolt
Primary End Cap to Main
Shaft 3/4-16 90-100
Screw
#6 7-10
Screw
#8 15 - 22
Screw
#10 25-35 2-3
Screw
#12
35 - 40
3-4
Screw 1/4 60-80 5-7
Screw 5/ 16
120- 140
10-12
Screw
3/8
220-240
18 - 20
Specifications and features may be changed at any time without notice
and without obligation towards vehicles previously manufactured.
*Use Torque Wrench
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3- 1
SECTION 3
GENERAL SNOWMOBILE INFORMATION
TABLE OF CONTENTS
TWO-CYCLE ENGINE OPERATION. . . . . .. 3-2
CARBURETION .•.•................ 3-3
IGNITION. . . . . . . . . . . . . . . . . . . . . . . .. 3-3
POWER FLOW. . . . • . . . . . . . . . . . . . . .. 3-4
PRIMARY DRIVE ................. 3-4
NEUTRAL LOCKOUT. . . . . . . . . . . . . .. 3-5
SECONDARY DRIVE .............•. 3-5
REVERSE TRANSMISSION ........... 3-5
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3-2
POWER STROKE - DOWN
COMBUSTION OF
FUEL-AIR
MIXTURE
CONNECTING COUNTER-
ROD BALANCE
AXIS OF
ROTATION
F i gur e 3-1
171 33
FUEL INTAKE AND EXHAUST
LEAF VALVES
EXHAUST
PORT OPEN
Figur e 3-2
17134
COMPRESSION STROKE - UP
FUEL FROM
EXHAUST
PORT CLOSED
Figure 3-3
17 13 5
TWO CYCLE ENGINE THEORY
An internal combustion engine is one in which fuel is burned inside the
engine: a charge of fuel is introduced into a combustion chamber
(cylinder) within the engine and ignited. The energy released by the
expansion of the burning fuel is converted to torque by the piston, con-
necting rod, and crankshaft.
Internal combustion engines are classified as either four-cycle or
two-cycle engines. The "four" and the "two" refers to the number of
piston strokes required to complete a power cycle of intake, compres-
Sion, power , and exhaust. A piston stroke is piston travel in one direc -
tion only; up is one stroke, down is another. In a four - cycle engine,
two crankshaft revolutions, or four strokes, are required for each power
cycle. In a two-cycle engine only one crankshaft revolution is required
per power cycle.
In a two-cy c le engine , the ignition of the fuel-air mixture occurs as
the piston reaches the top of each stroke. The expansion of gases drives
the piston downward (see Figure 3-1). Toward the end of the downward
stroke, ports which lead from the cylinder to the exhaust system are
uncovered. The expanding exhaust gases flow into these ports, reducing
pressure in the cylinder. Immediately after, intake ports are opened.
These ports connect the cylinder with the crankcase where a mixture
of fuel and air has been developed by carburetion. The downward motion
of the piston compresses this mixture and forces it through the intake
ports into the cylinder. See Figure 3-2.
The inrushing charge of the fuel-air mixture helps to eject (scavenge)
the last of the exhaust gases from the cylinder. At this point, the mo -
mentum of the flywheel is required to return the piston to the top of the
cylinder . As the piston begins its up-stroke, it closes the intake and
exhaust ports and begins to compress the fuel-air mixture trapped in the
cylinder. See Figure 3-3. The upward motion of the piston also re -
du
ce s the pressure in the crankcase. The resulting crankcase suction
opens leaf valves which admit a fresh charge of air and fuel from the
carburetor into the crankcase, thus preparing for the next power cycle.
Near the top of the piston stroke, the compressed fuel-air mixture is
ignited, the piston is driven downward, and the power cycle is repeated.
At full throttle, this cycle may be repeated more than five thousand
times every minute.
CARBURETION
The system which controls the intake of the fuel-air mixture in the
two cycle engine consists of a set of leaf valves which serve the same
purpose as the intake valves on a four cycle engine. The leaf valves
are thin, flexible metal strips mounted between the carburetor intake
manifold and crankcase.
When the piston is on the up-stroke, it creates a partial vacuum in
the crankcase. Atmospheric pressure forces the leaves away from the
body (see Figure 3-4), opening the passage between the carburetor and
crankcase. When the piston is on the down-stroke, it compresses the
crankcase charge, forcing the leaves against the passage opening, and
sealing off the crankcase from the carburetor. Since the opening and
clOSing may occur in excess of five thousand times per minute, the
leaves must be thin and flexible.
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Gasoline, in its liquid state, burns relatively slowly with an even
flame. . However, when gasoline is combined with air to form a vapor,
the mixture becomes highly inflammable and burns with an explosive
effect. To obtain best results, the fuel and air must be correctly pro-
portioned and thoroughly mixed. It is the fWlction of the carburetor to
accomplish this.
Gasoline vapor will burn when mixed with air in a proportion from
12:1 to 18:1 by weight. Mixtures of different proportions are required
for different purposes. Idling requires a relatively rich mixture; a
leaner mixture is desirable for maximum economy under normal load
conditions; avoid lean mixtures for high speed operation. The carbure-
tor is designed to deliver the correct proportion of fuel and air to the
engine for these various conditions.
The carburetor is essentially a Simple metering device. The float
chamber holds a limited quantity of fuel, regulated by a float valve.
Needle valves permit a precise amount of fuel to flow from the float
chamber to the carburetor throat. The upstroke of the piston creates a
suction which draws air through the leaf valves and the carburetor
throat. At a particular point the throat is restricted by a venturi (see
Figure 3-5). The venturi has the effect of reducing air pressure in the
air stream, creating a partial vacuum which draws fuel from the jet
nozzles. As it is rushed along to the firing chamber, the fuel is swirled
about in the air stream and vaporized.
A shutter or butterfly valve in the throat regulates the amount of air
drawn through the carburetor. To vary the speed of the engine, the
throttle shutter opens or closes, regulating the amount of fuel-air mix-
ture drawn into the engine.
A richer fuel mixture is required for starting a cold engine. A sec-
ond shutter, called a choke, is placed into the throat forward of the jets,
to restrict the flow of air. When the choke shutter is closed, more gas -
oline and less air is allowed into the air stream resulting in a richer
fuel-air mixture. When normal operating temperature is reached, the
choke is opened and the standard ratio of gasoline and air allowed to
flow from the carburetor.
IGNITION
The ignition system provides a high voltage electric current which
causes a spark to jump the spark plug gap within the cylinder and thus
ignite the compressed fuel-air mixture in the cylinder. The ignition
system consists of the magneto drive COilS, breaker points, and con-
denser, and the ignition coil assemblies. Permanent magnets built into
the flywheel revolve around the magneto drive coils. As the magnet
moves past the coils, the direction of the magnetic flux through the coil
is changed from one direction to the other (see Figure 3-6). Self-
inductance of the magneto drive coil circuit, completed through the
breaker pOints, prevents the flux in the coil laminations from changing
until the breaker pOints open. When the points open, the flux changes
direction very rapidly, inducing a current which flows through the
ignition coils' primary windings. The ignition coils transfrom this cur-
rent to a very high voltage which is sufficient to discharge across the
spark plugs' gap.
The lighting system coils produce alternating current which changes
in frequency and voltage in proportion to the engine speed. On models
having electric starting, this alternating current output is converted to
direct current by a diode bridge rectifier and used to charge the bat-
tery. Direct current from the battery is then used to power the head-
light and taillight, and the electric starter motor.
3-3
HOLE FOR FUEL
PUMP PULSE
ACTION
JNTAKE
MANIFOLD
GASKET
L EAVES
LEAF PLATE AND BASE ASSEM.
17088
Figure 3-4
INDUCED LOW ~VENTURI
PRESSURE
:::..~~~~ ..
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FUEL FORCED
UPWARDS
BY ATMOSPHERIC PRESSURE
Figure 3-5
P - PERMANENT MAGNET
N - NORTH POLE SHOE
S - SOUTH POLE SHOE
F - FLYWHEEL
C - LAMINATED COIL CORE
17136
- 17137
Figure 3-6

3-4
NEUTRAL LOCKOUT POSITION
Figure 3-7
A.
Neutral Lockout Plunger
B.
Emergency Starting
Sheave
C.
Garter (Activating)
Spring
D. End Cap
E.
Movable Half of Sheave
POWER FLOW
The transmission assembly transmits power from the engine to the
front axle which propels the vehicle along the track. The primary
sheave assembly is attached directly to the crankshaft. The secondary
sheave assembly has its own mounting pedestal and is larger in diam-
eter than the primary sheave assembly. The two are connected by a
transmission belt.
PRIMARY DRIVE
The primary sheave is centrifugally operated and engages the trans-
mission belt when the engine speed reaches approximately 2700 rpm.
When the engine is rotating at idle speed or below 2700 rpm, the trans-
mission belt rides on a ball bearing between the halves of the primary
sheave assembly (see Figure 3-7). The primary sheave assembly
halves are separated by a compression spring in the hub of the movable
sheave half.
As
the engine speed increases, centrifugal effect forces a garter
spring in the end cap outward against the contour of the end cap and
axially against the movable sheave half. As the sheaves are brought
together the transmission belt is forced outward to ride on a larger
diameter of the primary sheave assembly, increasing belt speed (see
Figure 3-8). Since the belt length remains constant, the secondary
sheave halves spread apart, allowing the belt to ride on a smaller
diameter. In this way, the engine transmits power through a variable
ratiO, presenting the engine with a mechanical advantage most favorable
for the speed at which it is operating.
ENGAGED POSITION
17171
171n
Figure 3-8
F. Transmission Belt
M. Splined Shaft
G.
Fixed Half of Sheave N. Bolt, End Cap to Splined
H.
Crankshaft Shaft
I.
Ball Bearing
O.
Spring
J.
Spring Cup
P. Neutral Lockout Rod
K.
Compression Spring
R.
Spring
L. Neutral Lockout Balls
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NEUTRAL LOCKOUT
A neutral lockout mechanism is used to prevent the drive from en-
gaging during starting, warm-up period, and idle. When the neutral
lockout plunger is actuated, a cone on the end of the plunger raises two
balls through the splines of the primary sheave assembly and into the
path of the movable sheave half, preventing it from engaging the belt.
The neutral lockout is spring actuated and will engage only when the
engine is below approximately 2000 rpm.
When the engine is running above approximately 2000 rpm, the
garter spring will expand by centrifugal effect. See Figure 3-9. The
garter spring will then ride up the ramp of the end cap and push the
movable sheave toward the fixed sheave. In doing this, the movable
sheave has covered the holes in the splined shaft. When the neutral
lockout knob is now pushed in, the plunger cannot move in because of
the interference by the neutral lockout balls. Spring (R) will therefore
be compressed as shown in Figure 3-9. If the engine speed is now
reduced to idle, 1300-1600 rpm, the garter spring will close and allow
the movable half of the sheave to move away from the fixed sheave.
Spring (R) will then push the plunger inward. The neutral lockout balls
will then move outward, through the splined shaft. The movable sheave
will now be locked out in the neutral position. See Figure 3-7.
SECONDARY DRIVE
The secondary drive mechanism incorporates a torque sensing device
that detects the need for more power for steep inclines or deep snow.
The mechanism immediately forces the secondary sheaves closer to-
gether to lower the transmission ratio and provide a higher torque to
the drive chain and track.
The drive ratio varies from 3.3 to 1 in low to .67 to 1 in high which
yields an overall drive range to approximately 5 to 1. Power is trans-
mitted from the secondary sheave assembly through a drive chain to
the front axle.
The ratio between the secondary sheave assembly and the front axle
is 16:42. Optional sprockets are available to change these ratios for
special applications.
REVERSE TRANSMISSION
The reverse gear is designed as part of the secondary drive. In
"Forward" gear, the input shaft drives the output shaft directly by
means of a "dog" type clutch. In "Reverse" gear, the dog clutch is
released while a gear set engages to reverse rotation of output shaft.
Shifting must be done with the engine at idle and machine at rest.
POSITION AT APPROXIMATELY
2000 RPM WITH NEUTRAL LOCKOUT
ROD DEPRESSED
Figure 3-9
17275
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4-1
SECTION 4
TROUBLE SHOOTING
TABLE OF CONTENTS
DESCRIPTION. . . . . . . . . . . . . . . . . . . . .• 4-2
COMPRESSION . . . . . . . . . . . • . . . . . • • •• 4- 2
SPARK PLUGS .•.....•.•.•......... 4-3
TROUBLE SHOOTING PROCEDURES. . . . . .. 4-4
TROUBLE SHOOTING GUIDE ........... 4- 7

4-2
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
)
)
)

(
r
(elongate) as engine temperature rises during operation. Insufficient
gap clearance will cause the ring to bend or warp as it flexes and ex-
pands; excessive gap clearance will permit loss of compression.
Compression leakage may also occur at the spark plugs. A cracked
spark plug insulator will cause similar trouble. Although compression
is primarily dependent on the piston, rings, and cylinder, these other
sources of leakage should be investigated when compression loss is
noted.
Compression leakage will occur if the compression relief valve link-
age is adjusted with insufficient clearance on the cable ends. The re-
lief valves vent combustion chamber pressure through a by-pass port
into the exhaust system.
Compression may also be affected by the fuel induction and exhaust
systems. Since the fuel vapor is first compressed in the crankcase,
leakage here will affect engine performance. Possible trouble spots
include leaf valve assemblies, seals between crankcase halves, and
crankshaft bearing seals. Exhaust ports which have become clogged
because of excessive deposits of carbon will hinder the efficient transfer
of exhaust gases.
Excessive carbon build-up on piston heads or elsewhere in the cyl-
inder walls can result in a loss of power.
Following the trouble check chart provided at the end of this section
and the recommended tune-up procedures given in Section 5 will assure
that all areas affecting fuel induction, compression, and exhaust will be
considered as part of every trouble shooting procedure. An engine with
low
or uneven compression cannot be successfully tuned for peak per-
formance. It is essential that improper compression be corrected be-
fore proceeding with an engine tune-up.
SPARK PLUGS
The spark plug provides a gap inside the combustion chamber across
which the high voltage from the ignition coil can be discharged. The
resulting spark ignites the compressed mixture of fuel vapor and air in
the cylinder. See Figure 4-1.
Spark plugs are made in a number of heat ranges to satisfy a variety
of operating conditions. The heat range of a spark plug refers to its
ability to dissipate heat from its firing end to the cylinder head. The
heat range established for any spark plug is determi.ned in design by the
length of the path which the heat from the tip must travel to the thread
and seat area where it is transferred to the cylinder. Spark plugs
having a short gap between the firing end of the center electrode and the
thread and seat area are used for hot running engines (see Figure 4-2).
Snowmobiles
used in heavy load conditions (ie. deep snow or sled
towing) will run the engine temperature higher, and in this case, a
colder plug might be recommended. Spark plugs operating under these
conditions must remain cool enough to avoid preignition and excessive
gap erosion. Spark plugs having a long gap transfer heat sl ower and are
used on cooler running engines. See Figure 4-3. Cooler running en-
gines have a relatively low combustion chamber temperature, therefore
a high spark plug temperature must be sustained in order to burn off
normal combustion deposits and avoid fouling. For most effective
sparking through any rpm range and under all conditions of operation,
the electrode and insulator tip temperature must be kept high enough
to vaporize or burn off particles of fuel mixture which collect on the
insulator. Low plug temperatures result in electrode fouling by an
accumulation of unburned fuel particles, carbon bits, sludge, etc. Se-
lection of the correct spark plugs for an engine depends on the type of
service to which it will be subjected. A cold running engine will require
a hot plug and a hot running engine, a cold plug. Spark plug recom-
mended for use on the Ski-Horse snowmobile is the Champion J7J or
equivalent. See page 7-8 for additional information on spark plugs.
GROUND
ELECTRODE
Figure 4-1
COOL PLUG
Figure 4 -2
HOT PLUG
Figure 4-3
4-3
CENTER
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17139
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4-4
TROUBLE SHOOTING
PROCEDURES
Trouble shooting to determine the cause of any
operating problem may be broken down into the
following steps:
a. Obtaining an accurate description of the trouble.
b. Quick tune-up.
c. Use of Trouble Check Chart to analyze engine
performance.
An accurate description of the trouble is essential
for trouble shooting. The owner's comments may
provide valuable information which will serve as a
clue to the cause of the problem. Find out pertinent
facts such as:
a. Correct spark plugs
b. Throttle linkage properly adjusted
c. Tank filled with fresh, clean fuel of the proper
mixture
d. Spark at each spark plug
e. Carburetor adjusted correctly
f. Compression. Turn flywheel by hand or with
recoil starter. If compression is present, it
can be felt when turning through one complete
revolution of the flywheel. If little or no com-
pression exists in both cylinders, engine will
spin very easily.
STARTING
1. Hard to start or won't start
a. Empty gas tank
b. Incorrect gas-lubricant ratio
c. Old fuel, or water or dirt in fuel system
d. Fuel line improperly connected
e. Fuel line kinked or severely pinched
f. Engine not primed
g. Clogged fuel line or fuel filter
h. Clogged check valve
i. Carburetor adjustments too lean
j. High speed needle bent or bowed
k. Engine flooded
1. Leaf valves not functioning properly
m. Faulty gaskets
n. Spark plugs fouled, improperly gapped, dirty
or broken
o. Loose or broken wire or frayed insulation in
electrical system
p. Sheared flywheel key
q. Faulty coils
r. Faulty condenser
s. Binding in engine
t. Weak or reversed polarity of flywheel magnets
2. Engine won't crank over
a. Cylinder wall corrosion
b. Broken connecting rod, crankshaft, or drive
shaft
c. Engine improperly assembled after repair
3. Cranks over extremely easily
a. Spark plug loose
b. Cylinder or pistons scored
c. Hole burned in piston head
d. Rings worn
4. Won't start, but kicks back and backfires
a. Flywheel key sheared
b. Timing out of adjustment
c. Leaf valves broken or not seating
STARTING -MANUAL STARTER
1. Manual starter pulls out, but starter does not
engage flywheel
a. Friction spring bent or burred
b. Excess or incorrect grease on pawls or spring
c. Pawls bent or burred
d. Pawls frozen (water) in place
2. Starter rope does not return
a. Recoil spring broken or binding
b. Starter housing bent