Evinrude Bobcat E251C User manual

25 HORSEPOWER · MODELS E251C E251M
32 HORSEPOWER · MODEL E251 HP
743 10-70 PART NO_ 261888 LlTHO U.S.A. © OUTBOARD MARINE CORPORATION 1970 ALL RIGHTS RESERVE:D

AVAILABLE FROM YOUR
EVINRUDE DEALER
FUEL RECOMMENDATIONS
The correct fuel mixture ratio is 24 parts of a good grade gasoline to
one part oil. Use regular gasoline in Model 251 and premium gasoline
in the 251 HP. For ease of measurement, this is equivalent to one
quart of oil to six gallons of gasoline, one pint of oil to three gallons of
gasoline, or 1/ 3 pint of oil to each gallon of gasoline.
Use only Evinrude Lubricant or a reputable auto motive engine oil,
SAE 30 MS or MM. Avoid use of low price third grade (ML light duty)
oils. DO NOT USE MULTIPLE VISCOSITY OILS, SUCH AS 10W30, OR
ANY OUTBOARD MOTOR OILS OTHER THAN EVINRUDE OUTBOARD
LUBRICANT.*
*EVEN THOUGH EVINRUDE 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 aSEPARATE,
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 24:1.
Again,
thoroughly agitate the mixture. A clean funnel equipped with a
fine screen should be used when pouring the fuel mixture 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 it is necessary to mix fuel and lubricant at temperatures
below O°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 0 °F.
Do not use kerosene or fuel oils for pre-mixing.

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SECTION 1
INTRODUCTION
INTROOUCTION
SPECIFICATIONS
GENERAL SNOWMOBILE INFORMATION
TROUBLE SHOOTING
TUNE-UP PROCEOURES
FUEL SYSTEM
IGNiTrON AND
ELECTRICAL SYSTEM
MANUAL STARTER
ENGINE
~ __ O_R_IV_E_T_R_A_IN __________________ ~
STEERING, TRACK ANO SUSPENSION
LUBRICATION ANO STORAGE

1-2
17000
Figure 1-1
The snow machine has been designed and built for dependable, high )
performance. It is important to every snow machine owner to be able
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 1971 model 251 and 251HP snowmobiles. Pictured be-
low left is the model 251 and to the right, the high performance, 251HP.
An effort has been made to produce a manual that will not only serve
as a ready reference book for the experienced service man, but will
also provide more basic information for the guidance of the less ex -
perienced 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.
17022
Figure 1-2
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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
inter mixed. This makes it unnecessary for the experienced service-
man to reread discussions of theory along with specific service in-
formation. illustrations 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 snow machine owner, and thereby build or
maintain areputation for reliable service.
This service manual covers all phases of servlcmg the 1971 snow-
mobiles, without reference to other information; 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 Depart-
ment for additional help. Always be sure to give complete information,
including model number and vehicle se rial 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.
17002
1. Hood Latch (Wing Nut)
6. Hand Brake with
2. Neutral Lockout Knob Parking Lock
3. Headlight Connector 7. Throttle Lever
4. Ignition/Lights Switch 8.
Primer
5. Compression Relief
9. Manual Start Handle
Figure 1-3
1-3
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SECTION 2
SPECIFICATIONS
TABLE OF CONTENTS
SPECIFICATIONS ............•.•... 2- 2
TORQUE SPECIFICATIONS . . . . . . . • . . . . 2-3
2-1

2-2
For your own protection, before
proceeding with any repair or
adjustments on these snowmo-
biles, see
on pages 4-7,6-6,7-7, 8-2, 9-2,
9-6 and 11-4.
SPECIFICATIONS
CAUTION: Snowmobiles are not manufactured for highway use and the
manufacturer does not represent that they are equipped with all the de-
vices legally required for such use.
Length Model 251 ........................... 96 inches
Model 251 HP . . . . . . . . . . . . . . . . . . . . . .. 98 inches
Width ........................... 32-9 /16 inches
Height Model 251 ........................... 42 inches
Engine
Rating
Model 251 HP . . . . . . . . . . . . . . . . . . . . .. 37-1/2 inches
. . . . . . . . . . . . . . . . . . OMC 2-cycle opposed twin
Model
251 .............. Maximum 25 HP at 5300 RPM
Model 251 HP .......... . . Maximum 32 HP at 6200 RPM
Starter ................................ Manual, rewind
Clutch ............... Centrifugal operated sheave engages belt
Drive ..................... Variable-speed, V-belt and chain
Drive Sprocket Ratio (Standard) . . . . . . . . . . . . . . . . . . . .. 16 to 30
Options .. . . . . . . . . . . . . . . . . . .. 11, 13, 15 or 18 to 30
Transmission ......... Variable speed, 3:1 to 1:1 ratio Forward only
Mufflers Model 251 ... Single Tuned engine muffler for quiet operation
Model 251 HP .. Dual Tuned engine mufflers for maximum
Brake .
Throttle ....
Track .... .
Width:
.. .
Skis ..... .
performance
· . . . . . . . ....... Disc type, hand operated
· . . . . . . . . . . . . . . . . . . . . . . . . thumb operated
· . . . . . . . . . Specially designed fully adjustable
· . . . . . . . . . . . . . . . . . . . . . . . . .. 15-3/8 inches
. . . . . . Formed steel, equipped with shock-absorbing
Seating capacity ...
leaf springs and replaceable wear runners
.. .... Two adults. Vinyl coated twill cover,
molded urethane foam cushion
Hood ................. Molded fiberglass with built-in headlights
Lighting ........................ 2 Headlights, and taillight
Fuel tank . . . . . . . . . . . . . . . . . . . . . . . . . .. 6 U.S. gallons
Fuel Model 251 ..................... good grade regular
Model 251 HP . . . . . . . . . . . . . . . . . . good grade premium
Lubrication. . . . . . . . . . . . . . 24:1 using Evinrude 24 to 1 lubricant or
SAE 30 MM or MS oil
Carburetor Needle Adjustment
High speed ........ .
Low
speed .............. .
RPM Ratings
. . . . . .. 1 turn off seat
1 to 1-1/4 turns off seat
Idle ................................... . 1100 -1300
2300 -2400
2600 -2800
Transmission Belt Engaging Speed Model 251 ....... .
Model 251
HP
Ignition
Breaker point gap. . . . . . . . . . . . . . . . . . . . . . . . .. .022 to .024
Spark plug Model 251 ........... Champion J7J or equivalent
Model 251 HP ........ Champion J4J or equivalent
Spark plug gap. . . . . . . . . . . . . . . . . . . . . . . . .. .028 -.032 inch
Condenser capacity ................... .18 -.22 mfd
Magneto driv~ coil resistance. . . . . . . . . . . . . . . . . . .. .8 ohm
Ignition coil primary resistance ...... . . . . . . . . . . . .. 1. 5 ohm
Ignition coil secondary resistance . . . . . . . . . . . .. 10,000 ohms
Lighting Coil resistance .... .678 to .562 ohm
Engine
Bore and stroke ...... '. . . . 2-3/4 x 2-1/4 inches
Piston displacement . . . . . . . . . . . . .. 437 cc or 26.7 cubic inches
Compression ratio Model 251 ..................... 7 to 1
Ring
diameter ....
Ring thickness . . . .
Cylinder Compression
Model 251 HP . . . . ......... 8 to 1
. . . . . . . . . . . . . . . . .
.. 2-3/4 inches
. . . . . . . . . . . . . . . . . . . .. 1/16 inch
Model 251 ......................... .
minimum 105 PSI
minimum 120 PSI
Model 251 HP .................. .
Clearances
Piston - wrist pin . . . . . . . . . . . . . . . . . . ....... Press fit
Piston ring gap. . . . . . . . . . . . . . . . . . . .007 - .017
Piston ring - ring groove . . . . . . . . . . . . .002 -.004
Cylinder - piston ........... Top of piston to cylinder .012 -.014
Bottom of piston to cylinder .006 -.010
Specifications and features may be changed at any time without notice and without obligation
towards vehicles previously manufactured.
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TORQUE SPECIFICATIONS
PART APPLICATION
SIZE TORQUE
IN./
FT. /
LBS. LBS.
Screw
Brake Hand Lever
#10-32 13-15
Screw
Coil Clamp to Main
Frame
35-45
* Screw
Connecting Rod 29-31
* Screw
Crankcase
60-80
6-7
* Nut
Cylinder Base
16-20
* Screw
Engine to Engine Frame
Assembly
33-38
* Nut
Flywheel
40-45
* Nut
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
5/8-18
20-25
Bolt and Nut Rear Axle Pivot
12-15
Setscrew Rear Sprocket
18-20
Nut Rear Suspension to
Frame
12-15
* Nut
Runner to Ski
90-100
* Nut
Saddle to Spring
3/8-24
25-30
Spark Plug 20-20-1 / 2
Nut Throttle Control Cable
30-40
* Nut
Tie Rod 3/ 8-24
18-20
* Screw
Truck to Frame
(Cleated Track Models)
3/ 8-16
25-30
* Screw
Truck to Frame
(Molded Track Models)
7/16
25-30
* Nut
Steering Arm to
Ski Column 3/ 8-24
18-20
* Bolt (Early
Primary Bolt to End Cap
70-80
MOdels)
* Bolt (Later
Primary Bolt to End Cap
90-100
MOdels) (Has
annular groove in
Screw
head). See Figure 2-1
#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
2- 3
ANNULAR GROOVE
1 7192
Figure 2-1

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SECTION 3
GENERAL SNOWMOBILE INFORMATION
TABLE OF CONTENTS
TWO-CYCLE ENGINE THEORY . . . . . . . . . . 3- 2
CARBURETION. . . . . . . . . . . . . . . . . . . •• 3-3
IGNITION . . . . . . . • . . . • . . . . . • . . . . . •. 3-3
POWER FLOW. . . . . . . . . . . . • . . . . . . . .. 3-4
3-1

3-2
POWER STROKE - DOWN
COMBUSTION OF
FUEL-AIR
AXIS OF
ROTATION
Figure 3-1
17133
FUEL INTAKE + EXHAUST
LEAF VALVES
CLOSED
EXHAUST
PORT OPEN
Figure 3-2
17134
COMPRESSION STROKE - UP
LEAF VALVE FUEL FROM
PORT CLOSED
17135
Figure 3-3
TWO CVCLE ENGINE THEORV
An interna! combustion engine is one in which fuel is burned inside the
engine: acharge of fuel is introduced into a combustion chamber (cyl-
inder) within the engine and ignited. The energy released by the expan -
sion of the burning fuel is converted to torque by the piston, connecting
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 apower 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-cycle 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, re-
ducing 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 down-
ward 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
momentum 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
reduces the pressure inthe 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. In the snow machine engine, the leaves
do not have to seat exactly flush with the body to permit normal operation.
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 function 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 mete ring device. Needle
valves permit a precise amount of fuel to flow to the carburetor throat.
The upstroke of the piston creates a suction which dra ws 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 is allowed to
flow
from the carburetor.
IGNITION
The ignition system provides the electrical energy which jumps the
spark plug gap within the cylinder and ignites the compressed fuel-air
mixture in the cylinder. The ignition system consists of the magneto
drive coils, breaker points, condenser, and the ignition coil assemblies.
3-3
HOLE FOR FUEL
INTAKE
MANIFOLO
STOP LEAVES
LEAF PLATE ANO BASE ASSEM.
170 88
Figure 3-4
-
~~~~~
FUEL FORCEO UPWAROS
BY ATMOSPHERIC PRESS URE
Figure 3-5
P - PERMANENT MAGNET
N - NORTH POLE SHOE
S - SOUTH POLE SHOE
F - FLYWHEEL
17 13 6
C - LAMINATEO ' COIL CORE
1 7137
Figure 3-6

3-4
PRIMARY SHEAVE
ASSEMBLY
Figure 3-7
A - BELT
682301 R I
B - IDLER BALL BEARING
C - COMPRESSION SPRING
D - GARTER (ACTIVATING )
SPRING
E - END CAP ASSEM
F - MOVABLE HALF OF SHEAVE
G - FIXED HALF OF SHEAVE
H - NEUTRAL LOCKOUT BALLS
I - CRANKSHAFT
~
BEL T ENGAGED
DRIVE
POSITION
PRIMARY SHEAVE ASSEMBL Y
682.30Z Rl
Figure 3-8
A permanent magnet is built into the flywheel. The magnet passes by
the magneto drive coil inducing a current flow through the drive coil
(see Figure 3-6). When the points open, the current flows into the pri-
mary of the ignition coil causing a magnetic flux build -up around the
secondary windings of the ignition coil. When the points elose, the flux
field collapses cutting through the secondary windings resulting in a high
voltage in the secondary. This high volta ge is discharged across the
spark plug gap.
The
lighting system coils produce alternating current which changes
in frequency and voltage in proportion to the engine speed.
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 is mounted on the chain case and is larger in diameter
than the primary assembly. The two are connected by a transmission
belt.
The primary sheave is centrifugally operated and engages when the
engine speed reaches approximately 2300-2400 RPM for the Model 251
and 2600-2800 for the Model 251 HP. When the engine is rotating at idle
speed or below the speed stated above, the transmission belt rides on a
idler 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 half of the sheave.
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 half of sheave. As the sheaves are brought
together the transmission belt is forced outward to ride on a larger di-
ameter of the primary sheave assembly, increasing belt speed (see Fig-
ure 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, pre-
senting the engine with a mechanical advantage most favorable for the
speed at which it is operating.
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 will operate only when the engine is at idle speed.
The secondary drive mechanism incorporates a torque sensing device
that anticipates a need for more power for steep inclines or deep snow.
The
overall ratio from the crankshaft of the engine to the secondary
sheave is approximately variable from 3:1 to 1:1. Power is transmitted
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 16:30. Optional sprockets are available to change these ratios for
special applications (see page 2-2).

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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 machine 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 lubrication,
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 du ring
engine operation, clearance or gap must be provided between the ring
ends to prevent butting. The ring gap also allows the ring to expand
(elongate)
as engine temperature rises du ring operation. Insufficient
)
J

L
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 ac ross
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-l.
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 determined 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 transfer red 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 rurming engines (see Figure 4-2).
Snowmobiles used in heavy load conditions (ie. deep snow or sIed
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 slower and are
used on cooler rurming engines. See Figure 4-3. Cooler rurming 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 rurming engine will require
a hot plug and a hot running engine, a cold plug. Spark plug recom-
mended for use on the Model 251 is the Champion J7J or equivalent.
The Champion J4J or equivalent is recommended for use on Model 251
HP Snowmobile. See page 7-9 for additional information on spark plugs.
GROUND
ELECTRODE
Figure 4-1
COOL PLUG
Figure 4-2
HOT PLUG
Figure 4-3
4-3
17 138
17139
17140

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 -u p.
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. With compression relief valve closed, turn fly-
wheel by hand or with recoil starter. If com-
pression is present , it can be feit when turning
through one complete revolution of the flywheel.
If hUle or no compression exists in both cyl-
inders, 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. Fuelline improperly connected
e. Fuel line kinked or severely pinched
f. Engine not primed
g. Clogged fuel line or fuel pick up in tank
h. Clogged check valve
i. Carburetor adjustments too lean
j. High speed needle bent or bowed
k. Leaf valves not functioning properly
1. Faulty gaskets
m. Spark plugs fouled, improperly gapped, dirty
or broken
n. Loose or broken wire or frayed insulation in
electrical system
o. Sheared flywheel key
p. FauIty coils
q. Faulty condenser
r. Binding in engine
s. Weak or reversed polarity of flywheel
magnets
t. Engine flooded
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 be nt
c. Loose or missing parts

( 3.
Clattering manual starter
a. Friction spring be nt or burred
b. Starter housing bent
c. Excess or incorrect grease on pawls or
spring
d. Dry starter spindie
RUNNlNG -LOW SPEED ONLY
1. Low speed miss
a. Incorrect gas - lubricant ratio
b. Carburetor idle adjustment too lean or too
rich
c. Leaf valve standing open or preloaded shut
d. Spark plugs improperly gapped, dirty, or
broken
e. Loose or broken ignition wires
f. Spark plug terminal loose
g. Weak coil or condenser
h. Breaker points burned, dirty or improperly
gapped
i. Cylinder gasket or leaf plate gasket blown
j. Leaking crankcase seals
k. Coil lead grounded on chassis
RUNNING -HIGH SPEED ONLY
1. High speed miss
a. Water in fuel
b. Spark plug he at range incorrect
c. Spark plugs improperly gapped or dirty,
cracked insulator
d. Ignition wires loose or broken or faulty
insulation
e. Coil or condenser weak
f.
Breaker points burned, dirty, or improperly
gapped
g. Engine improperly timed
h. Combustion chambers carboned or fouled
2.
Poor acceleration, low top rpm
a. Incorrect gas - lubricant ratio
4- 5
b. Old fuel
c. Fuel hoses plugged or kinked
d. Fuel pick up restricted
e. Fuel pump faulty
f. Incorrect carburetor mixture adjustments
g. Inlet needle and seat worn or sticky
h. Timing out of adjustment
i. Spark plugs dirty or improperly gapped
j. Loose, broken, or badly insulated high ten -
sion leads
k. Coil or condenser weak
1. Breaker points worn or improperly gapped
m. Leaf valves not properly seated, or broken
n. Piston rings stuck or scored
o. Excessive carbon on pistons and cylinder
head
p. Compression relief valve improperly ad -
justed or faulty
q. Carburetor high speed needle set too lean
3. Idles weH, but acceleration poor, dies at fuH
throttle
a. Incorrect gas - lubricant ratio
b. Fuel lines or passages obstructed
c. Fuel pick up closed
d. Faulty fuel pump
e. High speed nozzle or jet clogged
f. Dirt or packing behind needles and seats
g. Choke partly closed
h. High or low speed ne edle set too lean
i. Breaker points burned, dirty, or improperly
gapped
j. Timing out of adjustment
k. Fuel cap vent clogged
4. Engine runs at high speed only by using hand
primer
a. Fuel lines or passages obstructed

4-6
b. Fuel line leaks or fuel pick up obstructed
c. Fuel pump not supplying enough fuel
d. Dirt or packing behind needles or seats
e. Carburetor adjustments
f. Fuel cap vent clogged
g. Leaf block gasket reversed
RUNNING -HIGH AND LOW SPEED
1. Engine overheats
a. Incorrect gas lubricant ratio or improp-
erly mixed fuel
b. Engine not assembled correctly during re-
pair (binding)
c.
Lean mixture (carburetor adjustment)
2. Engine stops suddenly, or freezes up
a.
No lubricant in gas, or no fuel
b. Fuel connection faulty
c. Cylinder or crankshaft scored
d. Bent or broken rod, crankshaft, or stuck
piston
e. Ignition failure
f. Frozen bearing
3. Engine knocks excessively
a. Incorrect gas - lubricant ratio
b. Spark plug - wrong heat range
c. Flywheel loose
d. Crankshaft end play excessive
e. Carbon in combustion chambers and exhaust
ports, or on pistons
4.
f. Worn or loose bearings, pistons, rods, or
wrist pins
g. Loose assemblies, bolts, or screws
h. Manual starter not centered
Excessive fuel consumption
a. Hole
in fuel pump diaphragm
b.
Carburetor casting porous
c.
Deteriorated carburetor gaskets
d. Hole
in mete ring diaphragm
e.
Carburetor improperly adjusted
f. Carburetor flooding
5.
Vibrates excessively or runs rough and smokes
a. Too much lubricant mixed with gas
b. Idle or high speed needles too r ich
c. Air filter obstructed
d. Faulty ignition
6.
No power under heav~ load
a.
Faulty carburetion
b.
Faulty ignition
c. Breaker
points improperly gapped or dirty
d. Ignition timing too far retarded
e. Carbon build -up on piston head
(see Engi ne
Section)
f.
Cylinder scored or rings stuck
g. Compression relief valve open
h. High speed adjustment lean
i.
Spark plugs fouled or misfi ri ng
)