O.M.G. OPPOSED TWIN - Snowmobile Fanatics

ENGINES
Mercury/O.M.C.
OVERHAUL
Refer to Fig. 8 for exploded view of
crankshaft, crankcase and associated
parts. Cylinder is closed end type, install with exhaust ports to bottom.
Crankshaft and center main bearing
housing are removed from fanwheel
end of crankcase after removing fanwheel & housing, cylinders, rod &
piston units, flywheel, and the two
center main bearing through-bolts.
TIGHTENING TORQUES. Recommended tightening torques are as follows:
Fanwheel Cap Screw
40-45 ft.-lbs.
Flywheel Nut
60-65 ft.-lbs.
Spark Plug
20-ft.-lbs.
Backplate Nuts
200 in.-lbs.
Blower Housing
130 in.-lbs.
Carburetor Mtg. Nuts
130 in.-lbs.
Center Main Allen Screw .. 25 in.-lbs.
Center Main
Phillips Screw
Center Main
Retaining Bolt
Cylinder to
Crankcase Nuts
Connecting Rod Nuts
Manifold Nuts
Mounting Plate Bolt
PTO Side
Reed Stop Screws
Trigger Mtg. Plate Screws
60 ia.-Ibs.
110 in.-lbs.
350 in.-lbs.
180 in.-lbs.
75 in.-lbs.
200 in.-lbs.
20 in.-lbs.
. 75 in.-lbs.
O.M.G.
OPPOSED TWIN
CONDENSED SERVICE DATA
tJore—Inches
Stroke^Inches
No. of Cylinders
Displacement—(cc)
Cubic Inches
Horsepower (a RPM
Carburetor Model
Number Used
2.5
2.25
2 Opposed
362
22
16 (?< 4800
Carter
or Bendix
1
2.625
2.25
2 Opposed
399
24.35
21 (a 6000
OMC
2.75
2.25
2 Opposed
437
26.7
25 (a 4500
HD
Ignition:
Type
^ ^ - ^ ^ Energy Transfer*
Point Gap—Inch
0.020-0.022 O.U2U-0.022 0.020-0.022
Edge Gap
—Fixed
Fixed
Fixed—
Timing Advance?
Yes
Yes
Yes
Timing BTDC (mm)
- S e e Text
See Text
See T e x t Spark Plug:
Champion
J8J
J7J
J6J*
Electrode Gap—Inch . . . 0.028-0.030 0.028-0.033 0.028-0.030
Fuel/Oil Ratio
See Text
See Text
See Text
*1974 models use CD system and champion UL 77V surface gap plug which has no gap adjustment.
349
ENGINES
O.IVI.C.
MAINTENANCE
CARBURETOR. A Carter Model N
carburetor was used on models prior to
1968. Late 16 horsepower models use
the Bendix carburetor. Twenty-five
horsepower units use a Tillotson HD
carburetor, refer to overhaul section
under SERVICE FUNDAMENTALS
for overhaul data. Carburetors are similar in configuration and in adjustment
except for idle speed stop adjustment.
Refer to Fig. 1. Clockwise rotation of
either adjustment screw will lean the
mixture. Initial adjustment of high
speed mixture screw (1—Fig. 1) or low
speed mixture screw (2) is iy4 turns
open from the closed position. High
speed mixture needle (1) must be open
at least 1 1/6 turns to assure adequate
lubrication. Recommended slow idle
speed of 1500 rpm is adjusted by idle
speed stop screw.
With carburetor inverted and fuel
bowl removed, float should be parallel
with gasket surface of carburetor body.
Adjust by bending tab on float which
contacts needle valve. Refer also to
CARTER portion of SERVICE FUNDAMENTALS section of this manual.
REED VALVES. Reed valves (Fig.
3) used on 16 hp models can be removed
for service or inspection when carburetor is off by removing primer line
from fitting (1) and unbolting and removing intake manifold (2).
Examine the six valve reeds (4) visually. Do not attempt to straighten a
bent reed or to bend reeds by hand to
change performance. Renew reeds or
reed body (3) if any damage is apparent.
On 25 Hp models, twin vee valve
bodies are used as shown in Fig. 3A.
Leaf unit, leaf stop and valve body is
only available as an assembly and the
entire unit should be renewed if any
part is worn or damaged.
FUEL PUMP. The diaphragm type
Fig. 1-Assembled view of eariy carburetor
showing points of adiustment. Late models are
similar except for idle speed adiustment screw
(5).
350
fuel pump (Fig. 4) attaches to intake tank. Outlet line (2) connects to fitting
manifold (2—Fig. 3) and is actuated by (1—Fig. 3) located in inlet manifold,
pressure and vacuum pulsations in the by-passing the regular fuel pump and
crankcase. The fuel pump (2—Fig. 4) is carburetor.
To check the operation of primer
available only as a complete assembly,
however, filter unit (3 through 7) can pump, disconnect outlet hose (2—Fig.
be removed for cleaning or parts re- 5) at manifold. Depress plunger rod (6);
newal. NOTE: Parts can be assembled if a spurt of fuel is not ejected through
with gasket (3) reversed, but pump will outlet hose, check inlet line, outlet line
not work. Be sure gasket is properly and check valve (3) for plugging and, if
trouble is not corrected, remove, disasinstalled.
FUEL PRIMER. All models are semble and inspect the pump unit
equipped with a plunger type fuel using Fig. 5 as a guide.
MAGNETO AND TIMING. The mag
primer pump (Fig. 5). Inlet fuel line (1)
tees into main fuel line beneath fuel neto on all early models is the energy
transfer type, with low tension magneto driver coils beneath the flywheel
generating the primary current, and
individual automotive type ignition
coils connected in series to provide the
secondary current. Timing is controlled
by a single set of points located on
magneto stator plate and a centrifugal
advance breaker cam mounted on inner
side of flywheel provides a retarded
spark for starting only.
On 1974 models, CD ignition systems
are used. System is similar to that
described in Engine Sections for Rotary Combustion and Alternate Firing
Twin engines, and those sections can
be used for service procedures. On all
models, both spark plugs fire together.
3
4
Fig. 3-Bxploded view of inlet manifold and reed
valves.
1. Primer fitting
2. Manifold
3. Reed plate
4. Valve reed
5. Retainer
Fig. 4'Diaphragm type fuei pump (2} is available as an assembiy oniy, but fliter unit is
serviced.
1.
2.
3.
4.
Fig. SA-Expioded view of intake manifold and
(w/n vee ieaf vaive unit used on 25 horsepower
modeis.
i check valve
3. Leaf plate
2. Manifold
4. Leaf assembly
Gasket
Fuel pump
Gasket
Screen
5. Cap
6. Washer
7 Screw
Fig, 5-Exploded view of primer pump which
bypasses carburetor for coid starting.
1. Fuelline
2. Primer line
3. Outlet valve
4. Pump body
5. Pump plunger
6. Primer rod
ENGINES
O.M.C.
Timing is not adjustable except that,
on models with points, minor adjustment can be made by varying the point
gap. The fixed breaker contact point
is insulated and movable point is
grounded. To adjust or renew the
breaker points, first remove flywheel
using a suitable puller. Refer to
Figs. 6 and 7. To adjust the point
gap, remove retaining clamp and withdraw breaker cam (1—Fig. 6) from flywheel and temporarily install cam over
end of crankshaft. Turn cam to widest
point opening and adjust gap to 0.0200.022. Reinstall breaker cam in flywheel, making sure teeth of advance
weight and cam are fully meshed as
shown in Fig. 6. Turn crankshaft until
flywheel key is pointing directly away
from breaker point mounting post,
then reinstall flywheel assembly.
When flywheel is properly seated,
outer edge will be approximately 1/32inch above crankshaft shoulder. If difficulty is encountered in installing flywheel, do not use force. Breaker cam
may be catching on breaker point rub
block or oiler wick; recheck crankshaft
position and cam installation and eliminate the trouble before proceeding further. Tighten flywheel nut (5) to a
torque of 40-45 ft.-lbs.
Timing may be checked with a
timing light with engine at slow idle
speed (1500 rpm). One cooling fin on
flywheel (4) contains a raised boss and
paint mark. The marked fin should be
visible through timing hole (Fig. 8)
using a timing light, when timing is
correct.
Magneto drive coil resistance is 0.8
ohm; ignition coil primary resistance is
1.5 ohms.
LUBRICATION. Engine is lubricated by oil mixed with the fuel.
Johnson or Evinrude Snowmobile Oil
or standard grade, SAE 30 automotive
motor oil should be used. Do not use
inferior grade automotive oils or multiviscosity oil. Outboard motor oil is not
recommended. Use only good grade
regular gasoline.
Recommended fuel/oil ratio was 16:1
in 1965 and 1966; and 24:1 in 1967
through 1972. In 1973 a 50:1 fuel/oil
r a t i o is recommended ONLY if
JOHNSON or EVINRUDE 50:1 Snowmobile Oil is used. If other oils are
used, mix at standard 24:1 ratio. Mix
fuel and oil together in a separate
container before pouring mixture into
fuel tank. For cold weather blending,
pre-mix the oil with gasoline and shake
thoroughly until the mixture is liquid;
then blend in remainder of fuel. Do not
use kerosene or fuel oil for pre-mixing.
REPAIRS
TIGHTENING TORQUES. Recommended tightening torques are as follows:
Flywheel nut
40- 45 ft.-lbs.
Connecting rod
screws
29- 31 ft.-lbs.
1
2 3
Fig. $-Exploded view of ftywheel assembly
showing timing advance cam and associated
parts. Bottom view is shown in inset.
1. Breaker cam
2. Spring
3. Advance weight
4. Flywheel
5. Flywheel nut
Cylinder head
100-120 in.-lbs.
Crankcase screws . . . 60- 80 in.-lbs.
Spark plugs
20 ft.-lbs.
DISASSEMBLY AND REASSEMBLY. Refer to Fig. 9 for an
exploded view of engine. When service
is required, first remove engine as outlined in vehicle section of this manual
and remove carburetor, drive sheave,
starter and flywheel.
Remove exhaust manifold (1), rear
fan housing and cylinder shrouds; cylinders (2), intake manifold (16) and
reed valve (15). Drive out the three
tapered alignment pins (18) toward
flywheel side of crankcase, remove
crankcase screws and separate the
crankcase halves, using a rawhide
mallet if necessary to break gasket
seal. If crankcase halves cannot now be
separated, heat bearing area with a
heat gun or other means, until crankcase halves can be removed.
When reassembling, remove dried
cement and old gasket material from
joint faces, using a suitable solvent if
necessary. Mating surfaces of crankcase and gasket surfaces of cylinders
must be absolutely flat and free from
nicks and burrs. Surfaces may be
lapped if necessary, to remove minor
surface imperfections. Use new gaskets, seals and a suitable sealant when
parts are reassembled. Crankcase
halves should be heated to approximately 350° F. to allow easy entry of
main bearings into crankcase bores.
Tighten crankcase screws finger tight,
then install the three tapered pins from
flywheel side to assure perfect alignment of crankcase halves. With tapered pins installed, tighten crankcase
retaining screws to recommended
torque.
Crankshaft end play should not exceed 0.025 inch. If end play is excessive
install shims on drive side of crankshaft between bearing and thrust face
of shaft.
Use a suitable hinge-type ring compressor when installing cylinders.
INTAKC MANirOLO
PUCLOAUQC
AND CAP
CHECK
VALVK
ruCU PUMP
AND FILTCR
Fig. SA-Schematic view of fuei primer assembly used on late modeis.
Fig. 7-Expioded view of tow tension magneto.
\. Ignitior
2. Conden
Condenser
3. Point set
4. Lighting coils
5. Oiler wick
Fig. 8-View of engine showing location of
timing hole.
351
ENGINES
O.M.C.
Tighten cylinder retaining stud nuts
alternately in a criss-cross fashion to
the recommended torque of 100-120
inch-pounds. Complete the assembly
by reversing the disassembly procedure,
PISTONS, RINGS AND CYLINDERS. Pistons, rings and connecting
rods may be installed either side up
when new. Used assemblies should be
marked before disassembly and reassembled in the same relative position.
Piston skirt should be round at room
temperature, and should be rounded if
distorted, before piston to cylinder
clearance is measured. Measure clearance at right angles to piston pin at
bottom of cylinder. On engines used in
1972 and later models, piston rings are
positively positioned in ring grooves by
locating pins. On earlier engines, rings
are not pinned and ring end gaps
should be staggered during piston installation. Check the components
against the values which follow:
Piston ring end gap
0.010-0.013
Piston ring
side clearance
0.001-0.002
Piston skirt clearance .. 0.009-0,012
COMPRESSION RELIEF. Beginning in 1969, engines are equipped
with a compression relief valve which
is installed in one cylinder only. When
knob is pulled (for starting) part of the
compression escapes into the exhaust
port. Be sure to push knob in after
engine starts and make sure some
clearance exists at valve-end of cable
with knob pushed in. Adjust at cable
housing anchor.
PISTON PIN. Piston pin is a press
fit in piston boss bores at room temperature, and is retained in piston by snap
rings. Piston pin needle bearing uses
connecting rod bore and piston pin for
bearing races. Check piston pin for
heat discoloration, ridging, scoring or
wear in needle bearing area. Install
retaining snap rings with marked (lettered) side to outside.
CONNECTING ROD. A 5/16-inch,
12 point deep socket is required to
remove the connecting rod cap screws.
Rod and cap are marked for correct
assembly as are the needle bearing
cage halves. Use special OMC Needle
Bearing Grease (Part No. 378642) or a
non-fibrous, low melting point grease
to position the loose needle rollers
when installing connecting rod. Bearings, cages, and connecting rod must
turn freely on crankpin after cap
screws are tightened. Tighten screws
evenly to a torque of 29-31 ft.-lbs.
When installing bearings on shaft,
support the shaft under the counterweight next to bearing.
MANUAL STARTER. The manual
starter can be removed for inspection
or repair by removing the four retaining cap screws and lifting off the
starter unit.
To disassemble the removed starter
unit, refer to Fig. 10. Pry anchor (11)
from handle grip (12) and remove the
anchor; then allow starter rope (13) to
retract into handle and housing until
recoil spring (10) is completely unwound. Remove the spindle retaining
nut, washers and through-bolt, then
lift out spindle, pulley and associated
parts, allowing recoil spring (10) to
remain in housing (14) if possible.
Starter rope (13) is 7/32-inch nylon
rope 72%-inch in length. Fuse both cut
ends with an open flame to prevent
unraveling. Inner end is retained in
pulley (9) by a single knot while outer
end is secured by anchor (11). A new
rope can be installed without disassembling the starter if pieces can be removed. Tie a knot in one end of rope
CRANKSHAFT AND BEARINGS. The crankshaft is carried in two
ball-type main bearings which are a
press fit on crankshaft and a tight fit in
crankcase halves at room temperature.
f1
Fig. lO^xploded view of recoii Starter showing
component parts
1.
2.
3.
4.
5.
6.
7.
Ratchet mount
Ratchet
Spindle
Spring washer
Friction ring
Spindle bushing
Retainer
8.
9.
10.
11.
12.
13.
14.
Pawl
Pulley
Recoil spring
Anchor
Handle
Rope
Housing
Fig. ^-Exploded view of 362 cc engine showing component parts. Larger engine is similar.
1.
2.
3.
4.
5.
Exhaust manifold
Cylinder
Piaton rings
Piston
Piston pin
352
6.
7.
8.
9.
10.
Retainer
Needle bearing
Connecting rod
Needle roller
Bearing cage
11.
12.
13.
14.
16.
Crankcase half
Crankshaft seal
Main bearing
Crankshaft
Reed plate
16.
17.
18.
19.
20.
Intake manifold
Rod cap
Dowel pin
Crankcase half
Chain oiler line
Fig. tf-OMC eiectric starter soienoid.
1. Plunger
2. WincSngs
3. Motor terminal
4. Battery terminal
5. Contact disc
ENGINES
O.M.C.
and place starter engine-side up on a
bench or in a vise. Turn pulley (9)
counter-clockwise until spring (10) is
completely wound; then let pulley unwind approximately one turn until
rope hole in pulley aligns with exit hole
in housing (14). Tie a slip knot in free
end of rope to prevent spring from
completely unwinding, while handle
grip (12) and anchor (11) are installed.
ELECTRIC STARTER. The electric starter system consists of the
starter, solenoid and circuitry. The solenoid, as shown in Fig. 11, opens and
closes the circuit between the battery
and starter motor. The solenoid winding, when energized, causes the contact
disc to move against the terminal contacts by magnetic pull on the solenoid
plunger. The starter motor drive pulley
engages the starter motor drive belt by
rotation of the motor armature. When
the engine is running the pulley
sheave is driven faster than the armature and becomes disengaged.
BELT TENSION. Belt tension must
be correct. Refer to Fig. 13. A loose belt
will slip and a tight belt will ruin the
starter through overspeeding. With the
belt between the pulleys, turn the
t
Fig. 12-Expioded view of
etectric starter motor assembly.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
17
II
Commutator end head
Brush
Brush spring
Brush plate
Thrust washer
Armature
Spirol pins
Field coil assy.
Adjusting hracket
Adjusting spacer
Drive end head
Drive end bearing
Rear belt housing
Stationary pulley
Movable pulley
Belt
Front belt housing
Housing seal
movable pulley counter-clockwise to
drive position. Move starter upward to
tighten belt. Pulley halves should be
closed and in drive position for this
adjustment. Belt must be free when
pulley is in fully clock-wise or running
position.
STARTER TEST DATA. Test data to
determine starter condition are as follows:
Volts
12
Rotation
CW
Brush spring tension (oz.)
35-90
No-Load Test
Volts
6
Amperes
60
RPM
4200
STARTER
BELT
TENSION
ADJUSTMENT
fBELT GUIDE STUD
Fig. 13-View of starter belt adjustment.
353