Lawn-Boy M Series User manual

LAWN-BOY M-SERIES WPM SERVICE MANUAL
Table of Contents – Page 1 of 3
INDEX
GENERAL INFORMATION
INTRODUCTION
SAFETY SYMBOLS AND NOTES
2 CYCLE PRINCIPLES OF OPERATION
1ST CYCLE - DOWN STROKE
2ND CYCLE - UP STROKE
MOWER SPECIFICATIONS
ENGINE SPECIFICATIONS
MODEL NUMBER INDENTIFICATION
"M"-SERIES FEATURES
LUBRICATION (OIL)
GASOLINE
TORQUE REFERENCE
STORAGE PROCEDURE
REMOVING FROM STORAGE
CARBURETION
OPERATION
AIR FILTERS
CARBURETOR REMOVAL
CARBURETOR DISASSEMBLY
CARBURETOR ASSEMBLY
CARBURETOR INSTALLATION
FRESHLUBE OIL INJECTION
OPERATION
INSTALLATION
ADJUSTMENT
TESTING
CHECK VALVE TEST
GOVERNOR CONTROL AND LINKAGES
OPERATION
ADJUSTMENT
ELECTRICAL SYSTEM
IGNITION SYSTEM
SPARK PLUG
IGNITION TROUBLESHOOTING
INTERLOCK SYSTEM
INTERLOCK SYSTEM TROUBLESHOOTING
ELECTRIC START SYSTEM

LAWN-BOY M-SERIES WPM SERVICE MANUAL
Table of Contents – Page 2 of 3
ELECTRICAL SYSTEM - continued
WIRING DIAGRAMS
STARTER
MANUAL STARTER - TOP MOUNT
REMOVAL
DISASSEMBLY
REASSEMBLY
INSTALLATION
ELECTRIC STARTER
REMOVAL
DISASSEMBLY
REASSEMBLY
INSTALLATION
ADJUSTMENT
POWER HEAD
REMOVAL FROM DECK
DISASSEMBLY
REASSEMBLY
TESTING
BLADE AND BLADE HOUSING
BLADE REMOVAL
BLADE SHARPENING
BLADE BALANCING
BLADE TRACKING
BLADE INSTALLATION
BROKEN OR CRACKED HOUSING
HANDLE AND CONTROL REPLACEMENT AND ADJUSTMENTS
THROTTLE CONTROL CABLE
SELF-PROPELLED CABLE
INSTALLATION
FLYWHEEL BRAKE CABLE (ZONE START MODELS)
REMOVAL
INSTALLATION
GEAR SELECT CABLE
BLADE BRAKE CLUTCH (BBC) CABLE
HANDLE WIRE HARNESS
HANDLE REPLACEMENT
HANDLE SHAFT ASSEMBLY REPLACEMENT

LAWN-BOY M-SERIES WPM SERVICE MANUAL
Table of Contents – Page 3 of 3
DRIVE SYSTEM
OPERATION
REMOVAL
DISASSEMBLY
DISASSEMBLY OF AXLE SHAFT ASSEMBLY
REASSEMBLY
SLIDER
SHIFT COLLAR ASSEMBLY
SHIFT KEY ASSEMBLY
AXLE SHAFT ASSEMBLY
TRANSMISSION CASE ASSEMBLY
REDUCTION SHAFT ASSEMBLY
INSTALLATION
DRIVE BELT REPLACEMENT
ZONE AND ELECTRIC START MODELS
BLADE BRAKE CLUTCH (BBC) MODELS
FLYWHEEL BRAKE SYSTEM
OPERATION
REMOVAL
INSTALLATION
FLYWHEEL BRAKE CABLE ADJUSTMENT
BLADE BRAKE CLUTCH (BBC)
OPERATION
REMOVAL
INSTALLATION
ENGINE TROUBLE SHOOTING
ENGINE TROUBLESHOOTING CHARTS
TOOLS AND SERVICE AIDS
"M" SERIES SERVICE TOOLS
SERVICE SHOP AIDS
MOWER MONITOR

LAWN-BOY
"M" SERIES
SERVICE
MANUAL
Part No. 492-0420

GENERAL INFORMATION
Introduction
1-1
Safety Symbols and Notes
1-1
Principles of Operation
1-3
Mower Specifications '
..........
Engine Specifications ..... .......... 1-4
Model Number Identification
1-5
"M" Series Features
1-6
Lubrication
1-7
Gasoline
1-8
Torque References
.......... 1-9
Storage Procedure
1-11
Removing From Storage
1-13
Rev 5-90

INTRODUCTION
All photographs, illustrations and specifications contained in this manual are based on the latest
product information available at the time of publication and are subject to change without notice.
SAFETY SYMBOLS AND NOTES
Twosymbolsare used in this manualtoattract your attentionto importantinformation.
A
SAFETY WARNING: Failureto obey a safety warningmay result in injury to
yourself or others.
The purpose
of the safety symbol is to attract your attention to possible danger. The
symbol, and the explanationwith
it, deservesyour carefulattention. Understanding a
safety warning, by itself, does not eliminate the danger. The Instruction or warning it
gives is not a substituteforproperaccidentprevention measures.
NOTE Thissymboladvisesyouof information important to theoperationormaintenance
of your equipment.
Safety is No accident. .. . BeAlert!
Never let shop rags, used for cleaning, lay around to become fire hazards.
Always use safety glasses when servicing or inspecting the mower.
Always disconnect spark plug wire to prevent accidental starting when setting up,
Always store gasoline or flammable solvents used for cleaning in closed containers
Before cleaning, servicing or inspecting mower, make certain blade andallmoving parts
Never operate mower without proper safety protective devices inplace.
Regularlycheckbladeandenginemountinghardwareforpropertightness.
Never store mower with gasolineinthe tank insideabuilding where fumes may reach an
Allow engine to cool before storing in any enclosure.
adjusting or making repairs to the mower.
specifically designed for that purpose.
have stopped and engine and exhaust assemblies have cooled down.
/-
open flame or spark.
1-1

DO NOT run engineinan enclosed area. Exhaust, gasses contain carbon monoxide, an
odorless and deadly poison.
To reducefire hazard, keep the engine free of grass, leaves, or excessive grease.
If the mower should start to vibrate abnormally, stop engine and check immediately for the
cause. Vibrationisgenerally a warning
of trouble
To prevent high voltage shock
DO NOT check for spark with spark plug or spark plug wire
removed. Use an approved tester.
DO NOT tamper with governor springs, governor rods, levers or other parts whichmay
increase the governed engine speed.
Be sure allparts are secure before starting mower.
Pull starter cord slowly until resistanceis
felt. Then pull rapidly to avoid kickback and
prevent handor arm injury.

2 CYCLE PRINCIPLES OF OPERATION
I Transfer Transfer
Port
31 SCAVENGING COMPRESSION/
INTAKE
Lawn-Boy "M" Series engines are 2 cycle design. They are piston ported loop scavenged
design.Thereareonly2strokes
of thepistontocompletetheentirepowercycle in one
revolution of the crankshaft. Four major events occur during each revolution of the crankshaft.
They are: Power, Exhaust, Scavenging, and Compression/IntakeIgnitionof theair-fuel
mixture will occur during compression. During
the 1st Cycle (first 1/2 revolution) Power and
Exhaust occur. During the 2nd Cycle (second1/2revolution) Scavenging and Compression/
Intake occur. Near Top Dead Center Ignition occurs and a fresh charge ofair-fuel mixtureis
drawn into the crankcase.
1stCYCLE Down Stroke
Power Theburningair-fuelmixtureexpands,pushingthepistonandconnecting
exhaust port, powerwill continue. The intake port has dosed.
Exhaust When the exhaust port begins to open, the intake port has closed. As
the piston continues to move down, the exhaust portisopened and
the burnt gases flow out the exhaust port. At the same time the volume
of the crankcaseisdecreasing, theair-fuel mixtureinthe crankcase is
increasinginpressure until the piston uncovers the transfer ports.
rod down,causingthecrankshafttorotate.Untilthepistonopensthe
2nd CYCLE Up Stroke
Scavenging Near the bottom of the down stroke and during the beginning of the up
stroke, the transfer ports open. The pressureishigher inthe crankcase
than inthe combustion chamber, theair-fuel mixtureinthe crankcase
moves from the crankcase through the transferports into the combustion
chamber. This action purges the remaining exhaust gases from the
combustion chamber replacingitwith fresh air-fuel mixture.
Compression/ As the piston moves upward it closes the transfer ports and then the
Intake exhaustport.Withtransferandexhaust ports dosed, thevolumeofthe
combustion chamber decreases and the air-fuel mixtureiscompressed.
Just before top dead center, ignition of the air-fuel mixture occurs to
begin the next complete cycle. At the same time the piston skirt opens
the intake port and the volume
of the crankcaseisincreasing. With higher
air pressure outside the carburetor thaninthe crankcase, the air rushes
inmixing with fuelinthe carburetor and moves into the crankcase.
1-3

MOWER SPECIFICATIONS
M21ZPR
M25,5 H.P. commer-
cial grade, 2cycle
engine. Cast iron
cylinder sleeve, large
diameter forged
steel
crankshaft, forged
steel connecting rod,
upper and lower ball
bearings, mechanical
governor, paper air
filter with pre cleaner.
MODEL
ENGINE
M21 ZMR
M25,5 H.P. commer-
cial grade, 2 CYCLE
engine. Cast iron
cylinder
sleeve, large
diameter forged steel
crankshaft, forged
steel connecting rod.
upper and lower
ball
bearings, mechanical
governor, paper air
filter with pre cleaner.
MOWER TYPE
Rear Bagger dedicated
Rear Bagger dedicated
MOWERDECK Magnesium,
21' cut, 7
cut heights 1'- 3 1/2'
Magnesium, 21' cut, 7
cut heights 1' 3 1/2'
Three speed gear
drive
DRIVE Push
STARTER Top mount Top mount
COMPLIANCE
SYSTEM
Zone Zone
FUEL CAPACITY
1.6 quart
1.6 quart 1.6 quart
HANDLES 3-position. adjustable,
extruded aluminum 3-positionadjustable,3-position,adjustable,
extrudedaluminumextrudedaluminum
8' Front and Rear,
radial tread
8' Front and Rear,
radial tread
Standard, rear Standard,rear
%position, adjustable,
extruded aluminum
B' Front and Rear,
radial tread
WHEELS
B' Front and Rear,
radial tread
CATCHER standard,rear standard, rear
ENGINE SPECIFICATIONS
HorsePower 5 HP @ 3600RPM
PISTON RINGS
ThicknessTopRing.06081.0592 ThicknessBottomRing.0625/.0615EndGap(InCylinder)TopRing.0071.021
End Gap (InCylinder)BottomRing.0071.021
Bore
Stroke
Displacement Cu. In.
Displacement CC's
Compression Ratio
Compression (normal) Cold
Hot
Compression (minimum)
Cold
Governor Speed High
Governor Speed Low
Fuel/OilRatio
2.380
2.00
8.84
145
6.96:l
115-135psi
110-130 psi
90 psi
3100-3300 RPM
2100-2300RPM
64:1
CYLlNDER
Diameter (Inside) 2.381/2.380
CRANKCASE
DiameterTopMainBearing 2.0469/2.0462
Through BoreIn
DiameterLowerMainBothEnds
of
BearingCylinder-Crankcase Assembly
CRANKSHAFT
Bearing Journals Top and Bottom
Crank Pin
Wrist Pin diameter
.98401.9835
.7874/.7870
.5630/.5628
FLYWHEELAIRGAP
C.D. Module Air Gap
Alternator Air Gap
SPARKPLUGGAP
Type Spark Plug: Champion
Canada: Champion
Spark Plug Gap
.010
.010
PlSTON
Diameter (Right Angle
Wrist Pin Hole-Bottom
of Skirt)
Diameter (Parallel with Wrist Pin
Hole Bottom of Skirt)
Diameter WristPinHole
2.3792/2.3782 CJ-14
QJ-12
.035
2.3752/2.3742
.5635/.5631
1-4

MODEL NUMBER INDENTIFICATION
A new numbering system was introduced for walk-behind mowers starting with the 1989
Lawn-Boy models. The "M" Series Mowers use
this new model numbering system.
SERIES: SERVICE CODE
LLAWN-BOY(STEELDECK)AZ(NotUsed
S = SUPREME
M
= M SERIES
C
= COMMERCIAL
on first
productionModels)
GRASS CATCHER ASSEMBLY:
N = NONE
S SIDECATCHER
R = REARCATCHER
TYPE
Z ZONE MANUAL
E ELECTRIC START
V = 110V. ELECTRIC
C
= COMMERCIAL
B BBC (Blade Brake Clutch)
DRIVE SYSTEM:
P PUSH
S SELF-PROPELLED
M MULTIPLE SPEED SELF-PROPELLED
Example:
M21ZPR in a "M" Series, 21" cut, manual zone start, push unit with rear grass catcher assembly.
1-5

"M"-SERIES FEATURES
SELF-PROPELLED
CONTROL BAIL KEY SWITCH
(ELECTRIC START)
BLADE-BRAKE CONTROL
LEVER (BBC)
THREE POSITION
SELF-PROPELLED
SPEED CONTROL
REAR GRASS
CATCHER
SAFETY INTERLOCK
SWITCH COMMERCIAL ENGINE
AIR CLEANER
8" RADIAL TREAD
FRONT AND REAR
TIRES

LUBRICATION (OIL)
Use only Lawn-Boy%-Cycle Ashless Oil. It is
formulatedtogivebestengineperformance
with less combustion chamber deposits, least
pistonvarnish,providemaximumsparkplug
life,ensurecorrosionprotectionandprovide
positive lubrication.
You canbesureyouareprovidingthebest
possible protection for your Lawn-Boy mower
whenusingLawn-Boy2-CYCLEASHLESS
OIL.
NOTE: Avoid the use of the following as they could damage your engine and/or shorten spark
plug life:
Automotivecrankcaseoils
Premixfuelofunknown oilquantity
Premixfuelricherthantherecommendedratio
Premixfuelleanerthantherecommendedratioof 64:l
All M-25 engines equipped with FreshLube™ oil injection DO NOT require premixed fuel/oil
mixture.
NOTE: OIL INJECTION MODELS: Avoid mixing different brands ofoil in the reservoir if atall
possible. Some oils (especially some low ash oils) when mixed together will form
soap deposits
which will clog the tank outlet screen reducingor stopping the flow of oil. Thiswill cause severe
engine damage.
All M-25enginesequippedwithFreshLube™oilinjectionrequirea 64:l fuel/oilmixture.
To achieve a 64:l fuel/oilratio:
Use:
4 ozs. oil with 2 gallons gasoline
8 ozs. oil with 4 gallons gasoline
20 ozs. oil with 10 gallons gasoline
NOTE: Running or storing an engine on a leaner than recommended fuel/oilmixture may
resultinenginedamageduetocorrosion.
To assure thorough mixing of oil and gasoline:
Fillapprovedcontainerpartiallywithgasoline
Add recommendedamountof oil
Shakecontainertomix
Slowly open container to relieve the pressure
Add balance of gasoline and shake vigorously to assure mixing
NOTE: If other than Lawn-Boy 2-CYCLE ASHLESS OIL is ,used, enginelifecouldbereduced
and potential failures could occur.
If engine failure occurs andithas been determined that Lawn-
Boy 2-CYCLE ASHLESS OIL or its equivalent was not used, the failure may not be covered by
the Lawn-BoyWarranty
1-7

Unacceptable Fuel: Do not use any regular unleaded, regular leaded, or premium unleaded
gasoline having more than
10% ETHANOL or more than 5% METHANOL even if it contains
cosolvents or corrosion inhibitor and regardlessof octane rating.
Alcohol extended fuels may result in engine running quality problems such as vapor lock, fuel
"starvation", or moisture absorption do to phase separation on premix fuel.
If running quality
problems are experienced, switch to a gasoline without alcohol.
Alcohol attracts and holds moisture which may cause corrosion of metallic parts of the fuel
system.
Phase Separation
Phase separation causes the engine to run without oil even thoughoil
was mixed properly to begin with. Engine failures due to lack of proper
lubrication are not covered by warranty.
To Engine
! Gasoline
1% By Volume Oil Mix
Water Absorption
Alcohol Water
Emulsion
of Water
and
Oil
To Engine
NOTE: Metallic and nonmetallicpartsof the entire fuelsystem should be inspected frequently
and replaced if signs of excessive stiffness, deterioration corrosion, or fuel leakageare found.
Inspect at least annually.
SAFETYWARNING:Fuelleakagecancontributeto a fire or explosion

TORQUE REFERENCE
M* Mower FastenerTorque Speclflcatlons
Description
Engine Group
Nut Flywheel
Locknut Flywheel Brake Assembly
Screw Starter Mounting
Screw CD Ignition Mounting
Screw Cylinder Head to Block
Screw Shroud to Cylinder
Screw Muffler Mounting
Screw Carburetor Mounting
Carburetor Nozzle
Carburetor Main Jet
Carburetor Pilot Air Jet
Carburetor Bowl Screw
Screw Fuel Tank Mounting
Screw Governor Lever
Screw Governor Bracket to Block
Screw Fuel Tank Bracket to Block
Bolt Air Filter Base to Cylinder
Bolt Shoulder Brake Arm to Actuator
Screw Muffler Guard to Cylinder
Screw Mounting Plate to Block
Bolt Shoulder Brake Actuator Crankcase
Screw Shoulder Muffler Guard
Bolt Shoulder Brake Arm to Crankcase
Screw Cylinder to Crankcase
Screw Muffler Guard to Bracket
Screw Pulley to Crankshaft
Spark Plug
Screw Oil Pump Mounting
Screw Governor Pivot
Screw Engine Mounting Plate to Block
Nut Oil Level Switch
Screw Electric Starter to Block
Screw Alternator Mounting
Screw Starter Assembly
Screw Electric Starter Cover Plate
1-9
-I-
Assy Torque
IN.-LBS.
390
75
25
100
190
65
170
65
7
6
60
55
30
155
155
6:
65
75
75
155
155
25
170 75
25
95
175
75
25
155
5
170
75
70
20
----
Assy Torque
FT-LBS.
33
6
2
8
16
5
14
5
0.6
0.5
0.5
5
5
3
13
13
5
6
6
13
13
2
6
14
2
8
15
6
2
13
14
6
6
2
Assy Torque
N.M
44
8
3
11
22
7
19
7
0.8
0.7
0.7
7
6
4
18
18
7
8
8
18
18
3
8
19
3
11
20
8
3
18
0.6
19
8
8
2

Description
Screw Chute Interlock Switch
(Stainless Steel)
Screw Access Door
Screw Baffle to Housing
Screw Magnet Mounting (Brass)
Screw Height Adjustor to Housing
Bolt Height Adjustor to Housing
Bolt Wheel
Nut Blade
Screw Drive Cover to Housing
Screw Mounting Plate to Housing
Screw Clutch Cable Anchor to Housing
Screw Belt Guide to Housing
Screw Blade to Clutch
Screw Clutch to Crankshaft
Screw BBC Clutch Mounting
Screw BBC Interlock Switch
Screw BBC Cable Adjustment
Screw BBC Cable Clevis
Screw Battery to Access Door
Screw Throttle Cable Bracket
Nut Transmission Input Pulley
Screw Transmission Cover
Screw Shift Cable Bracket to Transmission
Screw Shift Cable Clamp
Nut Self-Propelled Cable
Screw Handle Shaft to Handle Assembly
Screw Handle Console Cover
Screw Bail Pivot
Screw Rope Guide
Screw Bail Block Mounting
Nut Flywheel Brake Cable
Assy Torque Assy Torque Assy Torque
IN.-LBS.FT.-LBS.N.M.
34 3034 15
1 2
1318 2128 2028 4763 2
3
1318 5
7
605 7
32 43
I
8
6 75
7
5 65 3
2
20
4
3 30
22
20
7
5 60 8 6 75
3
2
25 19
14
170
1-10

STORAGE PROCEDURE
Store the mowerin a dry place. Clean out and
wash
off grass bag. Hang grass bag and let dry
completely before storing.
A
SAFETY WARNING: Do not store
gasoline (fuel) unnecessarily over
long periods of time.
To prevent possible
explosion, storeonly
In approved "safe"
contalners.
To prevent explosion ofvapor-
izedfuel, donot store themowerwlthfuel
In
the tank or carburetor in an enclosurewlth
an open flame. (Example: furnace or water
heater pilot light).
Allow theengine to cool
before storlng
in any enclosure.
Add Lawn-Boy
2 + 4®Fuel Conditioner to
stabilizefuelstored(Followinstructionson
container);availablefromyourLawn-Boy
dealer.
If possible, the last fuel fill of the year
should be treated with Lawn-Boy Fuel Condi-
tioner.
Do not store more than 3 months.
Plan ahead
and try to run the mower near empty
when mowing for thelasttime prior to storage.
The smaller the amount of fuelinthe tank, the
shorter the period required to run the fuel sys-
tem dry.
To runfuelsystem dry, move the mower to a
safe outside location and start the mower and
run until the engine stops.
Close the fuel shut-off valve.

NOTE: Oil Injection Models: To prevent pos-
siblepowerheaddamageresultingfromair
enteringoillineduringservicing;werecom-
menddrainingfueltankand running mower
using
a 64:1 fuel/oilpre-mix until no air bubbles
are observed in oil lines. Minimum
20 minutes.
If battery is fullychargedmakesurecase is
clean and place in storage.
If the batteryisina discharged condition
it should be charged before placinginstorage.
Never allow a discharged battery to sit more
than
24 hours without charging, damage to the
battery will result.
To charge battery, plug the
adapter end of the Lawn-Boy charger supplied
into the receptacle at the handle console and
plug the charger into a 115 voltAC outlet.
Allow the battery to charge for 10-12 hours. Do
notchargebatteryformorethan
12 hours
maximum. Battery should be recharged after
6
months of storage for 10-1 2 hours maximum.
Disconnectthebatterychargerfrommower
during storage.
SAFETYWARNING:Whencharging
a batteryuseonlytheLawn-Boy
charger supplied with the mower. If proper
charger is not used, damage may occur to
the battery and the battery may expiodeand
result in injury to you or others.
Remove the spark plug. Using an oil can,
squirtaboutonetablespoon ofLawn-Boy
2-
CYCLE ASHLESSOIL into the spark plug hole.
Replace the sparkplugbut do not connect the
spark plug lead.
Rotate
the engineseveraltimes,using
rope starter to distribute the oil.

REMOVING FROM
STORAGE
Remove the spark plug.
Spintheenginerapidlyusingthe pull rope
starter to blow excess oil from cylinder. Clean
the spark plug. Replace
if itscracked, broken or
badly burned. Install the spark plug, but do not
connect spark plug lead.
Check and tightenall fasteners.
Non-OilInjectionModels:Fillfueltank
with fresh fuel/oil mixture.
Oil Injection Models: Fill the fuel tank with
fresh gasoline and the
oil reservoir with Lawn-
BOY 2-CYCLE
ASHLESS OIL.
Open fuel shut-off valve.
Reconnect
the spark plug lead.
NOTE: Oil injection Models: To prevent pos-
siblepowerheaddamageresultingfromair
enteringoillineduringservicing;werecom-
menddrainingfueltank and runningmower
using
a 64: 1 fuel/oil pre-mix until no air bubbles
are observed in oil lines. Minimum
20 minutes.
1-13

CARBURE;TION
Operation
Air Filter
Removal
Assembly
Installation.
Disassembly.
2-2
2-4
2-5
2-6
2-9
2-11

REF.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
DESCRIPTION
Gasket
Gasket
Gasket
Gasket
Float
Shaft Float
Valve Inlet Needle
Clip
Pilot Jet
Main Jet
Nozzle
Seal Throttle Shaft
Seal Choke Shaft
Screw
Screw
Pivot Upper
Pivot Lower
REF.
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
2-1
DESCRIPTION
Idle Needle
Idle Spring
Idle Speed Screw
Idle Spring
Spacer
Fuel Line
Gasket
Air Filter Base
Bolt
Screw
Air Filter
Pre Filter
Air Filter Cover Assembly
Pushnut
Screw
Air filter Cover
Air Box Cover
Clamp