Yamaha MZ250 User manual

MZ250
MZ300
SERVICE MANUAL
7VB-F8197-10LIT-19616-01-80
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FOREWORD
This manual was written by the Yamaha Motor
Powered Products Co., Ltd. primarily for use
by Yamaha dealers and their qualified mechan-
ics. It is not possible to put an entire mechan-
ic’s education into one manual, so it is
assumed that persons using this book to per-
form maintenance and repairs on Yamaha
Multi-Purpose Engine have a basic under-
standing of the mechanical precepts and pro-
cedures inherent to Multi-Purpose Engine
repair technology. Without such knowledge,
attempted repairs or service to this model may
render it unfit for use and/or unsafe.
Yamaha Motor Powered Products Co., Ltd. is
continually striving to further improve all mod-
els manufactured by Yamaha. Modifications
and significant changes in specifications or
procedures will be forwarded to all Authorized
Yamaha dealers and will, where applicable,
appear in future editions of this manual.
TIP
This Service Manual contains information
regarding periodic maintenance to the
emission control system. Please read this
material carefully.
HOW TO USE THIS MANUAL
PARTICULARLY IMPORTANT
INFORMATION
Particularly important information is distin-
guished in this manual by the following nota-
tions.
This is the safety alert symbol. It is used to
alert you to potential personal injury haz-
ards. Obey all safety messages that follow
this symbol to avoid possible injury or
death.
WARNING
A WARNING indicates a hazardous situa-
tion which, if not avoided, could result in
death or serious injury.
NOTICE
A NOTICE indicates special precautions
that must be taken to avoid damage to the
machine or other property.
TIP
A TIP provides key information to make proce-
dures easier or clearer.
MZ250/MZ300
SERVICE MANUAL
©2012 by Yamaha Motor
Corporation, U.S.A.
1st Edition, September 2012
All rights reserved. Any reprinting or
unauthorized use without the written
permission of Yamaha Motor
Corporation, U.S.A.
is expressly prohibited.
LIT-19616-01-80

HOW TO USE THIS MANUAL
This Service Manual contains general information on service procedure and data for the Multi-
purpose engine. For complete information on service procedure and data, it is necessary to use
this Service Manual together with the Supplementary Service Manual.
Comprehensive explanations of all installation, removal, disassembly, assembly, repair and check
procedures are laid out with the individual steps in sequential order.
9 The manual is divided into chapters and each chapter is divided into sections. The current sec-
tion title is shown at the top of each page 1.
9 Sub-section titles appear in smaller print than the section title 2.
9 To help identify parts and clarify procedure steps, there are exploded diagrams at the start of
each removal and disassembly section 3.
9 Numbers are given in the order of the jobs in the exploded diagram. A number indicates a dis-
assembly step 4.
9 Symbols indicate parts to be lubricated or replaced 5.
Refer to “ILLUSTRATED SYMBOLS”.
9 A job instruction chart accompanies the exploded diagram, providing the order of jobs, names of
parts, notes in jobs, etc. 6.
9 Jobs requiring more information (such as special tools and technical data) are described
sequentially 7.
9 These are general tightening torques for all Multi-purpose engines. Tightening torque specifica-
tions for each model are provided on the Supplementary Service Manual 8.
3-21
ENG
CYLINDER HEAD COVER, CYLINDER HEAD
12 Nm (1.2 m · kgf, 8.7 ft · lbf)
50 Nm (5.0 m · kgf, 36 ft · lbf)
1st
2nd
6
5
4
3
1
2
78
9
9
78
7 Nm (0.7 m · kgf, 5.1 ft · lbf)
11 Nm (1.1m · kgf, 8.0 ft · lbf)
11 Nm (1.1m · kgf, 8.0 ft · lbf)
20 Nm (2.0 m · kgf, 14 ft · lbf)
2.3 Nm (0.23m · kgf, 1.7 ft · lbf)
Order Job/Parts to remove Q’ty Remarks
Removing the cylinder head cover
and cylinder head
Remove the parts in the order listed
below.
Fuel tank Refer to “FUEL TANK, CANISTER” on 3-5.
Muffler Refer to “MUFFLER” on 3-7.
Air filter Refer to “AIR FILTER” on 3-3.
Carburetor Refer to “CARBURETOR” on 4-1.
Flywheel cover Refer to “FLYWHEEL” on 3-13.
1 Spark plug cap 1
2 Spark plug 1
3 Breather hose 1
4 Cylinder head cover 1
5 Cylinder head cover gasket 1
6 Cylinder air shroud 1
7 Locknut 2
8 Adjuster 2
9 Screw 2
CYLINDER HEAD COVER, CYLINDER HEAD
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3-23
ENG
CYLINDER HEAD COVER, CYLINDER HEAD
CHECKING THE PUSH ROD
1. Check:
9 Push rod runout
Runout limit:
Refer to “SPECIFICATIONS” in
the Supplementary service
manual.
Out of specifications → Replace.
CHECKING THE CYLINDER HEAD
1. Check:
9 Cylinder head combustion chamber
Check the combustion chamber for car-
bon deposits.
Any carbon deposits → Eliminate.
TIP
Be sure not to damage the contact surface of
the cylinder.
2. Check:
9 Cylinder head
Cracks/damage around the hole of
spark plug → Replace.
CHECKING THE ROCKER ARM
1. Check:
9 Rocker arm
Wear/damage/cracks → Replace.
chap03.indd 23 2012/09/14 10:45:52
1
4
8
3
6
7
2
5

ILLUSTRATED SYMBOLS
(Refer to the illustration)
SYMBOL DEFINITION SYMBOL DEFINITION
General information Periodic checks and
adjustments
Engine Carburetor
Electrical Troubleshooting
Specifications Filling fluid
Lubricant Special tool
T
R
.
.
Tightening torque Wear limit, clearance
Engine speed Electrical data
Replace the part with a new one. LS Lithium-soap base grease
MMolybdenum disulfide grease
E
Engine oil
M
Molybdenum disulfide oil LT Apply locking agent (LOCTITE®)

INDEX
GENERAL INFORMATION GEN
INFO
PERIODIC CHECKS
AND ADJUSTMENTS CHK
ADJ
ENGINE ENG
CARBURETOR
ELECTRICAL
TROUBLESHOOTING
CARB
ELEC
TRBL
1
2
3
4
5
6

CHAPTER 1.
GENERAL INFORMATION
IMPORTANT INFORMATION ....................1-1
PREPARATION FOR REMOVAL AND
DISASSEMBLY .....................................1-1
CAUTION ON SERVICE .......................1-1
NOTES ON SERVICE...........................1-1
ALL REPLACEMENT PARTS ...............1-2
GASKETS, OIL SEALS,
AND O-RINGS ......................................1-2
BEARINGS AND OIL SEALS ...............1-2
BASIC SERVICE INFORMATION ..............1-3
ELECTRICAL SYSTEM ........................1-3
SPECIAL TOOLS AND TESTERS .............1-6
CHAPTER 2.
PERIODIC CHECKS AND
ADJUSTMENTS
INTRODUCTION ........................................2-1
MAINTENANCE INTERVALS CHART ......2-1
PERIODIC MAINTENANCE/
LUBRICATION INTERVALS ......................2-1
PERIODIC MAINTENANCE ......................2-3
SPARK PLUG .......................................2-3
FUEL LEAKAGE ...................................2-5
ENGINE OIL LEAKAGE .......................2-5
ENGINE OIL LEVEL .............................2-5
REPLACING THE ENGINE OIL ...........2-6
AIR FILTER ELEMENT .........................2-7
MUFFLER .............................................2-8
FUEL TANK FILTER ..............................2-9
FUEL COCK STRAINER ....................2-10
BREATHER HOSE..............................2-11
CYLINDER HEAD
DECARBONIZATION ..........................2-11
ADJUSTING THE VALVE
CLEARANCE ......................................2-12
ENGINE SPEED (WITH
THROTTLE LEVER MODEL) .............2-14
ENGINE SPEED .................................2-15
ADJUSTING THE ENGINE
SPEED (WITH THROTTLE
LEVER MODEL) .................................2-15
ADJUSTING THE ENGINE SPEED ...2-17
RECOIL STARTER .............................2-17
FITTINGS AND FASTENERS ............2-17
CHAPTER 3.
ENGINE
ENGINE INSPECTION ...............................3-1
MEASURING THE COMPRESSION
PRESSURE ..........................................3-1
AIR FILTER ................................................3-3
SILENT SEMI-DRY TYPE .....................3-3
SEMI-CYCLONE TYPE ........................3-4
FUEL TANK, CANISTER ...........................3-5
CHECKING THE CANISTER ................3-6
MUFFLER ..................................................3-7
INSTALLING THE MUFFLER ...............3-8
RECOIL STARTER ....................................3-9
DISASSEMBLING THE RECOIL
STARTER ............................................3-10
CHECKING THE RECOIL
STARTER ............................................3-11
ASSEMBLING THE RECOIL
STARTER ............................................3-11
FLYWHEEL ..............................................3-13
REMOVING THE FLYWHEEL ............3-14
INSTALLING THE FLYWHEEL ...........3-15
INSTALLING THE TCI UNIT ...............3-16
GOVERNOR .............................................3-17
REMOVING THE FLYWEIGHT
SHAFT ASSEMBLY AND
GOVERNOR SHAFT ..........................3-18
DISASSEMBLING THE
FLYWEIGHT SHAFT ASSEMBLY .......3-18
CHECKING THE FLYWEIGHT
SHAFT ASSEMBLY AND
GOVERNOR SHAFT ..........................3-18
ASSEMBLING THE FLYWEIGHT
SHAFT ASSEMBLY ............................3-19

INSTALLING THE FLYWEIGHT
SHAFT ASSEMBLY AND
GOVERNOR SHAFT ..........................3-19
ENGINE SWITCH AND OIL WARNING
UNIT .........................................................3-20
CYLINDER HEAD COVER, CYLINDER
HEAD .......................................................3-21
CHECKING THE ROCKER ARM .......3-23
CHECKING THE PUSH ROD .............3-23
CHECKING THE CYLINDER
HEAD ..................................................3-23
INSTALLING THE CYLINDER
HEAD ASSEMBLY ..............................3-24
VALVE ......................................................3-25
REMOVING THE VALVE AND
VALVE SPRING ..................................3-26
CHECKING THE VALVE AND
VALVE SPRING ..................................3-26
CHECKING THE VALVE SEAT ...........3-28
VALVE LAPPING ................................3-30
INSTALLING THE VALVE AND
VALVE SPRING ..................................3-31
PISTON, CAMSHAFT, CRANKCASE,
AND CRANKSHAFT ................................3-32
REMOVING THE CAMSHAFT AND
VALVE LIFTER ....................................3-34
CHECKING THE CAMSHAFT ............3-34
CHECKING THE VALVE LIFTER ........3-36
INSTALLING THE VALVE LIFTER
AND CAMSHAFT ...............................3-36
CHECKING THE CRANKCASE
COVER ...............................................3-36
INSTALLING THE CRANKCASE
COVER ...............................................3-37
CHECKING THE CYLINDER ..............3-37
CHECKING THE CRANKCASE .........3-38
CHECKING THE PISTON AND
PISTON PIN ........................................3-39
CHECKING THE PISTON RING ........3-41
CHECKING THE CRANKSHAFT .......3-42
CHECKING THE CONNECTING
ROD OIL CLEARANCE ......................3-44
INSTALLING THE PISTON AND
PISTON RING .....................................3-45
INSTALLING THE CRANKSHAFT ......3-46
SELECTING THE CRANKSHAFT
SHIM (P.T.O. SHAFT TYPE E) ............3-48
CHAPTER 4.
CARBURETOR
REMOVING THE CARBURETOR ..............4-1
DISASSEMBLING THE CARBURETOR ...4-2
MANUAL CHOKE MODEL ...................4-2
AUTO CHOKE MODEL.........................4-3
CHECKING THE CARBURETOR .........4-5
CHECKING THE CHOKE SOLENOID
(AUTO CHOKE MODEL) ......................4-6
CHAPTER 5.
ELECTRICAL
ELECTRICAL COMPONENTS ..................5-1
FUSE (ELECTRIC STARTING MODEL) ...5-2
CHECKING THE FUSE ........................5-2
SWITCHES .................................................5-4
CHECKING THE SWITCH
CONTINUITY .......................................5-4
IGNITION SYSTEM ....................................5-5
TROUBLESHOOTING CHART .............5-5
ELECTRIC STARTING SYSTEM
(ELECTRIC STARTING MODEL) ............5-11
TROUBLESHOOTING CHART ...........5-11
REMOVING THE STARTER
MOTOR ...............................................5-13
DISASSEMBLING THE STARTER
MOTOR ...............................................5-14
CHECKING THE ARMATURE
COIL ....................................................5-15
CHECKING THE BRUSH ...................5-16
CHECKING THE MAGNETIC
SWITCH ..............................................5-16
CHARGING SYSTEM
(WITH CHARGING COIL MODEL) .........5-18
TROUBLESHOOTING CHART ...........5-18

CHAPTER 6.
TROUBLESHOOTING
ENGINE ......................................................6-1

1-1
GEN
INFO
IMPORTANT INFORMATION
SVU1030
SVU1040
SVU1050
SVU1060
SVU1070
SVU1080
SVU1090
SVU1100
GENERAL INFORMATION
IMPORTANT INFORMATION
PREPARATION FOR REMOVAL AND DISASSEMBLY
CAUTION ON SERVICE
1.Fireprevention
 When servicing the engine, always keep the engine and your-
selfawayfromfire.
NOTES ON SERVICE
1.Correcttools
 Be sure to use the correct special tool for the job to guard
againstdamage.
2.Oil,greaseandseals
 BesuretousegenuineYamahaoils,greaseandsealers,orthe
equivalents.
3.Expendableparts
 Alwaysreplacethegaskets,O-rings,cotterpinsandcirclipswith
newpartswhenservicingengine.
4.Tighteningtorque
 Be sure to follow torque specifications. When tightening bolts,
nutsorscrews,startwiththelargest-diameterfastenerandwork
from an inner position to an outer position in a crisscross pat-
tern.
5.Notesondisassemblyandassembly
 a.Parts should be cleaned in solvent and blown dry with com-
pressedairafterdisassembly.
 b.Contactsurfacesofmovingpartsshouldbeoiledwhenreas-
sembled.
 c.Makesurethattheparts,movesmoothlyaftereachsectionof
themachineisassembled.
1
CHAPTER 1.

1-2
GEN
INFO
IMPORTANT INFORMATION
SVU1110
SVU1120
ALL REPLACEMENT PARTS
We recommend the use of genuineYamaha parts for all replace-
ments. Use oil and/or grease, recommended by Yamaha, for
assemblyandadjustment.
GASKETS, OIL SEALS, AND O-RINGS
1. Allgaskets,seals,andO-ringsshouldbereplacedwhenan
engineisoverhauled.Allgasketssurfaces,oilseallips,and
O-ringsmustbecleaned.
2. Properlyoilallmatingpartsandbearingsduringreassembly.
 Applygreasetotheoilseallips.
BEARINGS AND OIL SEALS
Installthebearing(s)1andoilseal(s)2withtheirmanufacture’s
marks or numbers facing outward. (In other words, the stamped
letters must be on the side exposed to view.) When installing oil
seal(s),applya light coating oflightweightlithium base greaseto
theseallip(s).Oilthebearingsliberallywheninstalling.
NOTICE
Do not use compressed air to spin the bearings dry. This
causes damage to the bearing surfaces.

1-3
GEN
INFO
BASIC SERVICE INFORMATION
BASIC SERVICE INFORMATION
ELECTRICAL SYSTEM
Checking the electrical system
TIP
Beforecheckingtheelectricalsystem,makesurethatthebattery
voltageisatleast12V.
NOTICE
Never insert the tester probes into the coupler terminal slots.
Always insert the probes from the opposite end a of the cou-
pler, taking care not to loosen or damage the leads.
a
Checking the connections
Check the leads, couplers, and connectors for stains, rust, mois-
ture,etc.
 1. Disconnect:
9 Lead
9 Coupler
9 Connector
NOTICE
9 When disconnecting a coupler, release the coupler lock,
hold both sections of the coupler securely, and then dis-
connect the coupler.
9 There are many types of coupler locks;
therefore, be sure to check the type of coupler lock before
disconnecting the coupler.
NOTICE
When disconnecting a connector, do not pull the leads. Hold
both sections of the connector securely, and then disconnect
the connector.

1-4
GEN
INFO
BASIC SERVICE INFORMATION
1
 2. Check:
9 Lead
9 Coupler
9 Connector
  Moisture→Drywithanairblower.
  Rust/stains→Connectanddisconnectseveraltimes.
 3. Check:
9 Allconnections
  Looseconnection→Connectproperly.
TIP
9 Ifthepin1ontheterminalisflattened,benditup.
9 Afterdisassemblingandassemblingacoupler,pullontheleads
tomakesurethattheyareinstalledsecurely.
 4. Connect:
9 Lead
9 Coupler
9 Connector
TIP
9 Whenconnectingacouplerorconnector,pushbothsectionsof
the coupler or connector together until they are connected
securely.
9 Makesureallconnectionsaretight.

1-5
GEN
INFO
 5. Check:
9 Continuity
  (withthepockettester)
Analog pocket tester:
YU-03112-C
Pocket tester:
90890-03112
TIP
9 Ifthereisnocontinuity,cleantheterminals.
9 Whencheckingthewireharness,performsteps(1)to(4).
9 As a quick remedy, use a contact revitalizer available at most
partstores.
BASIC SERVICE INFORMATION

1-6
GEN
INFO
SPECIAL TOOLS AND TESTERS
SPECIAL TOOLS AND TESTERS
Theproperspecialtoolsarenecessaryforcompleteandaccurate
tune-upandassembly.Usingthecorrectspecialtoolwillhelppre-
vent damage caused by the use of improper tools or improvised
techniques.
TIP
9 For U.S.A. and Canada, use part number starting with “YM-”,
“YU-”,or“ACC-”.
9 Forothers,usepartnumberstartingwith“90890-”.
2. Feelergaugeset
 P/N.YU-26900-9
 Thicknessgauge
 P/N.90890-03180
 Thisgaugeisusedtoadjustvalveclearance,pistonclearance
andpistonringendgap.
4. Digitaltachometer
 P/N.YU-39951-B,90890-06760
 Thistoolisneededforobservingenginer/min.
5. Enginecompressiontester
 P/N.YU-33223
 Compressiongauge
 P/N.90890-03081
 Thistoolisusedforcheckingenginecompression.
ø9ø8 ø10
ø3 ø4
YM-A5970 3. Sixpiecetappetset
 P/N.YM-A5970
 Tappetadjustingtool
 P/N.90890-01311
 Thistoolisusedtoadjustthevalveclearances.
90890-01311
3mm
SVU1230
1. Analogpockettester
 P/N.YU-03112-C
 Pockettester
 P/N.90890-03112
 Thisinstrumentisnecessaryforcheckingtheelectricalsystem.

1-7
GEN
INFO
SVU1160
SVU1190
10.Cylindergauge
 Commerciallyobtainable
 Thisinstrumentisusedforcheckingcylinderboresizeand
condition.
11.Dialindicatorgauge
 P/N.YU-A8428
 Dialgauge
 P/N.90890-03097
 Thisinstrumentisusedforcheckingcrankshaftsideclearance.
6. Extension
 P/N.90890-04082
 Thistoolisusedforcheckingenginecompression.
SPECIAL TOOLS AND TESTERS
7. Primaryclutchholder
 P/N.YS-01880-A
 Sheaveholder
 P/N.90890-01701
 Thistoolisnecessaryforholdingthemagnetorotor.
SVU1220
8. Heavydutypuller
 P/N.YU-33270-B
 Flywheelpuller
 P/N.90890-01362
 Thistoolisnecessaryforremovingthemagnetorotor.
9. Valvespringcompressor
 P/N.90890-01253
 Thistoolisusedtoremovethevalvesprings.
13.Oppamapet-4000sparkchecker
 P/N.YM-34487
 Ignitionchecker
 90890-06754
 Thisinstrumentisnecessaryforcheckingtheignitionsystem
 components.
12.Pistonringcompressor
 P/N.YM-08037,90890-05158
 Thistoolisusedtocompressthepistonringswheninstalling
thepiston.

2-1
CHK
ADJ
INTRODUCTION/MAINTENANCE INTERVALS CHART/
PERIODIC MAINTENANCE/LUBRICATION INTERVALS
PERIODIC CHECKS AND ADJUSTMENTS
INTRODUCTION
This chapter includes all information necessary to perform recommended checks and adjustments.
These preventive maintenance procedures, if followed, will ensure more reliable machine
operation and a longer service life. The need for costly overhaul work will be greatly reduced. This
information applies to machines already in service as well as new machines that are being pre-
pared for sale. All service technicians should be familiar with this entire chapter.
MAINTENANCE INTERVALS CHART
Proper periodic maintenance is important. Especially important are the maintenance services
related to emissions control. These controls not only function to ensure air filter but are also vital
to proper engine operation and maximum performance.
PERIODIC MAINTENANCE/LUBRICATION INTERVALS
Item Routine
Pre-
operation
check
Initial Every
1 month
or 20 Hrs
3 months
or 50 Hrs
6 months
or 100 Hrs
12 months
or 300 Hrs
Spark plug
• Check condition.
√
• Clean and replace if
necessary.
Fuel • Check fuel level and
leakage. √
Fuel hose
• Check fuel hose for
cracks or damage. √
• Replace if necessary.
Engine oil • Check oil level in engine. √
• Replace. √ √
Air filter
element
• Check condition. √ (*1)
• Clean.
Spark arrester
• Check condition.
√
• Clean and replace if
necessary.
Fuel tank filter • Clean and replace if
necessary. √
Fuel strainer • Clean and replace if
necessary. √
Crankcase
breather hose
• Check breather hose for
cracks or damage. √
• Replace if necessary.
Cylinder head
• Decarbonize cylinder
head. 
• More frequently if nec-
essary.
Valve
clearance
• Check and adjust when
engine is cold. 
Idle speed • Check and adjust idle
speed. 

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CHK
ADJ
2
PERIODIC MAINTENANCE/LUBRICATION INTERVALS
Item Routine
Pre-
operation
check
Initial Every
1 month
or 20 Hrs
3 months
or 50 Hrs
6 months
or 100 Hrs
12 months
or 300 Hrs
Recoil starter • Check recoil starter for
damage. 
Fittings /
fasteners
• Check all fittings and
fasteners. 
• Correct if necessary.
The point where abnormality was recog-
nized by use. √
*1····· The air filter element needs to be replaced more frequently when using in unusually wet or
dusty areas.
····· Since these items require special tools, data and technical skills, have a Yamaha dealer per-
form the service.

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CHK
ADJ
PERIODIC MAINTENANCE
PERIODIC MAINTENANCE
SPARK PLUG
WARNING
Check and adjust the areas around the cyl-
inder head after the engine has cooled
down completely.
1. Remove:
9 Spark plug cap
9 Spark plug
NOTICE
Before removing the spark plug, use com-
pressed air to clean the cylinder head
cover to prevent dirt from falling into the
engine.
2. Check:
9 Spark plug type
Not correct → Replace
Spark plug type:
Refer to “SPECIFICATIONS” in
the Supplementary service
manual.
9 Electrode 1
Wear/damage → Replace.
9 Insulator color 2
Not normal → Replace.
a1
2
3. Measure:
9 Spark plug gap a
Use a wire gauge or thickness gauge.
Out of specification → Regap.
If necessary, clean the spark plug with
a spark plug cleaner.