Yamaha MX250 User manual

SERVICE MANUAL
MX250
MX300
7VE-F8197-10LIT-19616-02-45
2017.03×1 !
7VE-F8197-10_Cover.indd 1 2017/03/16 16:24:49

7VE-F8197-10_Cover.indd 2 2017/03/16 16:24:50

MX250, MX300
SERVICE MANUAL
©2017 by Yamaha Motor
Corporation, U.S.A.
1st Edition, March 2017
All rights reserved. Any reprinting or
unauthorized use without the written
permission of Yamaha Motor
Corporation, U.S.A.
is expressly prohibited.
LIT-19616-02-45

IMPORTANT
This manual was produced by the Yamaha Motor Powered Products Co., Ltd. primarily for use by
Yamaha dealers and their qualified mechanics. It is not possible to include all the knowledge of a
mechanic in one manual. Therefore, anyone who uses this book to perform maintenance and
repairs on Yamaha machines should have a basic understanding of mechanics and the techniques
to repair these types of machines. Repair and maintenance work attempted by anyone without this
knowledge is likely to render the machine unsafe and unfit for use.
This model has been designed and manufactured to perform within certain specifications in regard
to performance and emissions. Proper service with the correct tools is necessary to ensure that the
machine will operate as designed. If there is any question about a service procedure, it is imperative
that you contact a Yamaha dealer for any service information changes that apply to this model. This
policy is intended to provide the customer with the most satisfaction from his machine and to con-
form to federal environmental quality objectives.
Yamaha Motor Powered Products Co., Ltd. is continually striving to improve all of its models. Modifi-
cations and significant changes in specifications or procedures will be forwarded to all authorized
Yamaha dealers and will appear in future editions of this manual where applicable.
TIP
• This Service Manual contains information regarding periodic maintenance to the emission control
system. Please read this material carefully.
• Designs and specifications are subject to change without notice.
IMPORTANT MANUAL INFORMATION
Particularly important information is distinguished in this manual by the following notations.
This is the safety alert symbol. It is used to alert you to potential personal injury hazards.
Obey all safety messages that follow this symbol to avoid possible injury or death.
A WARNING indicates a hazardous situation which, if not avoided, could result in death or
serious injury.
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 procedures easier or clearer.
WARNING
NOTICE

HOW TO USE THIS MANUAL
This manual is intended as a handy, easy-to-read reference book for the mechanic.
Comprehensive explanations of all installation, removal, disassembly, assembly, repair and check
procedures are laid out with the individual steps in sequential order.
• The manual is divided into chapters and each chapter is divided into sections. The current section
title is shown at the top of each page “1”.
• Sub-section titles appear in smaller print than the section title “2”.
• To help identify parts and clarify procedure steps, there are exploded diagrams at the start of each
removal and disassembly section “3”.
• Numbers are given in the order of the jobs in the exploded diagram. A number indicates a disas-
sembly step “4”.
• Symbols indicate parts to be lubricated or replaced “5”.
Refer to SYMBOLS.
• A job instruction chart accompanies the exploded diagram, providing the order of jobs, names of
parts, notes in jobs, etc. “6”. This step explains removal procedure only. For installation, reverse
the steps.
• Jobs requiring more information (such as special tools and technical data) are described sequen-
tially “7”.
4-3
1
2
3
5
6
7
8
9
10
DISASSEMBLING THE CARBURETOR
CHECKING THE CARBURETOR
1.
Measure:
• Float height “a”
Out of specification →Replace.
TIP
• Lift up the float so that the tip of the float valve lightly
contacts the float arm, and then measure the float height
“a”.
• Do not adjust the float height.
2.
Check:
•Jets
• Carburetor body
Clogged →Apply compressed air to clean the jet.
3.
Check:
• Valve seat
Wear/damage →Replace.
Dirt →Clean.
a
Float height:
13.8 mm (0.54 in)
a
b
a. Wear at groove
b. Dirt
4-2
DISASSEMBLING THE CARBURETOR
DISASSEMBLING THE CARBURETOR
Order Job/Parts to remove Q’ty Remarks
Disassembling the carburetor Disassemble the parts in the order listed.
1/1gnirpS/wercsniarD1
1tlobrebmahctaolF2
1teksaG3
1rebmahctaolF4
1teksaG5
1niptaolF6
1taolF7
1ylbmessaeldeeN8
1elzzonniaM9
1evlavelttorhT01
11 Throttle stop screw/Spring 1/1
1tejtoliP21
1tfahselttorhT31
1evlavekohC41
1revelekohC51
1
2
3
4
5
7
6
8
9
12
11
10
13
14
15
0.7 N · m (0.07 kgf · m, 0.52 lb · ft)
0.7 N · m (0.07 kgf · m, 0.52 lb · ft)1.6 N · m (0.16 kgf · m, 1.2 lb · ft)
1.3 N · m (0.13 kgf · m, 0.95 lb · ft)
1.1 N · m (0.11 kgf · m, 0.81 lb · ft)
7 N · m (0.7 kgf · m, 5.2 lb · ft)
1
3
7
2
5
4
6

SYMBOLS
The following symbols are used in this manual for easier understanding.
TIP
The following symbols are not relevant to every machine.
SYMBOL DEFINITION SYMBOL DEFINITION
Filling fluid Lubricant
Special tool Tightening torque
Wear limit, clearance Engine speed
Electrical data Replace the part with a new one.
Lithium-soap base grease Molybdenum disulfide grease
Engine oil Silicone fluid
Apply locking agent (LOCTITE®)
T
R
.
.
New
LS
M
E
S
LT

GENERAL INFORMATION
PERIODIC CHECKS AND
ADJUSTMENTS
ENGINE
CARBURETOR
ELECTRICAL
TROUBLESHOOTING
SPECIFICATIONS
1
2
3
4
5
6
7
8
9
10
INDEX

TABLE OF CONTENTS
GENERAL INFORMATION
MACHINE IDENTIFICATION ......................1-1
SERIAL NUMBER .................................1-1
STARTING SERIAL NUMBER...............1-1
DIMENSIONS .............................................1-2
DIMENSION CHART .............................1-2
TOP .......................................................1-3
MOUNTING BASE ................................1-3
LEFT SIDE ............................................1-4
FRONT ..................................................1-5
IMPORTANT INFORMATION .....................1-6
PREPARATION FOR REMOVAL AND
DISASSEMBLY......................................1-6
CAUTION ON SERVICE........................1-6
NOTES ON SERVICE ...........................1-6
ALL REPLACEMENT PARTS................1-7
GASKETS, OIL SEALS, AND
O-RINGS ...............................................1-7
BEARINGS AND OIL SEALS ................1-8
BASIC SERVICE INFORMATION ..............1-8
ELECTRICAL SYSTEM.........................1-8
SPECIAL TOOLS AND TESTERS ...........1-12
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....................................2-4
ENGINE OIL LEAKAGE ........................2-4
ENGINE OIL LEVEL..............................2-4
REPLACING THE ENGINE OIL ............2-5
AIR FILTER ELEMENT..........................2-6
MUFFLER..............................................2-7
FUEL TANK FILTER ..............................2-9
FUEL COCK STRAINER.....................2-10
BREATHER HOSE ..............................2-11
CYLINDER HEAD
DECARBONIZATION .......................... 2-11
ADJUSTING THE VALVE
CLEARANCE ...................................... 2-11
ENGINE SPEED ................................. 2-13
ADJUSTING THE ENGINE SPEED.... 2-14
RECOIL STARTER ............................. 2-15
FITTINGS AND FASTENERS............. 2-16
ENGINE
ENGINE INSPECTION............................... 3-1
MEASURING THE COMPRESSION
PRESSURE .......................................... 3-1
AIR FILTER ................................................ 3-3
FUEL TANK................................................ 3-4
CHECKING THE FUEL TANK CAP
(CANISTER).......................................... 3-5
MUFFLER .................................................. 3-6
INSTALLING THE MUFFLER ............... 3-7
RECOIL STARTER .................................... 3-8
DISASSEMBLING THE RECOIL
STARTER.............................................. 3-9
CHECKING THE RECOIL STARTER ... 3-9
ASSEMBLING THE RECOIL
STARTER............................................ 3-10
FLYWHEEL .............................................. 3-12
REMOVING THE FLYWHEEL............. 3-13
INSTALLING THE FLYWHEEL ........... 3-14
INSTALLING THE TCI UNIT ............... 3-14
GOVERNOR............................................. 3-16
THROTTLE LEVER ASSEMBLY......... 3-16
FLYWEIGHT SHAFT ASSEMBLY ....... 3-17
REMOVING THE FLYWEIGHT
SHAFT ASSEMBLY AND
GOVERNOR FORK ............................ 3-18
DISASSEMBLING THE FLYWEIGHT
SHAFT ASSEMBLY ............................ 3-18
CHECKING THE FLYWEIGHT
SHAFT ASSEMBLY ............................ 3-18
ASSEMBLING THE FLYWEIGHT
SHAFT ASSEMBLY ............................ 3-19

INSTALLING THE FLYWEIGHT
SHAFT ASSEMBLY AND
GOVERNOR FORK.............................3-19
ADJUSTING THE GOVERNOR ..........3-20
ENGINE SWITCH AND OIL WARNING
UNIT..........................................................3-21
CYLINDER HEAD COVER, CYLINDER
HEAD ........................................................3-22
CHECKING THE ROCKER ARM ........3-24
CHECKING THE PUSH ROD..............3-24
CHECKING THE CYLINDER HEAD ...3-24
INSTALLING THE CYLINDER HEAD
ASSEMBLY..........................................3-25
VALVE .......................................................3-26
REMOVING THE VALVES AND
VALVE SPRINGS.................................3-27
CHECKING THE VALVES AND
VALVE SPRINGS.................................3-27
CHECKING THE VALVE SEATS .........3-29
VALVE LAPPING .................................3-30
INSTALLING THE VALVES AND
VALVE SPRINGS.................................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 ........3-35
INSTALLING THE VALVE LIFTER
AND CAMSHAFT ................................3-36
CHECKING THE CRANKCASE
COVER ................................................3-36
INSTALLING THE CRANKCASE
COVER ................................................3-36
CHECKING THE CYLINDER AND
PISTON ...............................................3-37
CHECKING THE CRANKCASE ..........3-38
CHECKING THE PISTON PIN ............3-39
CHECKING THE PISTON RINGS.......3-39
CHECKING THE CRANKSHAFT ........3-41
CHECKING THE CONNECTING
ROD OIL CLEARANCE .......................3-41
INSTALLING THE PISTON AND
PISTON RING .....................................3-42
INSTALLING THE CRANKSHAFT ......3-43
CARBURETOR
REMOVING THE CARBURETOR.............. 4-1
DISASSEMBLING THE CARBURETOR ... 4-2
CHECKING THE CARBURETOR ......... 4-3
ELECTRICAL
CIRCUIT DIAGRAM ................................... 5-1
ELECTRICAL COMPONENTS .................. 5-3
CHECKING THE SWITCH
CONTINUITY ........................................ 5-4
CHECKING THE IGNITION SPARK
GAP....................................................... 5-4
CHECKING THE SPARK PLUG CAP
RESISTANCE........................................ 5-5
CHECKING THE AIR GAP ................... 5-6
CHECKING THE IGNITION COIL
(TCI UNIT, PRIMARY) RESISTANCE ... 5-6
CHECKING THE IGNITION COIL
(TCI UNIT, SECONDARY)
RESISTANCE........................................ 5-7
CHECKING THE ENGINE SWITCH ..... 5-8
CHECKING THE OIL LEVEL
SWITCH................................................ 5-8
CHECKING THE WIRE HARNESS ...... 5-9
TROUBLESHOOTING
ENGINE DOES NOT START...................... 6-1
ENGINE STARTS BUT STALLS................ 6-2
ENGINE SPEED DOES NOT
INCREASE ................................................. 6-3
ENGINE SPEED IS UNEVEN .................... 6-4
GOVERNOR OPERATION......................... 6-5
SPECIFICATIONS
GENERAL SPECIFICATIONS ................... 7-1
MAINTENANCE SPECIFICATIONS .......... 7-3
ENGINE ................................................ 7-3

ELECTRICAL ........................................7-7
TIGHTENING TORQUES ...........................7-8
GENERAL TORQUE
SPECIFICATIONS ....................................7-10
LUBRICATION POINTS AND TYPE OF
LUBRICANTS...........................................7-11
WIRE ROUTING DIAGRAM .....................7-12
UPPER SIDE VIEW.............................7-12
ENGINE SWITCH AND OIL
WARNING UNIT ..................................7-14
TCI UNIT .............................................7-16
OIL LEVEL SWITCH (CRANKCASE
COVER)...............................................7-18

1
2
3
4
5
6
7
8
9
10
GENERAL INFORMATION
1-1
GENERAL INFORMATION
MACHINE IDENTIFICATION
SERIAL NUMBER
The serial number is printed on the label “1” affixed to the
position of the multi-purpose engine as shown in the illus-
tration.
TIP
• The first four digits identify the model, and the remaining
digits indicates the production number.
• Designs and specifications are subject to change without
notice.
STARTING SERIAL NUMBER
MX300
296
7VE
7VE
1
Model Code Starting serial num-
ber
7KHJ-020 7KHJ 7KHJ-1000101-
7VEJ-020 7VEJ 7VEJ-1000101-

DIMENSIONS
1-2
1
2
3
4
5
6
7
8
9
10
GENERAL INFORMATION
DIMENSIONS
DIMENSION CHART
*1: UNF: Unified fine thread (Unit: in)
*2: UNC: Unified coarse thread (Unit: in)
in (mm)
4.5
5
72.2
28
56
88.5
25.4 -0.021
0
110 +0.054
0
146.08 -0.040
0
21.8 -0.1
0
6.31
7/16 -20UNF-2B
30
30
45
45
45
45
6.5(165.1)
5(127)
1.37
(35)
1.37
(35)
4.05(103)
5.75
0
-0.0016
4.33 +0.0021
0
4 3/8-16UNC2B DEPTH0.71
4 5/16-24UNF-2B DEPTH0.71
2 5/16-24UNF-2B DEPTH0.71

1-3
1
2
3
4
5
6
7
8
9
10
GENERAL INFORMATION
DIMENSIONS
TOP
MOUNTING BASE
in (mm)
in (mm)
17.49 (444.5)
4.13 (105)
3.56 (90.5)
2 0.43(11)
0.63
(16)
0.43 (11)
2
2.32 (59)
1.46
(37)

DIMENSIONS
1-4
1
2
3
4
5
6
7
8
9
10
GENERAL INFORMATION
LEFT SIDE
in (mm)
4.06 (103)
2.76(70)
18.85(479)

1-5
1
2
3
4
5
6
7
8
9
10
GENERAL INFORMATION
DIMENSIONS
FRONT
in (mm)
15.36 (390.3)
7.2 (183) 8.16 (207.3)
3.48 (88.5)

IMPORTANT INFORMATION
1-6
1
2
3
4
5
6
7
8
9
10
GENERAL INFORMATION
IMPORTANT INFORMATION
PREPARATION FOR REMOVAL AND
DISASSEMBLY
CAUTION ON SERVICE
Fire prevention
When servicing the engine, always keep the engine and
yourself away from fire.
NOTES ON SERVICE
Correct tools
Be sure to use the correct special tool for the job to guard
against damage.
Oil, grease and seals
Be sure to use genuine Yamaha oils, grease and sealers,
or the equivalents.
Expendable parts
Always replace the gaskets, O-rings, cotter pins and cir-
clips with new parts when servicing engine.

1-7
1
2
3
4
5
6
7
8
9
10
GENERAL INFORMATION
IMPORTANT INFORMATION
Tightening torque
Be sure to follow torque specifications. When tightening
bolts, nuts or screws, start with the largest-diameter fas-
tener and work from an inner position to an outer position
in a crisscross pattern.
Notes on disassembly and assembly
• Parts should be cleaned in solvent and blown dry with
compressed air after disassembly.
• Contact surfaces of moving parts should be oiled when
reassembled.
• Make sure that the parts, move smoothly after each sec-
tion of the machine is assembled.
ALL REPLACEMENT PARTS
We recommend the use of genuine Yamaha parts for all
replacements. 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 reas-
sembly. Apply grease to the oil seal lips.

BASIC SERVICE INFORMATION
1-8
1
2
3
4
5
6
7
8
9
10
GENERAL INFORMATION
BEARINGS AND OIL SEALS
Install the bearing(s) “1” and oil seal(s) “2” with their manu-
facture’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.
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.
1
2
Do not use compressed air to spin the bearings dry.
This causes damage to the bearing surfaces.
NOTICE
a
Never insert the tester probes into the coupler termi-
nal slots. Always insert the probes from the opposite
end “a” of the coupler, taking care not to loosen or
damage the leads.
NOTICE

1-9
1
2
3
4
5
6
7
8
9
10
GENERAL INFORMATION
BASIC SERVICE INFORMATION
Checking the connections
Check the leads, couplers, and connectors for stains, rust,
moisture, etc.
1.
Disconnect:
• Lead
• Coupler
• Connector
2.
Check:
• Lead
• Coupler
• Connector
Moisture Dry with an air blower.
Rust/stains Connect and disconnect several
times.
• When disconnecting a coupler, release the coupler
lock, hold both sections of the coupler securely, and
then disconnect the coupler.
• There are many types of coupler locks; therefore, be
sure to check the type of coupler lock before discon-
necting the coupler.
NOTICE
When disconnecting a connector, do not pull the
leads. Hold both sections of the connector securely,
and then disconnect the connector.
NOTICE

BASIC SERVICE INFORMATION
1-10
1
2
3
4
5
6
7
8
9
10
GENERAL INFORMATION
3.
Check:
• All connections
Loose connection Connect properly.
TIP
• If the pin “1” on the terminal is flattened, bend it up.
• After disassembling and assembling a coupler, pull on
the leads to make sure that they are installed securely.
4.
Connect:
• Lead
• Coupler
• Connector
TIP
• When connecting a coupler or connector, push both sec-
tions of the coupler or connector together until they are
connected securely.
• Make sure all connections are tight.
1