transmission YAMAHA YZ125LC 2012 Owners Manual

HOW TO READ DESCRIPTIONS
To help identify parts and clarify pro-
cedure steps, there are exploded dia-
grams at the start of each removal
and disassembly section.
1. An easy-to-see exploded diagram
"1" is provided for removal and
disassembly jobs.
2. Numbers "2" are given in the or-der of the jobs in the exploded di-
agram. A number that is enclosed
by a circle indicates a disassem-
bly step.
3. An explanation of jobs and notes
is presented in an easy-to-read
way by the use of symbol marks
"3". The meanings of the symbol
marks are given on the next page.4. A job instruction chart "4" accom-
panies the exploded diagram,
providing the order of jobs, names
of parts, notes in jobs, etc.
5. For jobs requiring more informa-
tion, the step-by-step format sup-
plements "5" are given in addition
to the exploded diagram and job
instruction chart.
ILLUSTRATED SYMBOLS (Refer to
the illustration)Illustrated symbols "1" to "7" are used
to identify the specifications appear-
ing in the text.
1. With engine mounted
2. Filling fluid
3. Lubricant
4. Special tool
5. Tightening
6. Specified value, Service limit
7. Resistance (Ω), Voltage (V),
Electric current (A)
Illustrated symbols "8" to "14" in the
exploded diagrams indicate grade of
lubricant and location of lubrication
point.
8. Apply engine mixing oil
9. Apply transmission oil
10. Apply molybdenum disulfide oil
11. Apply brake fluid
12. Apply lightweight lithium-soap
base grease
13. Apply molybdenum disulfide
grease
14. Apply silicone greaseIllustrated symbols "15" to "16" in the
exploded diagrams indicate where to
apply a locking agent and where to in-
stall new parts.
15. Apply locking agent (LOC-
TITE
®)
16. Use new one
1
23
45

CONTENTS
CHAPTER 1
GENERAL INFOR-
MATION
LOCATION OF IMPORTANT
LABELS............................ 1-1
DESCRIPTION ................. 1-5
CONSUMER
INFORMATION................. 1-6
INCLUDED PARTS .......... 1-6
IMPORTANT
INFORMATION................. 1-6
CHECKING OF
CONNECTION .................. 1-7
SPECIAL TOOLS ............. 1-8
CONTROL
FUNCTIONS ................... 1-11
STARTING AND
BREAK-IN ...................... 1-12
TORQUE-CHECK
POINTS........................... 1-13
CLEANING AND
STORAGE ...................... 1-14
CHAPTER 2
SPECIFICATIONS
GENERAL
SPECIFICATIONS ............ 2-1
MAINTENANCE
SPECIFICATIONS ............ 2-2
TIGHTENING TORQUES . 2-7
CABLE ROUTING
DIAGRAM ....................... 2-13
CHAPTER 3
REGULAR INSPEC-
TION AND AD-
JUSTMENTS
MAINTENANCE
INTERVALS ...................... 3-1
PRE-OPERATION
INSPECTION AND
MAINTENANCE................ 3-5
ENGINE ............................ 3-6
CHASSIS .......................... 3-9
ELECTRICAL ................. 3-19
CHAPTER 4
ENGINE
SEAT, FUEL TANK AND
SIDE COVERS.................. 4-1
EXHAUST PIPE AND
SILENCER ........................ 4-3
RADIATOR ....................... 4-5
CARBURETOR AND
REED VALVE ................... 4-7
CYLINDER HEAD, CYLIN-
DER AND PISTON ......... 4-12
CLUTCH ......................... 4-19
KICK SHAFT AND
SHIFT SHAFT ................. 4-23
YPVS GOVERNOR......... 4-28
WATER PUMP................ 4-30
CDI MAGNETO............... 4-33
ENGINE REMOVAL ....... 4-35
CRANKCASE AND
CRANKSHAFT ............... 4-39
TRANSMISSION, SHIFT
CAM AND SHIFT
FORK .............................. 4-44
CHAPTER 5
CHASSIS
FRONT WHEEL AND
REAR WHEEL ..................5-1
FRONT BRAKE AND
REAR BRAKE ..................5-6
FRONT FORK .................5-16
HANDLEBAR..................5-24
STEERING ......................5-29
SWINGARM ....................5-33
REAR SHOCK
ABSORBER ....................5-38
CHAPTER 6
ELECTRICAL
ELECTRICAL
COMPONENTS AND
WIRING DIAGRAM ...........6-2
IGNITION SYSTEM...........6-3
CHAPTER 7
TUNING
ENGINE.............................7-1
CHASSIS ..........................7-6

1-5
DESCRIPTION
DESCRIPTION
• The machine you have purchased may differ slightly from those shown in the following.
• Designs and specifications are subject to change without notice.
1. Clutch lever
2. Engine stop switch
3. Front brake lever
4. Throttle grip
5. Radiator cap
6. Fuel tank cap
7. Kickstarter crank
8. Fuel tank
9. Radiator
10. Coolant drain bolt
11. Check bolt (Transmission oil level)
12. Rear brake pedal
13. Valve joint14. Fuel cock
15. Air filter
16. Drive chain
17. Shift pedal
18. Starter knob
19. Front fork

1-11
CONTROL FUNCTIONS
CONTROL FUNCTIONS
ENGINE STOP SWITCH
The engine stop switch "1" is located
on the left handlebar. Continue push-
ing the engine stop switch till the en-
gine comes to a stop.
CLUTCH LEVER
The clutch lever "1" is located on the
left handlebar; it disengages or en-
gages the clutch. Pull the clutch lever
to the handlebar to disengage the
clutch, and release the lever to en-
gage the clutch. The lever should be
pulled rapidly and released slowly for
smooth starts.
SHIFT PEDAL
The gear ratios of the constant-mesh
6 speed transmission are ideally
spaced. The gears can be shifted by
using the shift pedal "1" on the left
side of the engine.KICKSTARTER CRANK
Rotate the kickstarter crank "1" away
from the engine. Push the starter
down lightly with your foot until the
gears engage, then kick smoothly
and forcefully to start the engine. This
model has a primary kickstarter crank
so the engine can be started in any
gear if the clutch is disengaged. In
normal practices, however, shift to
neutral before starting.
THROTTLE GRIP
The throttle grip "1" is located on the
right handlebar; it accelerates or de-
celerates the engine. For accelera-
tion, turn the grip toward you; for
deceleration, turn it away from you.
FRONT BRAKE LEVER
The front brake lever "1" is located on
the right handlebar. Pull it toward the
handlebar to activate the front brake.
REAR BRAKE PEDAL
The rear brake pedal "1" is located on
the right side of the machine. Press
down on the brake pedal to activate
the rear brake.FUEL COCK
The fuel cock supplies fuel from the
tank to carburetor and also filters the
fuel. The fuel cock has the two posi-
tions:
OFF:
With the lever in this position, fuel will
not flow. Always return the lever to
this position when the engine is not
running.
ON:
With the lever in this position, fuel
flows to the carburetor. Normal riding
is done with the lever in this position.
STARTER KNOB (CHOKE)
When cold, the engine requires a
richer air-fuel mixture for starting. A
separate starter circuit, which is con-
trolled by the starter knob "1", sup-
plies this mixture. Pull the starter
knob out to open the circuit for start-
ing. When the engine has warmed
up, push it in to close the circuit.

1-12
STARTING AND BREAK-IN
STARTING AND BREAK-IN
FUEL
Mix oil with the gas at the ratio speci-
fied below. Always use fresh, name-
brand gasoline, and mix the oil and gas
the day of the race. Do not use premix
that is more than a few hours old.
If knocking or pinging occurs, use a
different brand of gasoline or higher
octane grade.
Never mix two types of oil in the
same batch; clotting of the oil
could result. If you wish to change
oil types, be sure to drain the fuel
tank and the carburetor float bowl
of old premix prior to filling with
the new type.
HANDLING NOTE
Before starting the machine, per-
form the checks in the pre-opera-
tion check list.
Never start or run the engine in a
closed area. The exhaust fumes are
poisonous; they can cause loss of
consciousness and death in a very
short time. Always operate the ma-
chine in a well-ventilated area.
AIR FILTER MAINTENANCE
According to "CLEANING THE AIR
FILTER ELEMENT" section in the
CHAPTER 3, apply the foam-air-filter
oil or its equivalent to the element.
(Excess oil in the element may ad-
versely affect engine starting.)
STARTING A COLD ENGINE
1. Shift the transmission into neutral.
2. Turn the fuel cock to "ON" and full
open the starter knob (CHOKE).
3. With the throttle completely
closed start the engine by kicking
the kick starter forcefully with firm
stroke.
4. Run the engine at idle or slightly
higher until it warms up: this usu-
ally takes about one or two min-
utes.
5. The engine is warmed up when it
responds normally to the throttle
with the starter knob (CHOKE)
turned off.
Do not warm up the engine for ex-
tended periods of time.
STARTING A WARM ENGINE
Do not operate the starter knob
(CHOKE). Open the throttle slightly
and start the engine by kicking the
kick starter forcefully with firm stroke.
Observe the following break-in
procedures during initial operation
to ensure optimum performance
and avoid engine damage.
BREAK-IN PROCEDURES
1. Before starting the engine, fill the
fuel tank with a break-in oil-fuel
mixture as follows.
2. Perform the pre-operation checks
on the machine.
3. Start and warm up the engine.
Check the idle speed, and check
the operation of the controls and
the "ENGINE STOP" button.
4. Operate the machine in the lower
gears at moderate throttle open-
ings for five to eight minutes. Stop
and check the spark plug condi-
tion; it will show a rich condition
during break-in.5. Allow the engine to cool. Restart
the engine and operate the ma-
chine as in the step above for five
minutes. Then, very briefly shift to
the higher gears and check full-
throttle response. Stop and check
the spark plug.
6. After again allowing the engine to
cool, restart and run the machine
for five more minutes. Full throttle
and the higher gears may be
used, but sustained full-throttle
operation should be avoided.
Check the spark plug condition.
7. Allow the engine to cool, remove
the top end, and inspect the pis-
ton and cylinder. Remove any
high spots on the piston with #600
grit wet sandpaper. Clean all
components and carefully reas-
semble the top end.
8. Drain the break-in oil-fuel mixture
from the fuel tank and refill with
the specified mix.
9. Restart the engine and check the
operation of the machine through-
out its entire operating range.
Stop and check the spark plug
condition. Restart the machine
and operate it for about 10 to 15
more minutes. The machine will
now be ready to race.
• After the break-in or before each
race, you must check the entire
machine for loose fittings and
fasteners as per "TORQUE-
CHECK POINTS". Tighten all
such fasteners as required.
• When any of the following parts
have been replaced, they must
be broken in.
CYLINDER AND CRANKSHAFT:
About one hour of break-in oper-
ation is necessary.
PISTON, RING AND GEARS:
These parts require about 30
minutes of break-in operation at
half-throttle or less. Observe the
condition of the engine carefully
during operation.
Recommended fuel:
Premium unleaded
gasoline only with a re-
search octane number
of 95 or higher.
Fuel tank capacity:
8.0 L (1.76 Imp gal, 2.11
US gal)
Mixing oil:
Recommended oil:
Yamalube "2-R"
(Yamalube racing 2-
cycle oil)
Mixing ratio: 30:1
If unavailable, use an
equivalent type of oil.
Mixing oil:
Yamalube "2-R"
Mixing ratio:
15:1

2-1
GENERAL SPECIFICATIONS
SPECIFICATIONS
GENERAL SPECIFICATIONS
Model name: YZ125B (USA, CDN, AUS, NZ)
YZ125 (EUROPE, ZA)
Model code number: 1SR1 (USA, CDN)
1SR2 (EUROPE)
1SR4 (AUS, NZ, ZA)
Dimensions: USA, AUS, NZ, ZA EUROPE, CDN
Overall length 2,135 mm (84.1 in) 2,139 mm (84.2 in)
Overall width 827 mm (32.6 in)←
Overall height 1,315 mm (51.8 in) 1,318 mm (51.9 in)
Seat height 997 mm (39.3in) 998 mm (39.3 in)
Wheelbase 1,443 mm (56.8 in)←
Minimum ground clearance 386 mm (15.2 in) 388 mm (15.3 in)
Weight:
Curb weight 94 kg (207 lb)
Engine:
Engine type Liquid cooled 2-stroke, gasoline
Cylinder arrangement Single cylinder
Displacement 124 cm
3 (4.36 Imp oz, 4.19 US oz)
Bore × stroke 54 × 54.5 mm (2.126 × 2.146 in)
Compression ratio 8.6–10.7 : 1
Starting system Kick starter
Lubrication system: Premix (30 : 1)(Yamalube 2-R)
Oil type or grade (2-stroke):
Transmission oil Recommended brand: YAMALUBE
SAE10W-40
API service SG type or higher
JASO standard MA
Periodic oil change 0.66 L (0.58 Imp qt, 0.69 US qt)
Total amount 0.70 L (0.62 Imp qt, 0.74 US qt)
Coolant capacity (including all routes): 0.9 L (0.79 Imp qt, 0.95 US qt)
Air filter: Wet type element
Fuel:
Type Premium unleaded gasoline only with a research octane
number of 95 or higher.
Tank capacity 8.0 L (1.76 Imp gal, 2.11 US gal)
Carburetor:
Type/Manufacturer TMXχ38SS/MIKUNI
Spark plug:
Type/Manufacturer BR9EVX/NGK (resistance type)
Gap 0.6–0.7 mm (0.024–0.028 in)
Clutch type: Wet, multiple-disc
2

2-2
MAINTENANCE SPECIFICATIONS
MAINTENANCE SPECIFICATIONS
ENGINETransmission:
Primary reduction system Gear
Primary reduction ratio 3.368 (64/19)
Final drive Chain
Secondary reduction ratio 3.692 (48/13)
Transmission type Constant mesh, 6-speed
Operation Left foot operation
Gear ratio:
1st 2.385 (31/13)
2nd 1.933 (29/15)
3rd 1.588 (27/17)
4th 1.353 (23/17)
5th 1.200 (24/20)
6th 1.095 (23/21)
Chassis: USA, ZA, AUS, NZ EUROPE, CDN
Frame type Semi double cradle←
Caster angle 25.50 ° 25.60 °
Trail 105 mm (4.13 in) 107 mm (4.21 in)
Tire:
Type With tube
Size (front) 80/100-21 51M
Size (rear) 100/90-19 57M
Tire pressure (front and rear) 100 kPa (1.0 kgf/cm
2, 15 psi)
Brake:
Front brake type Single disc brake
Operation Right hand operation
Rear brake type Single disc brake
Operation Right foot operation
Suspension:
Front suspension Telescopic fork
Rear suspension Swingarm (link type monocross suspension)
Shock absorber:
Front shock absorber Coil spring/oil damper
Rear shock absorber Coil spring/gas, oil damper
Wheel travel:
Front wheel travel 300 mm (11.8 in)
Rear wheel travel 315 mm (12.4 in)
Electrical:
Ignition system CDI magneto
Item Standard Limit
Cylinder head:
Combustion chamber volume 8.4 cm
3 (0.296 Imp oz, 0.284 US oz) ----
Warp limit ---- 0.03 mm
(0.0012 in)

2-4
MAINTENANCE SPECIFICATIONS
Clutch:
Friction plate thickness 2.9–3.1 mm (0.114–0.122 in) 2.8 mm (0.110
in)
Quantity 8 ----
Clutch plate thickness 1.5–1.7 mm (0.059–0.067 in) ----
Quantity 7 ----
Warp limit ---- 0.2 mm (0.008
in)
Clutch spring free length 40.1 mm (1.579 in) 38.1 mm
(1.500 in)
Quantity 5 ----
Clutch housing thrust clearance 0.15–0.26 mm (0.006–0.010 in) ----
Clutch housing radial clearance 0.014–0.046 mm (0.0006–0.0018 in) ----
Clutch release methodInner push, cam push ----
Transmission:
Main axle deflection limit ---- 0.01 mm
(0.0004 in)
Drive axle deflection limit ---- 0.01 mm
(0.0004 in)
Shifter:
Shifting type Cam drum and guide bar ----
Guide bar bending limit ---- 0.05 mm
(0.0020 in)
Kick starter type: Kick and mesh type ----
Kick clip friction force P=0.8–1.2 kg (1.8–2.6 lb) ----
Air filter oil grade (oiled filter): Foam-air-filter oil or equivalent oil ----
Carburetor: USA, CDN EUROPE AUS, NZ, ZA
Type/Manufacturer TMXχ38SS/
MIKUNI←←
I.D. mark 1C37 51 1C36 41←----
Main jet (M.J.) #430←←----
Jet needle-clip position (J.N.) 6BFY43-74-3←←----
Cutaway (C.A.) 4.0←←----
Pilot jet (P.J.) #40 #45←----
Pilot air screw (P.A.S.) 2-1/4←←----
Valve seat size (V.S.) ø3.8 mm (0.15
in)←←----
Starter jet (G.S.) #80←←----
Fuel level (F.L.) 9.5–10.5 mm
(0.37–0.41 in)←←----
Reed valve:
Thickness 0.47 mm (0.019 in) ----
Valve stopper height 8.2–8.6 mm (0.323–0.339 in) ----
Valve bending limit ---- 0.2 mm (0.008
in) Item Standard Limit

3-1
MAINTENANCE INTERVALS
REGULAR INSPECTION AND ADJUSTMENTS
MAINTENANCE INTERVALS
• The following schedule is intended as a general guide to maintenance and lubrication. Bear in mind that such factors as
weather, terrain, geographical location, and individual usage will alter the required maintenance and lubrication intervals.
If you are a doubt as to what intervals to follow in maintaining and lubricating your machine, consult your Yamaha dealer.
• Periodic inspection is essential in making full use of the machine performance. The service life of the parts varies sub-
stantially according to the environment in which the machine runs (e.g., rain, dirt, etc.). Therefore, earlier inspection is
required by reference to the list below.
ItemAfter
break-in
Every
race
(about
2.5
hours)Every
third
(about
7.5
hours)Every
fifth
(about
12.5
hours)
As re-
quiredRemarks
PISTON
Inspect and clean●●Inspect crack.
Replace●●Inspect carbon deposits and
eliminate them.
PISTON RING
Inspect●●Check ring end gap.
Replace●●
PISTON PIN, SMALL END
BEARING
Inspect●
Replace●
CYLINDER HEAD Inspect carbon deposits and
eliminate them.
Inspect and clean●●Check gasket.
Retighten●●
CYLINDER
Inspect and clean●●Inspect score marks.
Replace●Inspect wear.
YPVS
Inspect and clean●●Inspect carbon deposits and
eliminate them.
CLUTCH
Inspect and adjust●●Inspect housing, friction plate,
clutch plate and spring.
Replace●
TRANSMISSION
Replace oil●●Recommended brand: YA-
MALUBE
SAE10W-40
API service SG type or higher
JASO standard MA
Inspect●
Replace bearing●
SHIFT FORK, SHIFT CAM,
GUIDE BAR
Inspect●Inspect wear

3-5
PRE-OPERATION INSPECTION AND MAINTENANCE
PRE-OPERATION INSPECTION AND MAINTENANCE
Before riding for break-in operation, practice or a race, make sure the machine is in good operating condition.
Before using this machine, check the following points.
GENERAL INSPECTION AND MAINTENANCE
Item Routine Page
CoolantCheck that coolant is filled up to the radiator cap. Check the cool-
ing system for leakage.P. 3-6 – 7
FuelCheck that a fresh mixture of oil and gasoline is filled in the fuel
tank. Check the fuel line for leakage.P. 1-12
Transmission oilCheck that the oil level is correct. Check the crankcase for leak-
age.P. 3-8 – 9
Gear shifter and clutchCheck that gears can be shifted correctly in order and that the
clutch operates smoothly.P. 3-7
Throttle grip/HousingCheck that the throttle grip operation and free play are correctly
adjusted. Lubricate the throttle grip and housing, if necessary.P. 3-7 – 8
Brakes Check the play of front brake and effect of front and rear brake. P. 3-9 – 12
Drive chainCheck drive chain slack and alignment. Check that the drive chain
is lubricated properly.P. 3-12 – 13
WheelsCheck for excessive wear and tire pressure. Check for loose
spokes and have no excessive play.P. 3-16
SteeringCheck that the handlebar can be turned smoothly and have no ex-
cessive play.P. 3-17
Front forks and rear shock
absorberCheck that they operate smoothly and there is no oil leakage.
P. 3-13 – 16
Cables (wires)Check that the clutch and throttle cables move smoothly. Check
that they are not caught when the handlebars are turned or when
the front forks travel up and down.—
Muffler Check that the muffler is tightly mounted and has no cracks. P. 4-3 – 4
Rear wheel sprocket Check that the rear wheel sprocket tightening bolt is not loose. P. 3-12
Lubrication Check for smooth operation. Lubricate if necessary. P. 3-18
Bolts and nuts Check the chassis and engine for loose bolts and nuts. P. 1-13
Lead connectorsCheck that the CDI magneto, CDI unit, and ignition coil are con-
nected tightly.P. 1-7
SettingsIs the machine set suitably for the condition of the racing course
and weather or by taking into account the results of test runs be-
fore racing? Are inspection and maintenance completely done?P. 7-1 – 11