2012 YAMAHA YZ125LC low oil pressure

1-14
CLEANING AND STORAGE
CLEANING AND STORAGE
CLEANING
Frequent cleaning of your machine
will enhance its appearance, maintain
good overall performance, and ex-
tend the life of many components.
1. Before washing the machine,
block off the end of the exhaust
pipe to prevent water from enter-
ing. A plastic bag secured with a
rubber band may be used for this
purpose.
2. If the engine is excessively
greasy, apply some degreaser to
it with a paint brush. Do not apply
degreaser to the chain, sprockets,
or wheel axles.
3. Rinse the dirt and degreaser off
with a garden hose; use only
enough pressure to do the job.
Do not use high-pressure washers
or steam-jet cleaners since they
cause water seepage and deterio-
ration seals.
4. After the majority of the dirt has
been hosed off, wash all surfaces
with warm water and a mild deter-
gent. Use an old toothbrush to
clean hard-to-reach places.
5. Rinse the machine off immediate-
ly with clean water, and dry all
surfaces with a soft towel or cloth.
6. Immediately after washing, re-
move excess water from the
chain with a paper towel and lubri-
cate the chain to prevent rust.
7. Clean the seat with a vinyl uphol-
stery cleaner to keep the cover
pliable and glossy.
8. Automotive wax may be applied
to all painted or chromed surfac-
es. Avoid combination cleaner-
waxes, as they may contain abra-
sives.
9. After completing the above, start
the engine and allow it to idle for
several minutes.STORAGE
If your machine is to be stored for 60
days or more, some preventive mea-
sures must be taken to avoid deterio-
ration. After cleaning the machine
thoroughly, prepare it for storage as
follows:
1. Drain the fuel tank, fuel lines, and
the carburetor float bowl.
2. Remove the spark plug, pour a ta-
blespoon of SAE 10W-40 motor
oil in the spark plug hole, and re-
install the plug. With the engine
stop switch pushed in, kick the en-
gine over several times to coat the
cylinder walls with oil.
3. Remove the drive chain, clean it
thoroughly with solvent, and lubri-
cate it. Reinstall the chain or store
it in a plastic bag tied to the frame.
4. Lubricate all control cables.
5. Block the frame up to raise the
wheels off the ground.
6. Tie a plastic bag over the exhaust
pipe outlet to prevent moisture
from entering.
7. If the machine is to be stored in a
humid or salt-air environment,
coat all exposed metal surfaces
with a film of light oil. Do not apply
oil to rubber parts or the seat cov-
er.
Make any necessary repairs before
the machine is stored.

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

3-6
ENGINE
ENGINE
CHECKING THE COOLANT LEVEL
Do not remove the radiator cap
"1", drain bolt and hoses when the
engine and radiator are hot. Scald-
ing hot fluid and steam may be
blown out under pressure, which
could cause serious injury. When
the engine has cooled, place a
thick towel over the radiator cap,
slowly rotate the cap counter-
clockwise to the detent. This pro-
cedure allows any residual
pressure to escape. When the
hissing sound has stopped, press
down on the cap while turning
counterclockwise and remove it.
Hard water or salt water is harmful
to the engine parts. You may use
distilled water, if you can't get soft
water.
1. Place the machine on a level
place, and hold it in an upright po-
sition.
2. Remove:
• Radiator cap
3. Check:
• Coolant level "a"
Coolant level low→Add coolant.
1. Radiator
CHANGING THE COOLANT
Do not remove the radiator cap
when the engine is hot.
Take care so that coolant does not
splash on painted surfaces. If it
splashes, wash it away with water.
1. Place a container under the en-
gine.
2. Remove:
• Coolant drain bolt "1"
3. Remove:
• Radiator cap
Drain the coolant completely.
4. Clean:
• Cooling system
Thoroughly flush the cooling sys-
tem with clean tap water.
5. Install:
• Copper washer
• Coolant drain bolt
6. Fill:
• Radiator
•Engine
To specified level.
• Do not mix more than one type of
ethylene glycol antifreeze con-
taining corrosion inhibitors for
aluminum engine.
• Do not use water containing im-
purities or oil.
Handling notes of coolant:
The coolant is harmful so it should be
handled with special care.
• When coolant splashes to your
eye.
Thoroughly wash your eye with
water and see your doctor.
• When coolant splashes to your
clothes.
Quickly wash it away with water
and then with soap.
• When coolant is swallowed.
Quickly make him vomit and take
him to a doctor.
7. Install:
• Radiator cap
Start the engine and warm it up
for a several minutes.
8. Check:
• Coolant level
Coolant level low→Add coolant.
CHECKING THE RADIATOR CAP
1. Inspect:
• Seal (radiator cap) "1"
• Valve and valve seat "2"
Crack/damage→Replace.
Exist fur deposits "3" →Clean or
replace.
CHECKING THE RADIATOR CAP
OPENING PRESSURE
1. Attach:
• Radiator cap tester "1" and adapt-
er "2"
Apply water on the radiator cap seal.
Coolant drain bolt:
10 Nm (1.0 m•kg, 7.2
ft•lb)
Recommended coolant:
High quality ethylene
glycol anti-freeze con-
taining anti-corrosion
for aluminum engine
Coolant "1" and water
(soft water) "2" mixing ra-
tio:
50%/50%
Coolant capacity:
0.9 L (0.79 Imp qt, 0.95
US qt)
Radiator cap tester:
YU-24460-01/90890-
01325
Radiator cap tester
adapter:
YU-33984/90890-01352

4-41
CRANKCASE AND CRANKSHAFT
REMOVING THE SEGMENT
1. Remove:
• Bolt (segment) "1"
• Segment "2"
Turn the segment counterclockwise
until it stops and loosen the bolt.
If the segment gets an impact, it
may be damaged. Take care not to
give an impact to the segment
when removing the bolt.
DISASSEMBLING THE
CRANKCASE
1. Remove:
• Crankcase (right) "1"
Use the flywheel puller "2".
• Make appropriate bolts "3" as
shown available by yourself and at-
tach the tool with them.
• Fully tighten the tool holding bolts,
but make sure the tool body is par-
allel with the case. If necessary,
one screw may be backed out
slightly to level tool body.
• As pressure is applied, alternately
tap on the engine mounting boss
and transmission shafts.
Use soft hammer to tap on the
case half. Tap only on reinforced
portions of case. Do not tap on
gasket mating surface. Work slow-
ly and carefully. Make sure the
case halves separate evenly. If one
end "hangs up", take pressure off
the push screw, realign, and start
over. If the cases do not separate,
check for a remaining case bolt or
fitting. Do not force.
REMOVING THE CRANKSHAFT
1. Remove:
• Crankshaft "1"
Use the crankcase separating
tool "2".
Make appropriate bolts "3" as shown
available by yourself and attach the
tool with them.
Do not use a hammer to drive out
the crankshaft.
REMOVING THE CRANKCASE
BEARING
1. Remove:
•Bearing "1"
• Remove the bearing from the
crankcase by pressing its inner race
as shown in "A".
• If the bearing is removed together
with the crankshaft, remove the
bearing using a general bearing
puller "2" as shown in "B".
• Do not use the removed bearing.
CHECKING THE CRANKCASE
1. Inspect:
• Contacting surface "a"
Scratches→Replace.
• Engine mounting boss "b", crank-
case
Cracks/damage→Replace.
2. Inspect:
• Bearing "1"
Rotate inner race with a finger.
Rough spot/seizure→Replace.
3. Inspect:
• Oil seal "1"
Damage→Replace.
CHECKING THE CRANKSHAFT
1. Measure:
• Runout limit "a"
• Small end free play limit "b"
• Connecting rod big end side
clearance "c"
• Crank width "d"
Out of specification→Replace.
Use the dial gauge and a thick-
ness gauge. Flywheel puller:
YU-1362-A/90890-
01362
Crankcase separating
tool:
YU-1135-A/90890-
01135
Dial gauge and stand:
YU-3097/90890-01252

5-20
FRONT FORK
6. Tighten:
• Locknut "1"
Fully finger tighten the locknut onto
the damper assembly.
7. Loosen:
• Compression damping adjuster
"1"
• Loosen the compression damping
adjuster finger tight.
• Record the set position of the ad-
juster (the amount of turning out the
fully turned in position).
8. Install:
• Base valve "1"
To damper assembly "2".
First bring the damper rod pressure to
a maximum. Then install the base
valve while releasing the damper rod
pressure.
9. Check:
• Damper assembly
Not fully stretched → Repeat the
steps 2 to 8.
10. Tighten:
• Base valve "1"
Hold the damper assembly with the
cap bolt ring wrench "2" and use the
cap bolt wrench "3" to tighten the
base valve with specified torque.
11. After filling, pump the damper as-
sembly "1" slowly up and down
more than 10 times to distribute
the fork oil.
12. While protecting the damper as-
sembly "1" with a rag and com-
pressing fully, allow excessive oil
to overflow on the base valve
side.
Take care not to damage the damp-
er assembly.
13. Allow the overflowing oil to es-
cape at the hole "a" in the damper
assembly.
The overflow measures about 8
cm3(0.28 Imp oz, 0.27 US oz).
14. Check:
• Damper assembly smooth move-
ment
Tightness/binding/rough spots →
Repeat the steps 2 to 13.
15. Install:
• Dust seal "1"
• Stopper ring "2"
• Oil seal "3"
• Oil seal washer "4"
• Slide metal "5"
To inner tube "6".
• Apply the fork oil on the inner tube.
• When installing the oil seal, use vi-
nyl seat "a" with fork oil applied to
protect the oil seal lip.
• Install the oil seal with its manufac-
ture's marks or number facing the
axle holder side.
16. Install:
• Piston metal "1"
Install the piston metal onto the slot
on inner tube.
Base valve:
29 Nm (2.9 m•kg, 21 ft•
lb)
Cap bolt wrench:
YM-01500/90890-01500
Cap bolt ring wrench:
YM-01501/90890-01501

5-40
REAR SHOCK ABSORBER
HANDLING NOTE
• Support the machine securely so
there is no danger of it falling
over.
• This rear shock absorber is pro-
vided with a separate type tank
filled with high-pressure nitro-
gen gas. To prevent the danger
of explosion, read and under-
stand the following information
before handling the shock ab-
sorber. The manufacturer can
not be held responsible for prop-
erty damage or personal injury
that may result from improper
handling.
• Never tamper or attempt to dis-
assemble the cylinder or the
tank.
• Never throw the rear shock ab-
sorber into an open flame or
other high heat. The rear shock
absorber may explode as a re-
sult of nitrogen gas expansion
and/ or damage to the hose.
• Be careful not to damage any
part of the gas tank. A damaged
gas tank will impair the damp-
ing performance or cause a
malfunction.
• Take care not to scratch the
contact surface of the piston
rod with the cylinder; or oil
could leak out.
• Never attempt to remove the
plug at the bottom of the nitro-
gen gas tank. It is very danger-
ous to remove the plug.
• When scrapping the rear shock
absorber, follow the instruc-
tions on disposal.
NOTES ON DISPOSAL (YAMAHA
DEALERS ONLY)
Before disposing the rear shock ab-
sorber, be sure to extract the nitrogen
gas from valve "1". Wear eye protec-
tion to prevent eye damage from es-
caping gas and/or metal chips.
To dispose of a damaged or worn-
out rear shock absorber, take the
unit to your Yamaha dealer for this
disposal procedure.
REMOVING THE BEARING
1. Remove:
• Stopper ring (upper bearing) "1"
Press in the bearing while pressing its
outer race and remove the stopper
ring.
2. Remove:
• Upper bearing "1"
Remove the bearing by pressing its
outer race.
3. Remove:
• Lower bearing "1"
Remove the bearing by pressing its
outer race.
CHECKING THE REAR SHOCK
ABSORBER
1. Inspect:
• Damper rod "1"
Bends/damage→Replace rear
shock absorber assembly.
• Shock absorber "2"
Oil leaks→Replace rear shock
absorber assembly.
Gas leaks→Replace rear shock
absorber assembly.
• Spring "3"
Damage→Replace spring.
Fatigue→Replace spring.
Move spring up and down.
• Spring guide "4"
Wear/damage→Replace spring
guide.
• Spring seat "5"
Cracks/damage →Replace.
• Bearing "6"
Free play exists/unsmooth revolu-
tion/rust →Replace.
INSTALLING THE BEARING
1. Install:
• Upper bearing "1"
Install the bearing parallel until the
stopper ring groove appears by
pressing its outer race.
Do not apply the grease on the
bearing outer race because it will
wear the rear shock absorber sur-
face on which the bearing is press
fitted.

7-1
ENGINE
TUNING
ENGINE
CARBURETOR SETTING
• The role of fuel is to cool the engine,
and in the case of a 2-stroke en-
gine, to lubricate the engine in addi-
tion to power generation.
Accordingly, if a mixture of air and
fuel is too lean, abnormal combus-
tion will occur, and engine seizure
may result. If the mixture is too rich,
spark plugs will get wet with oil, thus
making it impossible to bring the en-
gine into full play or if the worst
comes to the worst, the engine may
stall.
• The richness of the air-fuel mixture
required for the engine will vary with
atmospheric conditions of the day
and therefore, the settings of the
carburetor must be properly suited
to the atmospheric conditions (air
pressure, humidity and tempera-
ture).
• Finally, the rider himself must make
a test-run and check his machine
for conditions (pick-up of engine
speed, road surface conditions)
and for the discoloration of the
spark plug(s). After taking these
into consideration, he must select
the best possible carburetor set-
tings.
It is advisable to make a note of set-
tings, atmospheric conditions, road
surface condition, lap-time, etc. so
that the memorandum can be used
as a reference useful for future.
ATMOSPHERIC CONDITIONS AND
CARBURETOR SETTINGS
The reason for the above tendency is
that the richness or leanness of a fuel
mixture depends on the density of the
air (i.e. the concentration of oxygen in
it).
• Higher temperature expands the air
with its resultant reduced density.• Higher humidity reduces the
amount of oxygen in the air by so
much of the water vapor in the
same air.
• Lower atmospheric pressure (at a
high altitude) reduces the density of
the air.
TEST RUN
After warming up the engine
equipped with the standard type car-
buretor(s) and spark plug(s), run two
or three laps of the circuit and check
the smooth operation of the engine
and discoloration of spark plug(s).
A. Normal
B. Over burned (too lean)
C. Oil fouled (too rich)
EFFECT OF SETTING PARTS IN
RELATION TO THROTTLE VALVE
OPENINGA. Closed
B. Full-open
1. Pilot air screw
2. Pilot jet
3. Jet needle
4. Diameter of straight portion
5. Clip position
6. Throttle valve
7. Main jet
ADJUSTING THE MAIN JET
The richness of air-fuel mixture with
1/2–4/4 throttle can be set by chang-
ing the main jet "1".
1. Spark plug is too hot.
• Select a main jet having higher
calibrating No. than standard. (To
be enriched)
2. Spark plug is wet.
• Select a main jet having lower cal-
ibrating No. than standard. (To be
leaned out)
ADJUSTING THE PILOT AIR
SCREW
The richness of the air-fuel mixture
with full closed to 1/4 throttle can be
set by turning the pilot air screw "1".
Turning in the pilot air screw will en-
rich the mixture at low speeds, and
turning out it will lean out the mixture. Air
temp
.Hu-
midi-
tyAir
pres-
sure
(alti-
tude)Mix-
tureSet-
ting
High HighLow
(high)Rich-
erLean
er
Low LowHigh
(low)Lean
erRich-
er
DiscolorationCondition of spark
plug
NormalInsulator is dry
and burnt brown.
Over burned
(too lean)Insulator is whit-
ish.
Oil fouled (too
rich)Insulator is sooty
and wet.
Standard main jet #430
Standard pilot
air screw posi-
tion2-1/4 turns out

7-6
CHASSIS
CHANGE OF THE HEAT RANGE
OF SPARK PLUGS
Judging from the discoloration of
spark plugs, if they are found improp-
er, it can be corrected by the following
two methods; changing carburetor
settings and changing the heat range
of spark plug.
• In principle, it is advisable to first
use spark plugs of standard heat
range, and judging from the discol-
oration of spark plugs, adjust carbu-
retor settings.
• If the calibration No. of the main jet
must be changed by ±30, it is advis-
able to change the heat range of
spark plugs and newly select the
proper main jet.
• When checking the discoloration of
spark plugs, be sure to stop the en-
gine immediately after a run and
check.
• Avoid racing.
• When changing the heat range of
spark plugs, never attempt to
change it more than ±1 rank.
• When using a spark plug other than
standard, check its heat range
against the standard and check that
it is a resistance type.
• Note that even if the discoloration
seems proper, it may slightly vary
with the spark plug maker and oil in
use.
CHASSIS
SELECTION OF THE SECONDARY
REDUCTION RATIO (SPROCKET) ondary gear reduction ratio>
• It is generally said that the second-
ary gear ratio should be reduced for
a longer straight portion of a speed
course and should be increased for
a course with many corners. Actual-
ly, however, as the speed depends
on the ground condition of the day
of the race, be sure to run through
the circuit to set the machine suit-
able for the entire course.
• In actuality, it is very difficult to
achieve settings suitable for the en-
tire course and some settings may
be sacrificed. Thus, the settings
should be matched to the portion of
the course that has the greatest ef-
fect on the race result. In such a
case, run through the entire course
while making notes of lap times to
find the best balance; then, deter-
mine the secondary reduction ratio.
• If a course has a long straight por-
tion where a machine can run at
maximum speed, the machine is
generally set such that it can devel-
op its maximum revolutions toward
the end of the straight line, with care
taken to avoid the engine over-rev-
ving.
Riding technique varies from rider to
rider and the performance of a ma-
chine also vary from machine to ma-
chine. Therefore, do not imitate other
rider's settings from the beginning but
choose your own setting according to
the level of your riding technique.
DRIVE AND REAR WHEEL
SPROCKETS SETTING PARTSTIRE PRESSURE
Tire pressure should be adjust to suit
the road surface condition of the cir-
cuit.
• Under a rainy, muddy, sandy, or
slippery condition, the tire pressure
should be lower for a larger area of
contact with the road surface.
• Under a stony or hard road condi-
tion, the tire pressure should be
higher to prevent a flat tire.
FRONT FORK SETTING
The front fork setting should be made
depending on the rider's feeling of an
actual run and the circuit conditions.
The front fork setting includes the fol-
lowing three factors:
1. Setting of air spring characteris-
tics
• Change the fork oil amount.
2. Setting of spring preload
• Change the spring.
3. Setting of damping force
• Change the compression damp-
ing.
• Change the rebound damping.
The spring acts on the load and
the damping force acts on the
cushion travel speed.
CHANGE IN AMOUNT AND
CHARACTERISTICS OF FORK OIL
Damping characteristic near the final
stroke can be changed by changing
the fork oil amount. Standard spark
plugBR9EVX/NGK
(resistance
type)
Secondary reduction ratio =
Number of rear wheel sprocket
teeth/Number of drive sprocket
teeth
Standard second-
ary reduction ratio3.692 (48/13)
Part name Size Part number
Drive
sprocket
"1"
(STD) 13T 9383B-13218
Rear wheel
sprocket
"2"
47T 1C3-25447-00
(STD) 48T 1C3-25448-00
49T 1C3-25449-00
50T 1C3-25450-00
51T 1C3-25451-00
52T 1C3-25452-00
Standard tire pressure:
100 kPa (1.0 kgf/cm2,
15 psi)
Extent of adjustment:
60–80 kPa (0.6–0.8 kgf/
cm
2, 9.0–12 psi)
Extent of adjustment:
100–120 kPa (1.0–1.2
kgf/cm
2, 15–18 psi)