2010 YAMAHA YZ125LC check engine

4-17
CYLINDER HEAD, CYLINDER AND PISTON
3. Install:
• Thrust plate "1"
• Screw (thrust plate) "2"
4. Check:
• Power valve smooth movement
Unsmooth movement→Repair or
replace.
5. Install:
• Gasket (power valve cover) "1"
• Power valve cover "2"
• Screw (power valve cover) "3"
• YPVS breather hose "4"
• Install the power valve cover so that
the arrow mark "a" faces upward.
• Install the clamp of the YPVS
breather hose with its opening por-
tion "b" facing backward.
INSTALLING THE PISTON RING
AND PISTON
1. Install:
• Piston ring "1"
• Take care not to scratch the piston
or damage the piston ring.
• Align the piston ring gap with the pin
"2".
• After installing the piston ring,
check the smooth movement of it.
2. Install:
• Gasket (cylinder) "1"
• Small end bearing "2"
• Dowel pin "3"
• Apply the engine oil onto the bear-
ing (crankshaft and connecting rod)
and connecting rod big end wash-
ers.
• Install the gasket with the seal print
side toward the crankcase.
3. Install:
•Piston "1"
• Piston pin "2"
• Piston pin clip "3"
• The arrow "a" on the piston dome
must point to exhaust side.
• Before installing the piston pin clip,
cover the crankcase with a clean
rag to prevent the piston pin clip
from falling into the crankcase cavi-
ty.
• When installing the piston pin
clip, use the hand so that it may
not be distorted.
• Do not allow the clip open ends
to meet the piston pin slot "b".
INSTALLING THE CYLINDER
HEAD AND CYLINDER
1. Apply:
• Engine oil
To piston "1", piston ring "2" and
cylinder surface.
2. Install:
• Cylinder "1"
Make sure the piston ring is prop-
erly positioned. Install the cylinder
with one hand while compressing
the piston ring with the other hand.
After installing, check the smooth
movement of the piston.
Screw (thrust plate):
4 Nm (0.4 m•kg, 2.9
ft•lb)
Screw (power valve cov-
er):
5 Nm (0.5 m•kg, 3.6
ft•lb)

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

4-43
CRANKCASE AND CRANKSHAFT
• Apply molybdenum disulfide grease
to the surface "a" where the crank-
shaft and bearing come in contact.
• Use two plain washers (Yamaha
genuine: 90201-243K3) "5" or the
ones of a size as shown one on the
other. (Except for USA and CDN)
• Install so that the plain washers do
not deviate from the crankshaft cen-
ter. (Except for USA and CDN)
• When installing the crankcase, the
connecting rod should be posi-
tioned at TDC (top dead center).
• Install while checking that the dowel
pin is in place.
A. For USA and CDN
B. Except for USA and CDN
6. Install:
• Clamp "1"
• Bolt (crankcase) "2"
Tighten the crankcase tightening
bolts in stage, using a crisscross pat-
tern.
7. Install:
• Holder "1"
• Bolt (holder) "2"
8. Install:
• Segment "1"
• Bolt (segment) "2"
• When installing the segment onto
the shift cam "3", align the punch
mark "a" with the dowel pin "b".
• Turn the segment clockwise until it
stops and tighten the bolt.
If the segment gets an impact, it
may be damaged. Take care not to
give an impact to the segment
when tightening the bolt.
9. Remove:
• Sealant
Forced out on the cylinder mating
surface.
10. Apply:
• Engine oil
To the crank pin, bearing, oil de-
livery hole and connecting rod big
end washer.11. Check:
• Crankshaft and transmission op-
eration.
Unsmooth operation→Repair.
If the crankshaft will not turn smooth-
ly, make an adjustment by gently tap-
ping its right end with a soft hammer.
Bolt (crankcase):
14 Nm (1.4 m•kg, 10
ft•lb)
(1.771.97 in)
(0.98
1.50 in)
(0.16
0.20 in)
Bolt (holder):
10 Nm (1.0 m•kg, 7.2
ft•lb)
Bolt (segment):
30 Nm (3.0 m•kg, 22
ft•lb)

5-28
HANDLEBAR
After tightening the bolts, check
that the throttle grip "2" moves
smoothly. If it does not, retighten
the bolts for adjustment.
13. Install:
• Engine stop switch "1"
• Clutch lever holder "2"
• Bolt (clutch lever holder) "3"
• Clamp "4"
• The engine stop switch, clutch lever
holder and clamp should be in-
stalled according to the dimensions
shown.
• Pass the engine stop switch lead in
the middle of the clutch lever hold-
er.
14. Install:
• Clutch cable "1"
Apply the lithium soap base grease
on the clutch cable end.
15. Adjust:
• Clutch lever free play
Refer to "ADJUSTING THE
CLUTCH CABLE FREE PLAY"
section in the CHAPTER 3.
Bolt (clutch lever holder):
4 Nm (0.4 m•kg, 2.9
ft•lb)
(0.55 in)

6-3
IGNITION SYSTEM
IGNITION SYSTEM
INSPECTION STEPS
Use the following steps for checking the possibility of the malfunctioning engine being attributable to ignition system failure
and for checking the spark plug which will not spark.
*marked: Only when the ignition checker is used.
• Remove the following parts before inspection.
1. Seat
2. Fuel tank
• Use the following special tools in this inspection.
Spark gap test Spark→*Clean or replace spark plug.
No spark↓
Check entire ignition system for connection. No good→Repair or replace.
OK↓
Check engine stop switch. No good→Replace.
OK↓
Check ignition coil. (primary coil and secondary
coil)No good→
Replace.
OK↓
Check spark plug cap. No good→Replace.
OK↓
Check CDI magneto. (pickup coil and charging
coil)No good→
Replace.
OK↓
Replace CDI unit.
Dynamic spark tester:
YM-34487
Ignition checker:
90890-06754
Pocket tester:
YU-3112-C/90890-03112

6-4
IGNITION SYSTEM
SPARK GAP TEST
1. Disconnect the spark plug cap
from spark plug.
2. Connect the dynamic spark tester
"1" (ignition checker "2") as
shown.
• Ignition coil "3"
• Spark plug "4"
A. For USA and CDN
B. Except for USA and CDN
3. Kick the kickstarter crank.
4. Check the ignition spark gap.
5. Start engine, and increase spark
gap until misfire occurs. (for USA
and CDN only)
CHECKING THE COUPLERS,
LEADS AND IGNITION COIL
CONNECTION
1. Check:
• Couplers and leads connection
Rust/dust/looseness/short-circuit
→Repair or replace.CHECKING THE ENGINE STOP
SWITCH
1. Inspect:
• Engine stop switch conduction
Not conductive while it is pushed→
Replace.
Conductive while it is freed→Re-
place.
Set the tester selection position to "Ω
× 1".
CHECKING THE IGNITION COIL
1. Inspect:
• Primary coil resistance
Out of specification→Replace.
2. Inspect:
• Secondary coil resistance
Out of specification→Replace.
• Remove the spark plug cap by turn-
ing it counterclockwise and inspect.
• Install the spark plug cap by turning
it clockwise until it is tight.
CHECKING THE SPARK PLUG
CAP
1. Inspect:
• Spark plug cap
Loose connection→Tighten.
Deteriorated/damaged→Re-
place.
• Spark plug cap resistance
Out of specification→Replace.
Minimum spark gap:
6.0 mm (0.24 in)
Tester (+) lead→Black/White lead
"1"
Tester (-) lead→Black lead "2"
Result
Conductive (while the
engine stop switch is
pushed)
Tester (+) lead→Yellow lead "1"
Tester (-) lead→Black lead "2"
Primary
coil resis-
tanceTester se-
lector posi-
tion
0.24–0.36 Ω at 20 °C
(68 °F) Ω × 1
Tester (+) lead→Spark plug lead
"1"
Tester (-) lead→Yellow lead "2"
Secondary
coil resis-
tanceTester se-
lector posi-
tion
5.7–8.5 kΩat 20 °C
(68 °F) kΩ × 1
Tester (+) lead→Spark plug lead
terminal "1"
Tester (-) lead→Spark plug termi-
nal "2"
Spark plug
cap resis-
tanceTester se-
lector posi-
tion
4–6 kΩ at
20 °C (68
°F) kΩ × 1

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#410
*#430
*Except for USA and CDN
Standard pilot
air screw posi-
tion2-1/4 turns out

7-4
ENGINE
ROAD CONDITION AND EXAMPLES OF CARBURETOR SETTING
A. For USA and CDN
B. Except for USA and CDN
SPECIFICATIONS OF JET NEEDLE (For USA and CDN)
SPECIFICATIONS OF JET NEEDLE (For EUROPE, AUS, NZ, and ZA)
EXAMPLES OF CARBURETOR SETTING DEPENDING ON SYMPTOMGeneral condition Sandy condition
Under 10°C
(50°F)15–25°C (59–
77°F)Over 30°C
(86°F)Under 10°C
(50°F)15–25°C (59–
77°F)Over 30°C
(86°F
(Winter) (Spring, Au-
tumn)(Summer) (Winter) (Spring, Au-
tumn)(Summer)
Main jet A #420 #410 #410 #440 #430 #430
B #440 #430 #420 #460 #450 #440
Jet needle A 6BFY43-74-3 6BFY42-74-3 6BFY42-74-3 6BFY43-74-4 6BFY42-74-4 6BFY43-74-3
B 6BFY44-74-3 6BFY43-74-3 6BFY44-74-26BFY43-74-4 6BFY44-74-3 6BFY43-74-3
Pilot jet A #42.5 #40 #40 #42.5 #40 #40
B #47.5 #45 #42.5 #47.5 #45 #42.5
Pilot air screw 2-1/4 2-1/4 2-1/4 2-1/4 2-1/4 2-1/4
Diameter of straight portion
ø2.72 mm
(0.1071 in)ø2.73 mm
(0.1075 in)ø2.74 mm
(0.1079 in)ø2.75 mm
(0.1083 in)ø2.76 mm
(0.1087 in)
Rich
1 richer6BFY44-72-3 6BFY44-73-3 6BFY44-74-3 6BFY44-75-3 6BFY44-76-3
6BFY42-72-4 6BFY42-73-4 6BFY42-74-4 6BFY42-75-4 6BFY42-76-4
0.5 richer 6BFY43-72-3 6BFY43-73-3 6BFY43-74-3 6BFY43-75-3 6BFY43-76-3
STD 6BFY42-72-3 6BFY42-73-3 6BFY42-74-3 6BFY42-75-3 6BFY42-76-3
0.5 leaner 6BFY43-72-2 6BFY43-73-2 6BFY43-74-2 6BFY43-75-2 6BFY43-76-2
1 leaner6BFY44-72-1 6BFY44-73-1 6BFY44-74-1 6BFY44-75-1 6BFY44-76-1
Lean 6BFY42-72-2 6BFY42-73-2 6BFY42-74-2 6BFY42-75-2 6BFY42-76-2
Diameter of straight portion
ø2.72 mm
(0.1071 in)ø2.73 mm
(0.1075 in)ø2.74 mm
(0.1079 in)ø2.75 mm
(0.1083 in)ø2.76 mm
(0.1087 in)
Rich 1 richer 6BFY43-72-4 6BFY43-73-4 6BFY43-74-4 6BFY43-75-4 6BFY43-76-4
0.5 richer6BFY44-72-3 6BFY44-73-3 6BFY44-74-3 6BFY44-75-3 6BFY44-76-3
6BFY42-72-4 6BFY42-73-4 6BFY42-74-4 6BFY42-75-4 6BFY42-76-4
STD 6BFY43-72-3 6BFY43-73-3 6BFY43-74-3 6BFY43-75-3 6BFY43-76-3
0.5 leaner6BFY44-72-2 6BFY44-73-2 6BFY44-74-2 6BFY44-75-2 6BFY44-76-2
6BFY42-72-3 6BFY42-73-3 6BFY42-74-3 6BFY42-75-3 6BFY42-76-3
Lean 1 leaner 6BFY43-72-2 6BFY43-73-2 6BFY43-74-2 6BFY43-75-2 6BFY43-76-2
Symptom Setting Checking
At full throttle
Stall at high speeds
*Hard breathing
Shearing noise
Whitish spark plug
↓
Lean mixtureIncrease main jet calibration no. (Gradual-
ly)Discoloration of spark plug→If tan color, it
is in good condition.
If cannot be corrected:
Clogged float valve seat
Clogged fuel hose
Clogged fuel cock