2007 YAMAHA YZ125LC check engine

4-27
ENGCYLINDER HEAD, CYLINDER AND PISTON
EC475280
Piston ring and piston
1. Install:
9Piston ring 1
NOTE:
9Take care not to scratch the piston or damage
the piston ring.
9Align the piston ring gap with the pin 2.
9After installing the piston ring, check the
smooth movement of it.
2. Install:
9Gasket (cylinder) 1
9Small end bearing 2
9Dowel pin 3
NOTE:
9Apply the engine oil onto the bearing (crank-
shaft and connecting rod) and connecting rod
big end washers.
9Install the gasket with the seal print side toward
the crankcase.
3. Install:
9Piston 1
9Piston pin 2
9Piston pin clip 3
NOTE:
9The arrow aon the piston dome must point to
exhaust side.
9Before installing the piston pin clip, cover the
crankcase with a clean rag to prevent the pis-
ton pin clip from falling into the crankcase cavi-
ty.
cC
9When installing the piston pin clip, use
the hand so that it may not be distorted.
9Do not allow the clip open ends to meet
the piston pin slot b.
1C3-9-32-4B 4/6/06 2:10 PM Page 20

4-28
ENGCYLINDER HEAD, CYLINDER AND PISTON
3. Install:
9Nut (cylinder) 1
NOTE:
Tighten the nuts in stage, using a crisscross pat-
tern.
4. Install:
9Collar 1
9Push rod 2
9Plain washer 3
9Bolt (push rod) 4
NOTE:
9Set the collar 5included in owner's tool kit to
install the bolt (push rod).
9Do not forget to remove the collar.
EC4753A2
Cylinder head and cylinder
1. Apply:
9Engine oil
To piston 1, piston ring 2and cylinder
surface.
2. Install:
9Cylinder 1
cC
Make sure the piston ring is properly posi-
tioned. Install the cylinder with one hand
while compressing the piston ring with the
other hand.
NOTE:
After installing, check the smooth movement of
the piston.
30 Nm (3.0 m•kg, 22 ft•lb)
5 Nm (0.5 m•kg, 3.6 ft•lb)
1C3-9-32-4B 4/6/06 2:10 PM Page 22

4-62
ENGCRANKCASE AND CRANKSHAFT
EC4N3212
Crankcase
1. Remove:
9Crankcase (right) 1
Use the crankcase separating tool 2.
NOTE:
9Make appropriate bolts 3as shown available
by yourself and attach the tool with them.
9Fully tighten the tool holding bolts, but make
sure the tool body is parallel with the case. If
necessary, one screw may be backed out
slightly to level tool body.
9As pressure is applied, alternately tap on the
engine mounting boss and transmission shafts.
cC
Use soft hammer to tap on the case half.
Tap only on reinforced portions of case. Do
not tap on gasket mating surface. Work
slowly and carefully. Make sure the case
halves separate evenly. If one end “hangs
up”, take pressure off the push screw, re-
align, and start over. If the cases do not
separate, check for a remaining case bolt
or fitting. Do not force.
Rotor puller:
YU-1362-A/90890-01362
EC4N3000
REMOVAL POINTS
Segment
1. Remove:
9Bolt (segment) 1
9Segment 2
NOTE:
Turn the segment counterclockwise until it stops
and loosen the bolt.
cC
If the segment gets an impact, it may be
damaged. Take care not to give an impact
to the segment when removing the bolt.
1C3-9-32-4D 5/1/06 10:41 AM Page 14

8. Install:
9Segment 1
9Bolt (segment) 2
NOTE:
9When installing the segment onto the shift cam
3, align the punch mark awith the dowel pin
b.
9Turn the segment clockwise until it stops and
tighten the bolt.
cC
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:
9Sealant
Forced out on the cylinder mating surface.
10. Apply:
9Engine oil
To the crank pin, bearing, oil delivery hole
and connecting rod big end washer.
11. Check:
9Crankshaft and transmission operation
Unsmooth operation Repair.
NOTE:
If the crankshaft will not turn smoothly, make an
adjustment by gently tapping its right end with a
soft hammer.
4-68
ENGCRANKCASE AND CRANKSHAFT
30 Nm (3.0 m•kg, 22 ft•lb)
1C3-9-32-4D 5/1/06 10:41 AM Page 26

5-48
CHASHANDLEBAR
13. Install:
9“ENGINE STOP” button 1
9Clutch lever holder 2
9Bolt (clutch lever holder) 3
9Clamp 4
NOTE:
9The “ENGINE STOP” button, clutch lever hold-
er and clamp should be installed according to
the dimensions shown.
9Pass the “ENGINE STOP” button lead in the
middle of the clutch holder.
4 Nm (0.4 m•kg, 2.9 ft•lb)
12. Install:
9Bolt (grip cap) 1
w
9Install the grip cap so that the gaps a a
and
b b
between the bolt (grip cap) and brake
lever 2 2
are equal. If you make a mistake in
the grip cap installation position, the brake
lever may contact the grip cap, resulting in
poor brake performance.
9After tightening the bolts, check that the
throttle grip 3 3
moves smoothly. If it does
not, retighten the bolts for adjustment.
4 Nm (0.4 m•kg, 2.9 ft•lb)
11. Install:
9Master cylinder 1
9Master cylinder bracket 2
9Bolt (master cylinder bracket) 3
NOTE:
9Install the bracket so that the arrow mark a
faces upward.
9First tighten the bolt on the upper side of the
master cylinder bracket, and then tighten the bolt
on the lower side.
9 Nm (0.9 m•kg, 6.5 ft•lb)
1C3-9-32-5C 4/14/06 12:05 PM Page 14

6-2
ELECIGNITION SYSTEM
EC620000
IGNITION SYSTEM
EC621003
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.
*Clean or replace
spark plug.
Primary coil
Secondary coil
Repair or replace.
Spark
Check entire ignition
system for connection.
Spark gap test
No good
No good
No good
No good
No good
No good No Spark
Check “ENGINE STOP”
button.
Pick-up coil
Source coil
Check ignition coil.
Check CDI magneto.
Replace CDI unit.
Replace.
Replace.
Replace.
Replace.
Replace.
*marked: Only when the ignition checker is used.
NOTE:
9Remove the following parts before inspection.
1) Seat
2) Fuel tank
9Use the following special tools in this inspection.
Dynamic spark tester:
YM-34487
Ignition checker:
90890-06754Pocket tester:
YU-3112-C/90890-03112
OK
OK
OK
OKReplace.
Check spark plug cap.No good
OK
1C3-9-32-6 4/6/06 5:32 PM Page 4

6-3
ELECIGNITION SYSTEM
EC622001
SPARK GAP TEST
1. Disconnect the spark plug cap from spark
plug.
2. Connect the dynamic spark tester 1(igni-
tion checker 2) as shown.
9Spark plug cap 3
9Spark plug 4
AFor USA and CDN
BExcept for USA and CDN
3. Kick the kick starter.
4. Check the ignition spark gap.
5. Start engine, and increase spark gap until
misfire occurs. (for USA and CDN only)
EC624000
COUPLERS AND LEADS CONNECTION
INSPECTION
1. Check:
9Couplers and leads connection
Rust/ Dust/ Looseness/Short-circuit 
Repair or replace.
EC625002
“ENGINE STOP” BUTTON INSPECTION
1. Inspect:
9“ENGINE STOP” button conduct
Not continuous while being pushed Replace.
Continuous while being freed Replace.
Minimum spark gap:
6.0 mm (0.24 in)
Tester (+) leadBlack/White lead 1
Tester (–) lead
Black lead 2
B/W B
12
PUSH IN
FREE
Tester selector
position
Ω× 1
å
∫
1C3-9-32-6 4/6/06 5:32 PM Page 8

7-1
TUNSETTING
EC700000
TUNINGEC710000
ENGINEEC711001
Carburetor setting
9The role of fuel is to cool the engine, and in the
case of a 2-stroke engine, to lubricate the
engine in addition to power generation.
Accordingly, if a mixture of air and fuel is too
lean, abnormal combustion will occur, and
engine seizure may result. If the mixture is too
rich, spark plugs will get wet with oil, thus mak-
ing it impossible to bring the engine into full play
or if the worst comes to the worst, the engine
may stall.
9The richness of the air-fuel mixture required for
the engine will vary with atmospheric conditions
of the day and therefore, the settings of the car-
buretor must be properly suited to the atmos-
pheric conditions (air pressure, humidity and
temperature).
9Finally, 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 tak-
ing these into consideration, he must select the
best possible carburetor settings.
It is advisable to make a note of settings,
atmospheric conditions, road surface condition,
lap-time, etc. so that the memorandum can be
used as a reference useful for future.
EC712000
Atmospheric conditions and carburetor
setting
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).
Air
Air temp. Humidity pressure Mixture Setting
(altitude)
High HighLow
Richer Leaner
(high)
Low LowHigh
Leaner Richer
(low)
1C3-9-32-7A 4/10/06 9:09 AM Page 2