light YAMAHA YZ125LC 2002 User Guide

3-38
ELECTRICAL/SPARK PLUG INSPECTION
INSP
ADJ
EC370000
ELECTRICAL
EC371001
SPARK PLUG INSPECTION
1. Remove:
9Spark plug
2. Inspect:
9Electrode 1
Wear/Damage Replace.
9Insulator color 2
Normal condition is a medium to light tan
color.
Distinctly different color Check the
engine condition.
NOTE:
When the engine runs for many hours at low
speeds, the spark plug insulator will become
sooty, even if the engine and carburetor are in
good operating condition.
3. Measure:
9Plug gap a
Use a wire gauge or thickness gauge.
Out of specification Regap.
4. Clean the plug with a spark plug cleaner if
necessary.
5. Tighten:
9Spark plug
NOTE:
9Before installing a spark plug, clean the
gasket surface and plug surface.
9Finger-tighten athe spark plug before
torquing to specification b.
Spark plug gap:
0.5~0.6 mm (0.020~0.024 in)
Standard spark plug:
BR9EG (NGK)
20 Nm (2.0 m•kg, 14 ft•lb)
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4-13
ENGCARBURETOR AND REED VALVE
2. Install:
9Reed valve assembly 1
9Reed valve spacer 2
NOTE:
Install the reed valve spacer with its wider con-
tact surface side ato the right.
3. Install:
9Carburetor joint 1
9Bolt (carburetor joint) 2
EC4652C2
Carburetor
1. Install:
9Throttle stop screw 1
9Locknut 2
9Pilot air screw 3
*For EUROPE
Pilot air screw:
2-1/2 turns out
*1-3/4 turns out
Note the following installation points:
9Screw in the pilot air screw until it is
lightly seated.
9Back out it by the specified number of
turns.
10 Nm (1.0 m•kg, 7.2 ft•lb)
2. Install:
9Starter plunger 1
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4-63
ENGCRANKCASE AND CRANKSHAFT
EC4N3000
REMOVAL POINTS
EC4N3100
Segment
1. Remove:
9Bolt (segment) 1
9Segment 2
NOTE:
Turn the segment counterclockwise until it stops
and loosen the bolt.
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
front 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
screw or fitting. Do not force.
Crankcase separating tool:
YU-1135-A/90890-01135
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4-67
ENGCRANKCASE AND CRANKSHAFT
2. Check:
9Shifter operation
9Transmission operation
Unsmooth operation Repair.
3. Apply:
9Sealant
On the crankcase (right) 1
NOTE:
Clean the contacting surface of crankcase (left
and right) before applying the sealant.
4. Install:
9Dowel pin 1
9O-ring 2
9Crankcase (right) 3
To crankcase (left) 4.
NOTE:
9Fit the crankcase (right) onto the crankcase
(left). Tap lightly on the case with soft hammer.
9When installing the crankcase, the connecting
rod should be positioned at TDC (top dead
center).
5. Install:
9Clamp 1
9Bolt (crankcase) 2
NOTE:
Tighten the crankcase tightening bolts in stage,
using a crisscross pattern.
Quick gasket®:
ACC-QUICK-GS-KT
Yamaha bond No. 1215:
90890-85505
14 Nm (1.4 m•kg, 10 ft•lb)
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4-70
ENGTRANSMISSION, SHIFT CAM AND SHIFT FORK
EC4H3000
REMOVAL POINTS
EC4H3200
Transmission
1. Remove:
9Main axle 1
9Drive axle 2
NOTE:
9Tap lightly on the transmission drive axle with a
soft hammer to remove.
9Remove assembly carefully. Note the position
of each part. Pay particular attention to the lo-
cation and direction of shift forks.
EC4H4000
INSPECTION
EC4H4200
Gears
1. Inspect:
9Matching dog a
9Gear teeth b
9Shift fork groove c
Wear/Damage Replace.
2. Check:
9Gears movement
Unsmooth movement 
Repair or replace.
EC4H4600
Bearing
1. Inspect:
9Bearing 1
Rotate inner race with a finger.
Rough spot/Seizure Replace.
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5-18
CHASFRONT BRAKE AND REAR BRAKE
3. Install:
9Brake hose holder 1
9Caliper 2
9Bolt (caliper) 3
NOTE:
Fit the brake hose holder cut aover the projec-
tion bon the front fork and clamp the brake
hose.
4. Tighten:
9Pad pin 4
5. Install:
9Pad pin plug 5
EC5A5120
Rear caliper
1. Install:
9Pad support 1
9Brake pad 2
9Pad pin 3
NOTE:
9Install the brake pads with their projections
ainto the caliper recesses b.
9Temporarily tighten the pad pin at this point.
2. Install:
9Copper washer 1
9Union bolt 2
w
Always use new copper washers.
cC
Install the brake hose so that its pipe por-
tion a a
directs as show and lightly touches
the projection b b
on the caliper.
30 Nm (3.0 m•kg, 22 ft•lb)
23 Nm (2.3 m•kg, 17 ft•lb)
18 Nm (1.8 m•kg, 13 ft•lb)
3 Nm (0.3 m•kg, 2.2 ft•lb)
5NY-9-30-5A 5/23/01 9:01 PM Page 36

5-24
CHASFRONT BRAKE AND REAR BRAKE
EC5A5501
Rear brake hose
1. Install:
9Copper washer 1
9Brake hose 2
9Union bolt 3
w
Always use new copper washers.
cC
Install the brake hose so that its pipe por-
tion
a adirects as show and lightly touches
the projection
b bon the master cylinder.
30 Nm (3.0 m•kg, 22 ft•lb)
4. Install:
9Copper washer 1
9Brake hose 2
9Union bolt 3
w
Always use new copper washers.
cC
Install the brake hose so that its pipe por-
tion a a
directs as show and lightly touches
the projection b b
on the master cylinder.
30 Nm (3.0 m•kg, 22 ft•lb)
5NY-9-30-5A 5/23/01 9:01 PM Page 48

7-10
TUNSETTING
EC71C020
Road condition and examples of carburetor setting
EC71D040
Examples of carburetor setting depending on symptom
Symptom Setting Checking
At full throttle Increase main jet calibration No. Discoloration of spark plug

Stall at high speeds (Gradually) If tan color, it is in good condition.
* Hard breathing If can not be normalized:
Shearing noise Clogged float valve seat
Whitish spark plug Clogged fuel hose
Clogged fuel cock
Lean mixture
At full-throttle Decrease main jet calibration No. Discoloration of spark plug 
Stop of speed pick-up (Gradually) If tan color, it is in good condition.
Slow speed pick-up *In case of racing slight enrichment If not effect:
of mixture reduces engine trouble. Clogged air cleaner
Slow response Fuel overflow from carburetor
Sooty spark plug Clogged main air passage or clogged
filter
Rich mixture
Lean mixture Lower jet needle clip position.
(1 groove down)
Rich mixture Raise jet needle clip position.
(1 groove up)
1/4~3/4 throttle Lower jet needle clip position.
* Hard breathing (1 groove down)
Lack of speed
1/4~1/2 throttle Raise jet needle clip position.
Slow speed pick-up (1 groove up)
White smoke
Poor acceleration
Conditions General condition Sandy condition
Under 10°C 15~25°C Over 30°C Under 10°C 15~25°C Over 30°C
(50°F) (59~77°F) (86°F) (50°F) (59~77°F) (86°F)
Parts (Winter)(Spring, Autumn)(Summer) (Winter)(Spring, Autumn)(Summer)
Main jet#430 #420 #410 #450 #440 #430
*#440 *#430 *#420 *#460 *#450 *#440
Jet needle6CHY6-79-3 6CHY5-79-3 6CHY6-79-2 6CHY5-79-4 6CHY6-79-3 6CHY5-79-3
*6CHY6-77-3 *6CHY5-77-3 *6CHY6-77-2 *6CHY5-77-4 *6CHY6-77-3 *6CHY5-77-3
Pilot jet #30 #30 #30 #30 #30 #30
Pilot air screw2-1/2 2-1/2 2-1/2 2-1/2 2-1/2 2-1/2
*1-3/4 *1-3/4 *2 *1-3/4 *1-3/4 *2
Leaner
(Standard)
Richer


Clip position indicates the position of jet
needle groove, to which the clip is fitted.
The position is numbered from the top.
If a change in the clip position (1
groove) is effective, try another jet nee-
dle that provides a difference of 0.5 in
the clip position.
*For EUROPE
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7-12
TUNSETTING
EC71M001
Change of the heat range of spark plugs
Judging from the discoloration of spark plugs, if
they are found improper, it can be corrected by
the following two methods; changing carburetor
settings and changing the heat range of spark
plug.
9In principle, it is advisable to first use spark
plugs of standard heat range, and judging from
the discoloration of spark plugs, adjust carbure-
tor settings.
9If the calibration No. of the main jet must be
changed by ±30, it is advisable to change the
heat range of spark plugs and newly select the
proper main jet.
NOTE:
9When checking the discoloration of spark
plugs, be sure to stop the engine immediately
after a run and check.
9Avoid racing.
9When changing the heat range of spark plugs,
never attempt to change it more than ±1 rank.
9When using spark plugs other than standard,
make sure of the difference in heat range and
find the equivalent to the standard.
9Note that even if the discoloration seems prop-
er, it may slightly vary with the spark plug
maker and oil in use.
Standard spark plug BR9EG/NGK
5NY-9-30-7A 5/23/01 9:19 PM Page 32