2005 YAMAHA YZ125LC check engine light

1-1
GEN
INFO
DESCRIPTION
EC100000
GENERAL INFORMATION
EC110000
DESCRIPTION
1Clutch lever
2“ENGINE STOP” button
3Front brake lever
4Throttle grip
5Radiator cap
6Fuel tank cap
7Kick starter
8Fuel tank
9Radiator
0Coolant drain bolt
qCheck bolt (Transmission oil level)
wRear brake pedal
eValve joint
rFuel cock
tAir cleaner
yDrive chain
uShift pedal
iStarter knob
oFront fork
NOTE:
9The machine you have purchased may differ
slightly from those shown in the following.
9Designs and specifications are subject to
change without notice.
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1-13
GEN
INFO
STARTING AND BREAK-IN
EC190000
STARTING AND BREAK-IN
cC
Before starting the machine, perform the
checks in the pre-operation check list.
w
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 machine in a well-ventilated area.
EC191001
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 usually takes about one or
two minutes.
5. The engine is warmed up when it responds
normally to the throttle with the starter knob
(CHOKE) turned off.
cC
Do not warm up the engine for extended peri-
ods.
EC193001
STARTING A WARM ENGINE
Do not operate the starter knob (CHOKE). Open
the throttle slightly and start the engine by kick-
ing the kick starter forcefully with firm stroke.
cC
Observe the following break-in procedures
during initial operation to ensure optimum
performance and avoid engine damage.
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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 (resistance type)
20 Nm (2.0 m•kg, 14 ft•lb)
1C3-9-30-3B 17/5/04 1:22 PM Page 40

4-62
ENGCRANKCASE AND CRANKSHAFT
EC4N3000
REMOVAL POINTS
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.
Rotor puller:
YU-1362-A/90890-01362
1C3-9-30-4D 25/5/04 7:15 PM Page 14

7-9
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#420 #410 #410 #440 #430 #430
*#440 *#430 *#420 *#460 *#450 *#440
Jet needle
6CHY6-80-4 6CHY5-80-4 6CHY5-80-46CHY5-80-5
6CHY6-80-4 6CHY6-80-4
6CHY18-80-3
Pilot jet #40 #40 #40 #40 #40 #40
Pilot air screw2-1/4 2-1/4 2-1/4 2-1/4 2-1/4 2-1/4
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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7-11
TUNSETTING
EC71M010
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 a spark plug other than standard,
check its heat range against the standard and
check that it is a resistance type.
9Note that even if the discoloration seems prop-
er, it may slightly vary with the spark plug
maker and oil in use.
Standard spark plugBR9EG/NGK(resistance type)
1C3-9-30-7A 25/5/04 7:26 PM Page 28