2004 YAMAHA YZ125LC fuel

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
5XE-9-30-6 4/15/03 11:33 AM Page 4

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
taking 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)
5XE-9-30-7A 4/30/03 9:39 AM Page 2

7-3
TUNSETTING
EC714060
Effect of setting parts in relation to throttle
valve opening
1Throttle valve opening
2Full-open
3Full-closed
EC715002
Main jet adjustment
The richness of air-fuel mixture with 1/2~4/4
throttle can be set by changing 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 calibrating No.
than standard. (To be leaned out)
Setting partThrottle valve openingFull-closed 1/4 1/2 3/4 Full-open
Pilot air screw
Pilot jet
Diameter of
straight portion
Clip position
Throttle valve
Main jet
Jet needle
Standard main jet
#410
5XE-9-30-7A 4/30/03 9:39 AM Page 6

7-4
TUNSETTING
EC716001
Pilot air screw adjustment
The richness of 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 enrich the mix-
ture at low speeds, and turning out it will lean out
the mixture.
EC71R010
Pilot jet adjustment
The richness of air - fuel mixture with the throttle
fully closed to 1/2 open can be set by turning the
pilot jet 1.
It is changed when adjustment cannot be made
by the pilot air screw alone.
Standard pilot air 1-3/4 turns out
screw position *2 turns out
Standard pilot jet#40
*#37.5
*For EUROPE
*For EUROPE
5XE-9-30-7A 4/30/03 9:39 AM Page 8

7-8
TUNSETTING
EC71B040
Relationship with throttle opening
The flow of the fuel through the carburetor main
system is controlled by the main jet and then, it is
further regulated by the area between the main
nozzle and the jet needle. On the relationship
between the fuel flow and the throttle opening,
the fuel flow relates to the straight portion of the
jet needle at full closed~1/8 throttle, to the 1st
tapered portion at 1/4 throttle, to the second
tapered portion at 1/2 throttle, to the third tapered
portion at 3/4 throttle and to the fourth tapered
portion at full open.
Therefore, the fuel flow is balanced at each stage
of throttle opening by a combination of the jet
needle diameter and clip position.

6CHY6-81-3rd groove
6CHY6-81-2nd groove
6CHY6-82-3rd groove
åLean (larger diameter)
∫Rich (smaller diameter)
1Full closed
21/4 throttle
31/2 throttle
43/4 throttle
5Full open
1st taper
2nd taper
3rd taper
4th taper
aMain nozzle
IV
III
II
I
5XE-9-30-7A 4/30/03 9:39 AM Page 20

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 #420 #410#400
#440 #430 #420
*#410
Jet needle6CHY5-81-4 6CHY6-81-3 6CHY5-81-3 6CHY6-81-4 6CHY5-81-4 6CHY6-81-3
*6CHY5-80-4 *6CHY6-80-3 *6CHY6-80-3 *6CHY6-80-4 *6CHY5-80-4 *6CHY5-80-4
Pilot jet#40 #40 #40 #40 #40 #40
*#37.5 *#37.5 *#37.5 *#37.5 *#37.5 *#37.5
Pilot air screw1-3/4 1-3/4 1-3/4 1-3/4 1-3/4 1-3/4
*2 *2 *2 *2 *2 *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
5XE-9-30-7A 4/30/03 9:39 AM Page 24