2007 YAMAHA YZ125LC engine

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 jetå#420 #410 #410 #440 #430 #430
∫, ç#440 #430 #420 #460 #450 #440
Jet needleå
6BFY43-74-3 6BFY42-74-3 6BFY42-74-3 6BFY43-74-4 6BFY42-74-4 6BFY43-74-3
∫, ç6BFY44-74-3 6BFY43-74-3 6BFY44-74-2 6BFY43-74-4 6BFY44-74-3 6BFY43-74-3
å#42.5 #40 #40 #42.5 #40 #40
Pilot jet∫#47.5 #45 #42.5 #47.5 #45 #42.5
ç#42.5 #40 #40 #45 #42.5 #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.
åFor USA and CDN
∫For EUROPE
çFor AUS, NZ and ZA
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7-11
TUNSETTING
Symptom Setting Checking
0~1/4 throttle Use jet needle having a smaller Number of turns-back 
* Hard breathing diameter. Correct properly
Speed down Overflow from carburetor
0~1/4 throttle Use jet needle having a larger
Poor acceleration diameter.
White smoke
Unstable at low speeds Lower jet needle clip position.
Pinking noise (1 groove down)
Turn in pilot air screw.
Poor response at extremely Reduce pilot jet calibration No. Dragging brake
low speed Turn out pilot air screw. Overflow from carburetor
If not effect, reverse the
above procedures.
Poor response in Raise jet needle clip position.
the range of low to If no effect, reverse the above
intermediate speeds procedures.
Poor response when Check overall settings. Check air cleaner for fouling.
throttle is opened quickly Use main jet having lower
calibration No.
Raise jet needle clip position.
(1 groove up)
If no effect, reverse the above
procedures.
Poor engine operation Turn in pilot air screw. Check throttle valve operation.
This should be taken simply for an example.
It is necessary to set the carburetor while
checking the operating conditions of the
engine and discoloration of spark plugs.
Normally, carburetor setting is made by
means of the main jet, jet needle clip position
(including one with 0.5 difference), pilot jet
and pilot air screw. If the result of setting is
still unsatisfactory, it is advisable to change
the diameter of the straight portion the jet
needle.
*In case of hard breathing, check the air vent hose for clogging.
1C3-9-32-7A 4/10/06 9:09 AM Page 25

7-12
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 plugBR9EVX/NGK(resistance type)
1C3-9-32-7A 4/10/06 9:09 AM Page 32

7-13
TUNSETTING
EC720000
CHASSISEC71P002
Selection of the secondary reduction ratio
(Sprocket)
Secondary Number of driven sprocket teethreduction =——————————————
ratioNumber of drive sprocket teeth
reduction ratio>
9It is generally said that the secondary gear ratio
should be reduced for a longer straight portion
of a speed course and should be increased for
a course with many corners. Actually, however,
as the speed depends on the ground condition
of the day of the race, be sure to run through
the circuit to set the machine suitable for the
entire course.
9In actuality, it is very difficult to achieve settings
suitable for the entire course and some settings
may be sacrificed. Thus, the settings should be
matched to the portion of the course that has
the greatest effect on the race result. In such a
case, run through the entire course while mak-
ing notes of lap times to find the best balance;
then, determine the secondary reduction ratio.
9If a course has a long straight portion where a
machine can run at maximum speed, the
machine is generally set such that it can devel-
op its maximum revolutions toward the end of
the straight line, with care taken to avoid the
engine over-revving.
NOTE:
Riding technique varies from rider to rider and
the performance of a machine also vary from
machine to machine. Therefore, do not imitate
other rider’s settings from the beginning but
choose your own setting according to the level of
your riding technique.
Standard secondary
48/13 (3.692)
reduction ratio
1C3-9-32-7B 4/28/06 10:39 AM Page 2

EC72P010
Front fork setting parts
9Front fork spring 1
NOTE:
The I.D. mark (slits) ais proved on the end of
the spring.
EC72B000
Rear suspension setting
The rear suspension setting should be made
depending on the rider’s feeling of an actual run
and the circuit conditions.
The rear suspension setting includes the follow-
ing two factors:
1. Setting of spring preload
9Change the set length of the spring.
9Change the spring.
2. Setting of damping force
9Change the rebound damping.
9Change the compression damping.
SPRING SPRINGI.D.
TYPE
RATE PART NUMBERMARK
(slits)
SOFT 0.398 1C3-23141-A0 I
STD 0.408 1C3-23141-N0 –
0.418 1C3-23141-C0 III
0.428 1C3-23141-D0 IIII
0.438 1C3-23141-E0 IIIII
STIFF 0.449 1C3-23141-F0 I-I
0.459 1C3-23141-G0 I-II
0.469 1C3-23141-H0 I-III
0.479 1C3-23141-J0 I-IIII
7-17
TUNSETTING
EC72C001
Choosing set length
1. Place a stand or block under the engine to
put the rear wheel above the floor, and mea-
sure the length abetween the rear wheel
axle center and the rear fender holding bolt.
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7-18
TUNSETTING
2. Remove the stand or block from the engine
and with a rider astride the seat, measure
the sunken length bbetween the rear wheel
axle center and the rear fender holding bolt.
3. Loosen the locknut 1and make adjustment
by turning the spring adjuster 2to achieve
the standard figure from the subtraction of
the length bfrom the length a.
Standard figure:
90~100 mm (3.5~3.9 in)
NOTE:
9If the machine is new and after it is broken in,
the same set length of the spring may change
because of the initial fatigue, etc. of the spring.
Therefore, be sure to make re-evaluation.
9If the standard figure cannot be achieved by
adjusting the spring adjuster and changing the
spring set length, replace the spring with an
optional one and make re-adjustment.
1C3-9-32-7B 4/28/06 10:39 AM Page 12