2004 YAMAHA YZ250LC engine

7-3
SETTINGTUN
EC714032
Effects of setting parts in relation to throt-
tle valve opening
NOTE:
The power jet closes at 8,500 rpm of the engine,
after which only the main jet dominates.
1Throttle valve opening
2Full-open
3Full-closed
EC715002
Main jet adjustment
The richness of air-fuel mixture with 3/4~4/4
throttle can be set by changing the main jet 1.
*For EUROPE
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)
Standard main jet#178
*#180
EC71V000
Power jet adjustment
The richness of air-fuel mixture under 8,500 rpm
to the extent of 1/2 to full opened throttle can be
set by changing the power jet 1. A larger size jet
results in a richer mixture, and a smaller size in a
leaner mixture.
Standard power jet#50
Setting partThrottle valve openingFull-closed 1/4 1/2 3/4 Full-open
Pilot jet
Pilot air screw
Diameter of
straight portion
Clip position
Throttle valve
Power jet
Main jet
Jet needle
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7-4
SETTINGTUN
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.
EC718001
Jet needle groove position adjustment
Should the engine be hard to run smoothly at
intermediate speeds, the jet needle 1must be
adjusted. If the mixture is too rich or too lean at
intermediate speed operation, irregular engine
operation and poor acceleration will result.
Whether or not the richness of the mixture is
proper is hard to be determined by means of the
spark plug and therefore, it should be judged
from your feeling of actual engine operation.
*For EUROPE
Standard pilot jet#50
*#52
EC716001
Pilot air screw adjustment
The richness of air-fuel mixture with full closed to
1/8 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.
*For EUROPE
*For EUROPE
1 turn out
Standard pilot air
*7/8 turns out
screw position
(for reference only)
Standard clip positionNo.2 groove
*No 3 groove
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7-5
SETTINGTUN
1. Too rich at intermediate speeds
Rough engine operation is felt and the
engine will not pick up speed smoothly. In
this case, step up the jet needle clip by one
groove and move down the needle to lean
out the mixture.
2. Too lean at intermediate speeds
The engine breathes hard and will not pick
up speed quickly.
Step up the jet needle clip by one groove
and move up the needle to enrich the mix-
ture.
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7-10
SETTINGTUN
EC71D021
Examples of carburetor setting depending on symptom
Symptom Setting Checking
At full throttle Increase main jet calibration No. Discoloration of spark plug 
Hard breathing (Gradually) If tan color, it is in good condition.
Shearing noise If can not be normalized:
Whitish spark plug Clogged float valve seat
Clogged fuel hose
Lean mixture Clogged fuel cock
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:
Slow response of mixture reduces engine trouble. Clogged air cleaner
Sooty spark plug Fuel overflow from carburetor
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. Clip position indicates the position of
Slow speed pick-up (1 groove up) jet needle groove, to which the clip is
White smoke fitted. The position is numbered from
Poor acceleration the top.
Leaner
(Standard)
Richer


EC71C030
Road condition and examples of carburetor setting
Conditions General condition Sandy condition
Under 10°C 10~25°C Over 25°C Under 10°C 10~25°C Over 25°C
(50°F) (50~77°F) (77°F) (50°F) (50~77°F) (77°F)
Parts (Winter)(Spring, Autumn)(Summer) (Winter)(Spring, Autumn)(Summer)
Main jet#178
#178 #178#180 #178
#178
*#180 *#182 *#180
Jet needle N3EW-3N3EJ-2 N3CJ-2 N3CW-3 N3CW-3 N3CW-3
*N3CW-3 *N3EW-2 *N3CW-4 *N3EW-3 *N3EW-3
Pilot jet #50 #50 #50 #52 #52 #52
Pilot air screw-1/4 Zero ZeroZero
Zero +1/4*-1/4
Power jet #50 #50 #50 #50 #50 #50
*For EUROPE
NOTE:
Optimum pilot air screw setting can be abtained by adding the ex-factory number of the same screw
back-out turns to any required value provided in the chart.
For example, if the ex-factory number is “1”, add “1” to the value chosen in the chart.
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7-11
SETTINGTUN
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 the Raise jet needle clip position.
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, needle clip position,
pilot jet and pilot air screw. If the result of
setting is still unsatisfactory, it is advisable to
change the sizes of the jet needle.
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7-12
SETTINGTUN
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 ±15, 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 plugBR8EG/NGK(resistance type)
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7-13
SETTINGTUN
EC720000
CHASSISEC71P002
Selection of the secondary reduction ratio
(Sprocket)
Secondary Number of driven sprocket teethreduction =
ratioNumber of drive sprocket teeth
*For EUROPE and CDN
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 50/14 (3.571)
reduction ratio *49/14 (3.500)
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7-19
SETTINGTUN
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.
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.
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)
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