Yamaha YZ250 Owner's Service Manual page 554

Yz series
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EC71C030
Road condition and examples of carburetor setting
Conditions
Under 10°C
(50°F)
Parts
(Winter)
#178
Main jet
*#180
Jet needle
N3EW-3
Pilot jet
#50
Pilot air screw
-1/4
Power jet
#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.
EC71D021
Examples of carburetor setting depending on symptom
Symptom
At full throttle
Hard breathing
Shearing noise
Whitish spark plug
Lean mixture
At full-throttle
Stop of speed pick-up
Slow speed pick-up
Slow response
Sooty spark plug
Rich mixture
Lean mixture
Rich mixture
1/4~3/4 throttle
Hard breathing
Lack of speed
1/4~1/2 throttle
Slow speed pick-up
White smoke
Poor acceleration
General condition
10~25°C
Over 25°C
(50~77°F)
(77°F)
(Spring, Autumn)
(Summer)
#178
N3EJ-2
N3CJ-2
*N3CW-3
*N3EW-2
#50
Zero
#50
Setting
Increase main jet calibration No.
(Gradually)
Decrease main jet calibration No.
(Gradually)
*In case of racing slight enrichment
of mixture reduces engine trouble.
Lower jet needle clip position.
(1 groove down)
Raise jet needle clip position.
(1 groove up)
Lower jet needle clip position.
(1 groove down)
Raise jet needle clip position.
(1 groove up)
7-10
SETTING
Under 10°C
(50°F)
(Winter)
#180
#178
*#182
N3CW-3
*N3CW-4
#50
#52
Zero
Zero
*-1/4
#50
#50
Discoloration of spark plug ¡
If tan color, it is in good condition.
If can not be normalized:
Clogged float valve seat
Clogged fuel hose
Clogged fuel cock
Discoloration of spark plug ¡
If tan color, it is in good condition.
If not effect:
Clogged air cleaner
Fuel overflow from carburetor
Clip position indicates the position of
jet needle groove, to which the clip is
fitted. The position is numbered from
the top.
TUN
Sandy condition
10~25°C
Over 25°C
(50~77°F)
(77°F)
(Spring, Autumn)
(Summer)
#178
#178
*#180
N3CW-3
N3CW-3
*N3EW-3
*N3EW-3
#52
#52
Zero
+1/4
#50
#50
Checking
Leaner
(Standard)
Richer

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