Yamaha YZ250(W)/W1, Owner'S Service Manual

Page: 558 / 596

7-9
SETTING
TUN
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
å
For USA and CDN
∫
For EUROPE
ç
For AUS, NZ and ZA
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.
Conditions General condition Sandy condition
Under 10°C 15~25°C Over 25°C Under 10°C 15~25°C Over 25°C
(50°F) (59~77°F) (77°F) (50°F) (59~77°F) (77°F)
Parts (Winter) (Spring, Autumn) (Summer) (Winter) (Spring, Autumn) (Summer)
Main jet
å
,
ç
#178 #178 #178 #180 #178 #178
∫
#180 #178 #178 #182 #180 #178
Jet needle
å
,
ç
N3CW-3 N3EW-2 N3EJ-2 N3CW-3 N3CW-3 N3CW-3
∫
N3EW-3 N3CW-3 N3EW-2 N3CW-4 N3EW-3 N3EW-3
Pilot jet #50 #50 #50 #52 #52 #52
Pilot air
å
,
ç
-1/4 Zero Zero Zero Zero +1/4
screw
∫
-1/4 Zero Zero -1/4 Zero +1/4
Power jet #50 #50 #50 #50 #50 #50
1P8-9-32-7 4/28/06 3:47 PM Page 18