Yamaha YZ250F(S) Owner's Service Manual page 622

SETTING
Jet needle adjustment
The jet needle is adjusted by changing it.
Standard jet needle
The tapered sections of all jet needles have
the same starting positions, but the needles
are available with different straight-portion
diameters.
<Example>
OBEKR - 4
Effects of changing the jet needle (refer-
ence)
(Diameter of the straight portion)
Changing the diameter of the straight portion
adjusts the air-fuel mixture when the throttle is
1/8 to 1/4 open.
Å
1/4
Å Idle
ı Fully open
7 - 7
T U N
OBEKR
Clip position
Diameter a of straight portion
1/2 3/4
Q
S
ı
+10 %
R
–10 %