Yamaha 2004 WR250FS Owner's Service Manual page 328

SETTING
Jet needle adjustment
The jet needle is adjusted by changing it.
Standard jet needle
* Except for USA
The tapered sections of all jet needles have
the same starting positions, but the needles
are available with different straight-portion
diameters.
<Example>
OBDUS - 4
Clip position
Diameter a of straight portion
Effects of changing the jet needle
(reference)
(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.
È
1/4
R
T
È Idle
É Fully open
7 - 7
T U N
OBDUS
* OBEKQ
É
1/2 3/4
+10 %
–10 %
S