Yamaha WR250F Manuel D'atelier page 652

SETTING
Jet needle adjustment
The jet needle is adjusted by changing it.
Standard jet needle
* For EUROPE
** For CDN, ZA, AUS and NZ
The tapered sections of all jet needles have
the same starting positions, but the needles
are available with different straight-portion
diameters.
<Example>
OBELQ - 3
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.
Å
1/4
Å Idle
ı Fully open
7 - 7
T U N
OBELQ
* OBELN
** OBEKP
Clip position
Diameter a of straight portion
1/2 3/4
P
R
ı
+10 %
Q
–10 %