Yamaha YZ250F(W) Owner's Service Manual page 638

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>
OBEPQ - 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.
È
1/4
È Idle
É Fully open
Relationship with throttle opening
The flow of the fuel through the carburetor
main system is controlled by the main jet and
then, it is further regulated by the area
between the main nozzle and the jet needle.
The fuel flow relates to the diameter of the
straight portion of the jet needle with the throt-
tle 1/8 to 1/4 open and relates to the clip posi-
tion with the throttle 1/8 to 3/4 open.
Therefore, the fuel flow is balanced at each
stage of throttle opening by the combination of
the jet needle straight portion diameter and clip
position.
7 - 7
T U N
OBEPQ
Clip position
Diameter a of straight portion
1/2 3/4
P
R
É
+10 %
Q
–10 %