Yamaha YZ426F(M)/LC Owner's Service Manual page 572

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 taper angles and
straight-portion diameters.
<Example>
OBEKR - 4
Clip position
Diameter a of straight portion
Taper angle b
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
(Taper)
A smaller taper affects the richness of the air-
fuel mixture when the throttle is fully open. To
change the air-fuel mixture only at 1/2 throttle,
also adjust the main jet.
1/4
Å
Å Idle ı Fully open
7 - 6
T U N
OBEKR
*OBEKQ
1/2 3/4
R
P
1/2 3/4
C
E
ı
+10%
Q
–10%
ı
+5%
D
–5%