Yamaha YZ125(W)/W1 Owner's Service Manual page 532

SETTING
EC71B040
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. On the relationship
between the fuel flow and the throttle opening,
the fuel flow relates to the straight portion of the
jet needle at full closed~1/8 throttle, to the 1st
tapered portion at 1/4 throttle, to the second
tapered portion at 1/2 throttle, to the third tapered
portion at 3/4 throttle and to the fourth tapered
portion at full open.
Therefore, the fuel flow is balanced at each stage
of throttle opening by a combination of the jet
needle diameter and clip position.
<Example>
(For USA and CDN)
6BFY42-74-3rd groove
6BFY42-74-2nd groove
6BFY42-75-3rd groove
(For EUROPE, AUS, NZ and ZA)
6BFY43-74-3rd groove
6BFY43-74-2nd groove
6BFY43-75-3rd groove
å Lean (larger diameter)
∫ Rich (smaller diameter)
1 Full closed
2 1/4 throttle
3 1/2 throttle
4 3/4 throttle
5 Full open
1st taper
I
2nd taper
II
3rd taper
III
IV
4th taper
a Main nozzle
7-8
TUN