Yamaha YZ450F(S) Owner's Service Manual page 598

1
SETTING
Pilot screw adjustment
The richness of the air-fuel mixture with the
throttle fully closed to 1/4 open can be set by
turning the pilot screw 1. Turning in the pilot
screw will make the mixture lean at low
speeds, and turning it out will enrich it.
Standard pilot
screw position
NOTE:
If the idling speed fluctuates, turn the pilot
G
screw only 1/2 of a turn in either direction.
To optimize the fuel flow at a smaller throttle
G
opening, each machine's pilot screw has
been individually set at the factory. Before
adjusting the pilot screw, turn it in fully and
count the number of turns. Record this num-
ber as the factory-set number of turns out.
Effects of adjusting the pilot screw
(reference)
Å
1/4
1
2
Å Idle
ı Fully open
1 2-1/2 turns out
2 1-1/2 turns out
3 2 turns out
7 - 5
T U N
2
(example)
ı
1/2 3/4
+5%
3
–5%