Yamaha YZ450F(V) Owner's Service Manual page 612

SETTING
Pilot jet adjustment
The richness of the air-fuel mixture with the
throttle open 1/4 or less can be set by adjust-
ing the pilot jet 1.
Standard pilot jet
Effects of adjusting the pilot jet (reference)
È
1/4
#45
#40
È Idle É Fully open
Jet needle groove position adjustment
Adjusting the jet needle 1 position affects the
acceleration when the throttle is 1/8 to 3/4
open.
1. Too rich at intermediate speeds
Rough engine operation is felt and the
engine will not pick up speed smoothly. In
this case, step up the jet needle clip by one
groove and move down the needle to lean
out the mixture.
2. Too lean at intermediate speeds
The engine breathes hard and will not pick
up speed quickly.
Step down the jet needle clip by one
groove and move up the needle to enrich
the mixture.
Standard clip position
Effects of changing the jet needle groove
position (reference)
È
1/4
1
2
È Idle
É Fully open
1 No.5 groove
2 No.3 groove
3 No.4 groove
7 - 6
T U N
#42
1/2 3/4
No.4 groove
É
1/2 3/4
É
+5%
#42
–5%
+10%
3
–10%