Yamaha YZ450FY 2009 Owner's Service Manual page 264

Effects of adjusting the pilot screw (refer-
ence)
A. Idle
B. Fully open
1. 2 turns out
2. 1 turn out
3. 1-1/2 turns out
ADJUSTING THE PILOT JET
The richness of the air-fuel mixture with the
throttle open 1/4 or less can be set by adjusting
the pilot jet "1".
Standard pilot jet
Effects of adjusting the pilot jet (reference)
A. Idle
B. Fully open
1. #48
2. #42
3. #45
ADJUSTING THE JET NEEDLE GROOVE
POSITION
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 en-
gine will not pick up speed smoothly.
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 mix-
ture.
Standard clip position
#45
Effects of changing the jet needle groove
position (reference)
A. Idle
B. Fully open
1. No.4 groove
2. No.2 groove
3. No.3 groove
ADJUSTING THE JET NEEDLE
The jet needle is adjusted by changing it.
Standard jet needle
The jet needle setting parts, having the same
taper angle, are available in different straight
portion diameters and in different taper starting
positions.
a. Diameter of the straight portion
b. Reference needle
c. 0.5 richer
7-3
ENGINE
No.3 groove
NFLR