Yamaha WR450F(S) Owner's Service Manual page 710

SETTING
Pilot system
The FLATCR carburetor is manufactured with
a pilot screw. The pilot screw adjustment
ranges from fully closed throttle to 1/4 open
throttle.
Main jet adjustment
The richness of the air-fuel mixture at full throt-
tle can be set by changing the main jet 1.
Standard main jet
* Except for USA
If the air-fuel mixture is too rich or too lean, the
engine power will drop, resulting in poor accel-
eration.
Effects of changing the main jet (reference)
Å
1/4
Å Idle ı Fully open
7 - 4
T U N
#165
* #160
ı
1/2 3/4
#175
#155
+10%
#165
–10%