Yamaha YZ250F(S) Owner's Service Manual page 616

SETTING
1 Jet needle
2 Pilot air jet
3 Needle jet
4 Main jet
5 Pilot jet
6 Pilot screw
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
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
#180
ı
1/2 3/4
#182
#178
+10%
#180
–10%