Yamaha YZ250F(P)/LC Owner's Service Manual page 620

SETTING
1 Jet needle
2 Main air jet
3 Pilot air jet
4 Needle jet
5 Main jet
6 Pilot jet
7 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
#178
ı
1/2 3/4
#188
#168
+10%
#178
–10%