Main Jet Adjustment; Pilot Screw Adjustment - Yamaha WR400F(M) Owner's Service Manual

SETTING

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)
A
1/4
Å Idle
ı Fully open

Pilot screw adjustment

The richness of the air-fuel mixture with the
throttle fully closed to 1/8 open can be set by
turning the pilot screw 1 . Turning in the pilot
screw will make the mixture lean at low
speeds, and turning it out will enrich it.
Standard pilot
screw position
* Except for USA
NOTE:
The pilot screw is set at the factory, there is no
need to adjust it.
If the idling speed fluctuates, turn the pilot
screw only 1/2 of a turn in either direction.
Effects of adjusting the pilot screw (refer-
ence)
A
1/4
1
2
Å Idle
ı Fully open
1 2-1/8 turns out
2 1-1/8 turns out
3 1-5/8 turns out
7 - 4
T U N
#165
B
1/2 3/4
#175
#155
1-5/8 turns out
* 1-1/2 turns out
(for reference only)
1/2 3/4
+10%
#165
–10%
B
+5%
3
–5%