Yamaha YZ250 2021, Owner'S Manual

Page: 131 / 148

ENGINE
8-2
EAM30593
EFFECTS OF THE SETTING PARTS ON THE
THROTTLE VALVE OPENING
TIP
The power jet closes at 8500 r/min of the engine,
after which only the main jet dominates.
EAM30594
MAIN JET ADJUSTMENT
The richness of air-fuel mixture with 3/4–4/4
throttle can be set by changing the main jet “1”.
1. Spark plug is too hot.
Select a main jet having higher calibrating
No. than standard. (To be enriched)
2. Spark plug is wet.
Select a main jet having lower calibrating No.
than standard. (To be leaned out)
EAM30604
ADJUSTING THE POWER JET
The richness of air-fuel mixture under 8500 r/min
to the extent of 1/2 to full opened throttle can be
set by changing the power jet “1”. A larger size
jet results in a richer mixture, and a smaller size
in a leaner mixture.
EAM30595
PILOT AIR 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 air screw “1”.
Turning in the pilot air screw will make the mix-
ture enrich at low speeds, and turning it out will
lean it.
A. Closed
B. Full-open
1. Pilot jet
2. Pilot air screw
3. Jet needle
4. Diameter of straight portion
5. Clip position
6. Throttle valve
7. Power jet
8. Main jet
Standard main jet #178
Standard power jet #50
Standard pilot air
screw position
1-1/4 turns out