Yamaha YZ250 2021 User Manual

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

Yamaha YZ250 2021 Specifications

General

Engine Type	249cc liquid-cooled 2-stroke; reed-valve inducted
Bore x Stroke	66.4mm x 72.0mm
Compression Ratio	8.9~10.6:1
Ignition	CDI
Transmission	5-speed; multiplate wet clutch
Final Drive	Chain
Suspension Rear	KYB® single shock; fully adjustable, 12.4-in travel
Brakes Front	Hydraulic disc, 270mm
Tires Front	80/100-21
Tires Rear	110/90-19
Width	32.5 in
Height	50.8 in
Wheelbase	58.5 in
Ground Clearance	14.2 in
Fuel Capacity	2.1 gal
Wet Weight	227 lb
Fuel Delivery	Keihin PWK38S carburetor
Suspension Front	KYB® Speed-Sensitive System inverted fork; fully adjustable, 11.8-in travel
Brakes Rear	Hydraulic disc, 245mm