Yamaha YZ250 Owner's Service Manual page 542

SETTING
EC716001
Pilot air screw adjustment
The richness of air-fuel mixture with full closed to
1/8 throttle can be set by turning the pilot air
screw 1.
Turning in the pilot air screw will enrich the mix-
ture at low speeds, and turning out it will lean out
the mixture.
Standard pilot air
screw position
*For EUROPE
EC71R010
Pilot jet adjustment
The richness of air - fuel mixture with the throttle
fully closed to 1/2 open can be set by turning the
pilot jet 1.
It is changed when adjustment cannot be made
by the pilot air screw alone.
Standard pilot jet
*For EUROPE
EC718001
Jet needle groove position adjustment
Should the engine be hard to run smoothly at
intermediate speeds, the jet needle 1 must be
adjusted. If the mixture is too rich or too lean at
intermediate speed operation, irregular engine
operation and poor acceleration will result.
Whether or not the richness of the mixture is
proper is hard to be determined by means of the
spark plug and therefore, it should be judged
from your feeling of actual engine operation.
Standard clip position
*For EUROPE
7-4
TUN
1 turn out
*7/8 turns out
(for reference only)
#50
*#52
No.2 groove
*No 3 groove