Yamaha yz250f 2012, Owner'S Service Manual

Page: 188 / 198

7-2
ENGINE
ADJUSTING THE PILOT SCREW
The richness of the air-fuel mixture
with the throttle fully closed to 1/4
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.
• If the engine idling speed fluctuates,
turn the pilot screw only 1/2 of a turn
in either direction.
• To optimize the fuel flow at a small-
er throttle opening, each machine's
pilot screw has been individually set
at the factory. Before adjusting the
pilot screw, turn it in fully and count
the number of turns. Record this
number as the factory-set number
of turns out.
Effects of adjusting the pilot screw
(reference)
A. Idle
B. Fully open
1. 2-1/2 turns out
2. 1-1/2 turns out
3. 2 turns out
ADJUSTING THE PILOT JET
The richness of the air-fuel mixture
with the throttle open 1/4 or less can
be set by adjusting the pilot jet "1".
Effects of adjusting the pilot jet
(reference)
A. Idle
B. Fully open
1. #40
2. #35
3. #38
ADJUSTING THE JET NEEDLE
GROOVE POSITION
Adjusting the jet needle "1" position
affects the acceleration when the
throttle is 1/8 to 3/4 open.
1. Too rich at intermediate speeds
• Rough engine operation is felt
and the engine will not pick up
speed smoothly.
Step up the jet needle clip by one
groove and move down the nee-
dle to lean out the mixture.
2. Too lean at intermediate speeds
• The engine breathes hard and will
not pick up speed quickly.
Step down the jet needle clip by
one groove and move up the nee-
dle to enrich the mixture.
Effects of changing the jet needle
groove position (reference)
A. Idle
B. Fully open
1. No.6 groove
2. No.4 groove
3. No.5 groove
ADJUSTING THE JET NEEDLE
The jet needle is adjusted by chang-
ing it.
The jet needle setting parts, having
the same taper angle, are available in
different straight portion diameters
and in different taper starting posi-
tions.
a. Diameter of the straight por-
tion
b. Reference needle
c. 0.5 leaner
Changing from NFPR-5 to NFLR-5
has the same effect as a rising of 0.5
clip position.
Effects of changing the jet needle
(reference)
(Diameter of the straight portion)
Changing the diameter of the straight
portion adjusts the air-fuel mixture
when the throttle is 1/8 to 1/4 open.
A. Idle
B. Fully open
RELATIONSHIP WITH THROTTLE
OPENING
The flow of the fuel through the car-
buretor main system is controlled by
the main jet and then, it is further reg-
ulated by the area between the main
nozzle and the jet needle.
The fuel flow relates to the diameter
of the straight portion of the jet needle
with the throttle 1/8 to 1/4 open and
relates to the clip position with the
throttle 1/8 to 3/4 open.
Therefore, the fuel flow is balanced at
each stage of throttle opening by the
combination of the jet needle straight
portion diameter and clip position.
Standard pilot screw
position (example)
2
* 2-1/4
* For EUROPE
Standard pilot jet #38
Standard clip posi-
tion
No.5
groove
Standard jet needle NFPR
* NFPQ
* For EUROPE