Yamaha yz250 2015, Owner'S Manual

Page: 164 / 174

7-2
ENGINE
ADJUSTING THE POWER JET
The richness of air-fuel mixture under
8,500 rpm to the extent of 1/2 to full
opened throttle can be set by chang-
ing the power jet "1". A larger size jet
results in a richer mixture, and a
smaller size in a leaner mixture.
ADJUSTING THE PILOT AIR
SCREW
The richness of the 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 en-
rich the mixture at low speeds, and
turning out it will lean out the mixture.
ADJUSTING THE PILOT JET
The richness of air-fuel mixture with
the throttle fully closed to 1/2 open
can be set by changing the pilot jet
"1". It is changed when adjustment
cannot be made by the pilot air screw
alone.
ADJUSTING THE JET NEEDLE
GROOVE POSITION
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 inter-
mediate speed operation, irregular
engine operation and poor accelera-
tion 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.
1. Too rich at intermediate speeds
• Rough engine operation is felt
and the engine will not pick up
speed smoothly.
In this case, step up the jet needle
clip by one groove and move
down the needle to lean out the
mixture.
2. Too lean at intermediate speeds
• The engine breathes hard and will
not pick up speed quickly.
Step up the jet needle clip by one
groove and move up the needle to
enrich the mixture.
ADJUSTING THE JET NEEDLE
On the carburetors used in the
YZ250, the main nozzle is press-fit-
ted, so it can not be replaced. There-
fore, carburetor setting requires the
change of the jet needle.
1. The jet needle setting parts, hav-
ing the same taper angle, are
available in different straight por-
tion diameters and in different ta-
per starting positions.
A. Difference in straight portion
diameter.
B. Difference in taper starting
position
a. Diameter of the straight por-
tion
b. Taper starting position
c. Rich
d. Lean
2. Effects of changing the jet needle
(reference)
• Diameter of straight portion
Changing the diameter of the
straight portion adjusts the air-fuel
mixture when the throttle is 1/8 to
1/4 open.
• Taper starting position
Changing the taper starting posi-
tion produces the same effect as
changing the clip position by 0.5
groove.
<Example>
In case of being 0.5 groove leaner
in relation to N3EW-3rd groove,
choose N3CW-3rd groove.
A. In case of being 0.5 groove
leaner in relation to N3EW-
3rd groove.
a. Difference of 0.5 groove
Standard power jet #50
Standard pilot
air screw posi-
tion
1-1/4 turns out
* 2-1/4 turns out
(for reference
only)
* Except for USA and CDN
Standard pilot jet
#50
*#52
* Except for USA and CDN
Standard clip
position
No.2 groove
*No.3 groove
*Except for USA and CDN
Standard jet
needle
N3EW