Yamaha YZ250 2021, Owner'S Manual

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ENGINE
8-4
EAM30598
JET NEEDLE GROOVE POSITION
ADJUSTMENT
Should the engine be hard to run smoothly at in-
termediate 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.
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.
In this case, step down the jet needle clip by
one groove and move up the needle to enrich
the mixture.
EAM30605
RELATIONSHIP WITH THROTTLE OPENING
The flow of the fuel through the carburetor main
system is controlled by the main jet and then, it
is further regulated by the area between the
main nozzle and the jet needle. On the relation-
ship between the fuel flow and the throttle open-
ing, the fuel flow relates to the jet needle straight
portion diameter around 1/8 to 1/4 throttle open-
ing, whereas around 1/4 to 1/1 throttle opening it
relates to the taper starting position and to the
clip position.
Therefore, the fuel flow is balanced at each
stage of throttle opening by the combination of
the jet needle straight portion diameter, taper
starting position and clip position.
Example:
A. In case of being 0.5 groove leaner in relation
to N3EW-3rd groove.
a. Difference of 0.5 groove
Standard clip position No. 2 groove
1
A. Lean (larger diameter)
B. Rich (smaller diameter)
1. 1/8 throttle
2. 1/4 throttle
3. 1/2 throttle
4. 1/1 throttle
a. Main nozzle
N 3 EW- 3
N 3 EW-4
N 3 EJ- 3
N 8 RW- 3