Jet Needle Adjustment; Jet Needle Groove Position Adjustment; Relationship With Throttle Opening - Yamaha YZ125X 2021 Owner's Service Manual

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JET NEEDLE ADJUSTMENT

On the carburetors used in the YZ125X the main
nozzle is a non disassembly type, so it can not
be replaced. Therefore, carburetor setting re-
quires the change of the jet needle.
1. The jet needle setting parts, having the same
taper angle, are available in different straight
portion diameters and in different taper start-
ing positions.
Standard jet needle
In the case of the same number of clip posi-
tion, changing from 6BFY43-74 to 6BFY42-
74 has the same effect as a lowering of 0.5-
clip position. And in the case of the same
number of clip position, changing from
6BFY43-74 to 6BFY44-74 has the same ef-
fect as a rising of 0.5-clip position.
A. Difference in straight portion diameter
B. Difference in clip position
a. Reference needle
b. Rich
c. Lean
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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
6BFY43-74
2. Too lean at intermediate speeds
6BFY43-72
6BFY43-73
6BFY43-74
6BFY43-75
6BFY43-76
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RELATIONSHIP WITH THROTTLE OPENING

6BFY44-74
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-
6BFY43-74
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-
6BFY42-74
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.
10-3
Standard clip position
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 or 0.5 groove and move down the
needle to lean out the mixture.
The engine breathes hard and will not pick up
speed quickly.
In this case, step down the jet needle clip by
one groove or 0.5 groove and move up the
needle to enrich the mixture.
ENGINE
No. 3 groove
1