Yamaha YZ125(D), Owner'S Service Manual

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7-2
ENGINE
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 or 0.5 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 or 0.5
groove and move up the needle to
enrich the mixture.
ADJUSTING THE JET NEEDLE
On the carburetors used in the
YZ125, the main nozzle is a non dis-
assembly type, so it can not be re-
placed. Therefore, 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.
In the case of the same number of clip
position, 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 effect as
a rising of 0.5-clip position.
A. Difference in straight portion
diameter.
B. Difference in clip position
a. Reference needle
b. 0.5 richer
c. 0.5 leaner
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. On the re-
lationship between the fuel flow and
the throttle opening, the fuel flow re-
lates to the straight portion of the jet
needle at full closed–1/8 throttle, to
the 1st tapered portion at 1/4 throttle,
to the second tapered portion at 1/2
throttle, to the third tapered portion at
3/4 throttle and to the fourth tapered
portion at full open.
Therefore, the fuel flow is balanced at
each stage of throttle opening by a
combination of the jet needle diame-
ter and clip position.
<Example>
A. Lean (larger diameter)
B. Rich (smaller diameter)
C. 1st taper
D. 2nd taper
E. 3rd taper
F. 4th taper
1. Full closed
2. 1/4 throttle
3. 1/2 throttle
4. 3/4 throttle
5. Full open
a. Main nozzle
Standard pilot jet
#40
*#45
*Except for USA and CDN
Standard clip
position
No.3 groove
Standard jet
needle
6BFY43-74
6BFY43-73
6BFY43-74
6BFY43-75
6BFY44-74
6BFY43-74
6BFY42-74
6BFY43-74-3
6BFY43-74-2
6BFY43-75-3
7