Yamaha YZ125 2009, Owner'S Service Manual

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7-7
ENGINE
EXAMPLES OF CARBURETOR SETTING DEPENDING ON SYMPTOM
Symptom Setting Checking
At full throttle
Stall at high speeds
*Hard breathing
Shearing noise
Whitish spark plug
↓
Lean mixture
Increase main jet calibration no.
(Gradually) Discoloration of spark plug
→
If tan
color, it is in good condition.
If cannot be corrected:
Clogged float valve seat
Clogged fuel hose
Clogged fuel cock
At full throttle
Stop of speed pick-up
Slow speed pick-up
Slow response
Sooty spark plug
↓
Rich mixture
Decrease main jet calibration no.
(Gradually)
*In case of racing slight enrichment
of mixture reduces engine trouble. Discoloration of spark plug
→
If tan
color, it is in good condition.
If not effect:
Clogged air filter
Fuel overflow from carburetor
Clogged main air passage or
clogged filter
Lean mixture Lower jet needle clip position. (1
groove down)
Clip position indicates the position
of jet needle groove, to which the
clip is fitted.
The position is numbered from the
top.
If a change in the clip position (1
groove) is effective, try another jet
needle that provides a difference of
0.5 in the clip position.
Rich mixture Raise jet needle clip position. (1
groove up)
1/4–3/4 throttle
*Hard breathing
Lack of speed
Lower jet needle clip position. (1
groove down)
1/4–1/2 throttle
Slow speed pick-up
White smoke
Poor acceleration
Raise jet needle clip position. (1
groove up)
0–1/4 throttle
*Hard breathing
Speed down
Use jet needle having a smaller di-
ameter. Number of turns-back
→
Correct
properly
Overflow from carburetor
0–1/4 throttle
Poor acceleration
White smoke
Use jet needle with a larger diame-
ter.
Unstable at low speeds
Pinking noise
Lower jet needle clip position.
(1 groove down)
Turn in pilot air screw.
Poor response at ex-
tremely low speed Reduce pilot jet calibration No.
Turn out pilot air screw.
If not effect, reverse the above pro-
cedures. Dragging brake
Overflow from carburetor
Poor response in the
range of low to intermedi-
ate speeds Raise jet needle clip position.
If no effect, reverse the above pro-
cedures.
Groove 1
Groove 2
Groove 3
Groove 4
Groove 5
Clip
Jet needle
Leaner
(Standard)
Richer