Yamaha YZ125 2019 Owner's Service Manual page 226

Symptom
Lean mixture
Rich mixture
1/4–3/4 throttle
*Hard breathing
Lack of speed
1/4–1/2 throttle
Slow speed pick-up
White smoke
Poor acceleration
0–1/4 throttle
*Hard breathing
Speed down
0–1/4 throttle
Poor acceleration
White smoke
Unstable at low speeds
Pinking noise
Poor response at ex-
tremely low speed
Poor response in the
range of low to intermedi-
ate speeds
Poor response when
throttle is opened quickly
Poor engine operation
* In case of hard breathing, check the air vent hose for clogging.
This should be taken simply for an example. It is necessary to set the carburetor while checking the
operating conditions of the engine and discoloration of spark plugs. Normally, carburetor setting is
made by means of the main jet, jet needle clip position (including one with 0.5 difference), pilot jet
and pilot air screw. If the result of setting is still unsatisfactory, it is advisable to change the diameter
of the straight portion the jet needle.
Setting
Lower jet needle clip position. (1
groove down)
Raise jet needle clip position. (1
groove up)
Lower jet needle clip position. (1
groove down)
Raise jet needle clip position. (1
groove up)
Use jet needle having a smaller di-
ameter.
Use jet needle with a larger diame-
ter.
Lower jet needle clip position.
(1 groove down)
Turn in pilot air screw.
Reduce pilot jet calibration No.
Turn out pilot air screw.
If not effect, reverse the above pro-
cedures.
Raise jet needle clip position.
If no effect, reverse the above pro-
cedures.
Check overall settings.
Use main jet having lower calibra-
tion no.
Raise jet needle clip position. (1
groove up)
If no effect, reverse the above pro-
cedures.
Turn in pilot air screw.
7-7
Checking
Groove 1
Clip
Groove 2
Groove 3
Leaner
Groove 4
Groove 5
(Standard)
Richer
Jet needle
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.
Number of turns-back → Correct
properly
Overflow from carburetor
Dragging brake
Overflow from carburetor
Check air filter for fouling.
Check throttle valve operation.
ENGINE