Yamaha YZ125 2016 Owner's Service Manual page 158

EXAMPLES OF CARBURETOR SETTING DEPENDING ON SYMPTOM
Symptom
At full throttle
Stall at high speeds
*Hard breathing
Shearing noise
Whitish spark plug
↓
Lean mixture
At full throttle
Stop of speed pick-up
Slow speed pick-up
Slow response
Sooty spark plug
↓
Rich mixture
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 extremely low
speed
Poor response in the range of
low to intermediate 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
Increase main jet calibration no. (Gradual-
ly)
Decrease main jet calibration no. (Gradual-
ly)
*In case of racing slight enrichment of mix-
ture reduces engine trouble.
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 diameter.
Use jet needle with a larger diameter.
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 procedures.
Raise jet needle clip position.
If no effect, reverse the above procedures.
Check overall settings.
Use main jet having lower calibration no.
Raise jet needle clip position. (1 groove up)
If no effect, reverse the above procedures.
Turn in pilot air screw.
7-5
Checking
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
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 fil-
ter
Groove 1
Clip
Groove 2
Groove 3
Groove 4
Groove 5
(Standard)
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 pro-
vides 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
Leaner
Richer