Yamaha YZ125(D), Owner'S Service Manual

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7-5
ENGINE
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.
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 pro-
vides 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 diameter. Number of turns-back → Correct properly
Overflow from carburetor
0–1/4 throttle
Poor acceleration
White smoke
Use jet needle with a larger diameter.
Unstable at low speeds
Pinking noise
Lower jet needle clip position.
(1 groove down)
Turn in pilot air screw.
Poor response at extremely low
speed Reduce pilot jet calibration No.
Turn out pilot air screw.
If not effect, reverse the above procedures. Dragging brake
Overflow from carburetor
Poor response in the range of
low to intermediate speeds Raise jet needle clip position.
If no effect, reverse the above procedures.
Poor response when throttle is
opened quickly 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. Check air filter for fouling.
Poor engine operation Turn in pilot air screw. Check throttle valve operation.
* In case of hard breathing, check the air vent hose for clogging.
Symptom Setting Checking
Groove 1
Groove 2
Groove 3
Groove 4
Groove 5
Clip
Jet needle
Leaner
(Standard)
Richer