Yamaha YZ250 2021, Owner'S Manual

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ENGINE
8-7
* marked: In case of hard breathing, check the carburetor breather hoses 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, pilot jet and pilot
air screw.
Lean mixture Lower jet needle clip position. (1
groove down)
1. Groove 1
2. Groove 2
3. Groove 3
4. Groove 4
5. Groove 5
6. Jet needle
7. Clip
Leaner
(Standard)
Richer
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)
The clip position is the jet needle
groove on which the clip is installed.
The positions are numbered from the
top.
•
Closed to 1/4 throttle
*Hard breathing
Speed down
Use jet needle having a smaller diam-
eter.
•
Closed to 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 proce-
dures.
Dragging brake
Overflow from carburetor
Poor response in the low to
intermediate speeds
Raise jet needle clip position.
If no effect, reverse the above proce-
dures.
Poor response when throt-
tle 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 proce-
dures.
Check air filter for fouling.
Poor engine operation Turn in pilot air screw. Check throttle valve operation.
Symptom Setting Checking