Yamaha yz250 2015, Owner'S Manual

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7-5
ENGINE
EXAMPLES OF CARBURETOR SETTING DEPENDING ON SYMPTOM
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, needle clip
position, pilot jet and pilot air screw. If the result of setting is still unsatisfactory, it is advisable to change the sizes of the jet
needle.
Symptom Setting Checking
At full throttle
Hard breathing
Shearing noise
Whitish spark plug
↓
Lean mixture
Increase main jet calibration no. (Gradual-
ly) 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. (Gradual-
ly)
*In case of racing slight enrichment of mix-
ture 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
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.
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.
Groove 1
Groove 2
Groove 3
Groove 4
Groove 5
Clip
Jet needle
Leaner
(Standard)
Richer