Yamaha YZ450F, Owner'S Service Manual

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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.
Symptom Setting Checking
At full throttle
Hard breathing
Shearing noise
Whitish spark plug
Lean mixture
Increase main jet calibration no.
(Gradually)
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
Check that the accelerator pump
operates smoothly.
At full throttle
Speed pick-up stops
Slow speed pick-up
Slow response
Sooty spark plug
Rich mixture
Decrease main jet calibration no.
(Gradually)
Discoloration of spark plug
→
If tan color, it is in good condition.
If cannot be corrected:
Clogged air cleaner
Fuel overflow from carburetor
Lean mixture Lower jet needle clip position.
(1 groove down)
The clip position is the jet needle
groove on which the clip is installed.
The positions are numbered from the
top.
Check that the accelerator pump oper-
ates smoothly (except for rich mixture
symptom).
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
Poor acceleration
Raise jet needle clip position.
(1 groove up)
Closed to 1/4 throttle
Hard breathing
Speed down
Use jet needle with a smaller diame-
ter. Slow-speed-circuit passage
Clogged
→
Clean.
Overflow from carburetor
Closed to 1/4 throttle
Poor acceleration
Use jet needle with a larger diameter.
Raise jet needle clip position. (1
groove up)
Poor response in the low to
intermediate speeds Raise jet needle clip position.
If this has no effect, lower the jet nee-
dle clip position.
Poor response when throt-
tle is opened quickly Check overall settings.
Use main jet with a lower calibration
no.
Raise jet needle clip position.
(1 groove up)
If these have no effect, use a main jet
with a higher calibration no. and lower
the jet needle clip position. Check air cleaner for fouling.
Check that the accelerator pump
operates smoothly.
Jet needle
Clip
Groove 7
Groove 6
Groove 5
Groove 4
Groove 3
Groove 2
Groove 1
Leaner
↑
(Standard)
↓
Richer
*
SETTING