Yamaha YZ250F(T) Owner's Service Manual page 636

Examples of carburetor setting depending on symptom
Symptom
At full throttle
Hard breathing
Shearing noise
Whitish spark plug
Lean mixture
At full throttle
Speed pick-up stops
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
Poor acceleration
Closed to 1/4 throttle
Hard breathing
Speed down
Closed to 1/4 throttle
Poor acceleration
Poor response in the low to
intermediate speeds
Poor response when throttle
is opened quickly
This should be taken simply for an example. It is necessary to set the carburetor while checking
*
the operating conditions of the engine.
Setting
Increase main jet calibration no.
(Gradually)
Decrease main jet calibration no.
(Gradually)
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 with a smaller diame-
ter.
Use jet needle with a larger diameter.
Raise jet needle clip position.
(1 groove up)
Raise jet needle clip position.
If this has no effect, lower the jet nee-
dle clip position.
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.
7 - 10
T U N
SETTING
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
Check that the accelerator pump oper-
ates smoothly.
Discoloration of spark plug →
If tan color, it is in good condition.
If cannot be corrected:
Clogged air filter
Fuel overflow from carburetor
Groove 1
Groove 2
Clip
Groove 3
Groove 4
Groove 5
Groove 6
Groove 7
Jet needle
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).
Slow-speed-circuit passage
Clogged → Clean.
Overflow from carburetor
Check air filter for fouling.
Check that the accelerator pump oper-
ates smoothly.
Leaner
↑
(Standard)
↓
Richer