Yamaha YZ250F(X), Owner'S Service Manual

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7-4
ENGINE
SPECIFICATIONS OF JET NEEDLE
EXAMPLES OF CARBURETOR SETTING DEPENDING ON SYMPTOM
Diameter of straight portion
Rich Lean
N P Q R S T U
Rich 1 richer NFPN-6 NFPP-6 NFPQ-6 NFPR-6 NFPS-6 NFPT-6 NFPU-6
0.5 richer NFLN-6 NFLP-6 NFLQ-6 NFLR-6 NFLS-6 NFLT-6 NFLU-6
STD NFPN-5 NFPP-5 NFPQ-5 NFPR-5 NFPS-5 NFPT-5 NFPU-5
0.5 leaner NFLN-5 NFLP-5 NFLQ-5 NFLR-5 NFLS-5 NFLT-5 NFLU-5
Lean 1 leaner NFPN-4 NFPP-4 NFPQ-4 NFPR-4 NFPS-4 NFPT-4 NFPU-4
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
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. (Gradual-
ly) Discoloration of spark plug
→
If tan color, it
is in good condition.
If cannot be corrected:
Clogged air filter
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 operates
smoothly. (except for rich mixture symp-
tom).
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 diameter. 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 in-
termediate speeds Raise jet needle clip position.
If this has no effect, lower the jet needle clip
position.
Poor response when throttle 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 filter 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