CARB YAMAHA YZ250F 2009 Owner's Guide

7-3
ENGINE
ADJUSTING THE LEAK JET
(ADJUSTING THE ACCELERATOR
PUMP)
The leak jet "1" is a setting part that
adjusts the flow of fuel discharged by
the accelerator pump. Since the ac-
celerator pump operates only when
throttle is open, the leak jet is used to
adjust a fuel mixture ratio for quick
throttle opening and is therefore dif-
ferent from other setting parts that ad-
just a fuel mixture for each throttle
opening (each engine speed).
1. When the engine breathes hard in
quick throttle opening, select a
leak jet having lower calibrating
No. than standard to enrich the
mixture. #70→#60
2. When rough engine operation is
felt in quick throttle opening, se-
lect a leak jet having higher cali-
brating No. than standard to lean
out the mixture. #70
→#80CARBURETOR SETTING PARTS
Standard leak jet #70
Main jet Size Part number
Rich #188 4MX-14943-95
#185 4MX-14943-44
#182 4MX-14943-94
#180 4MX-14943-43
(STD) #178 4MX-14943-93
#175 4MX-14943-42
#172 4MX-14943-92
#170 4MX-14943-41
Lean #168 4MX-14943-91
Pilot jet Size Part number
Rich #48 4MX-14948-06
* (STD) #45 4MX-14948-05
(STD) #42 4MX-14948-04
#40 4MX-14948-03
Lean #38 4MX-14948-02
Jet nee-
dleSize Part number
Rich NFPN 5TA-14916-PN
NFPP 5TA-14916-PP
NFPQ 5TA-14916-P1
NFPR 5TA-14916-PR
(STD) NHKR 5TA-14916-KR
NFPS 5TA-14916-PS
NFPT 5TA-14916-PT
Lean NFPU 5TA-14916-PU
Rich NFLN 5TA-14916-LN
NFLP 5TA-14916-LP
NFLQ 5TA-14916-L1
* (STD) NFLR 5TA-14916-LR
NFLS 5TA-14916-LS
NFLT 5TA-14916-LT
Lean NFLU 5TA-14916-LU
Leak jet Size Part number
Rich #40 4JT-1494F-03
#50 4JT-1494F-07
#60 4JT-1494F-11
(STD) #70 4JT-1494F-15
#80 4JT-1494F-19
#90 4JT-1494F-23
Lean #100 4JT-1494F-27
* Except for USA and CDN

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.
Symptom Setting Checking
At full throttle
Hard breathing
Shearing noise
Whitish spark plug
↓
Lean mixtureIncrease 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 mixtureDecrease 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 speedLower jet needle clip position. (1 groove
down)
1/4–1/2 throttle
Slow speed pick-up
Poor accelerationRaise jet needle clip position. (1 groove up)
Closed to 1/4 throttle
Hard breathing
Speed downUse jet needle with a smaller diameter. Slow-speed-circuit passage
Clogged→Clean.
Overflow from carburetor
Closed to 1/4 throttle
Poor accelerationUse jet needle with a larger diameter.
Raise jet needle clip position. (1 groove up)
Poor response in the low to in-
termediate speedsRaise jet needle clip position.
If this has no effect, lower the jet needle clip
position.
Poor response when throttle is
opened quicklyCheck 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 needleClip
Groove 7 Groove 6
Groove 5
Groove 4
Groove 3
Groove 2
Groove 1
Leaner
(Standard)
Richer