carb YAMAHA YZ450F 2008 Owner's Guide

6-7
THROTTLE POSITION SENSOR SYSTEM
10. Put the aligning marks "a" on the
throttle position sensor and car-
buretor.
11. Stop the engine.
12. Remove the carburetor.
13. Tighten:
• Screw (throttle position sensor)
"1"
Tighten the screw (throttle position
sensor) using the T25 bit.
14. Install the carburetor.
CHECKING THE THROTTLE
POSITION SENSOR INPUT
VOLTAGE
1. Disconnect the throttle position
sensor coupler.
2. Start the engine.
3. Inspect:
• Throttle position sensor input volt-
age
Out of specification
→Replace
the CDI unit.Throttle po-
sition sen-
sor output
voltageTester se-
lector posi-
tion
0.58–0.78 V DCV
Tester (+) lead
→Blue lead "1"
Tester (-) lead
→Black/Blue lead
"2"
Throttle po-
sition sen-
sor input
voltage Tester se-
lector posi-
tion
4–6 V DCV-20

7-1
ENGINE
TUNING
ENGINE
CARBURETOR SETTING
• The air/fuel mixture will vary de-
pending on atmospheric conditions.
Therefore, it is necessary to take
into consideration the air pressure,
ambient temperature, humidity,
etc., when adjusting the carburetor.
• Perform a test run to check for prop-
er engine performance (e.g., throt-
tle response) and spark plug(-s)
discoloration or fouling. Use these
readings to determine the best pos-
sible carburetor setting.
It is recommended to keep a record of
all carburetor settings and external
conditions (e.g., atmospheric condi-
tions, track/surface conditions, lap
times) to make future carburetor set-
ting easier.
• The carburetor is a part of the
fuel line. Therefore, be sure to in-
stall it in a wellventilated area,
away from flammable objects
and any sources of fire.
• Never look into the carburetor in-
take. Flames may shoot out from
the pipe if the engine backfires
while it is being started. Gasoline
may be discharged from the ac-
celerator pump nozzle when the
carburetor has been removed.
• The carburetor is extremely sen-
sitive to foreign matter (dirt,
sand, water, etc.). During instal-
lation, do not allow foreign mat-
ter to get into the carburetor.
• Always handle the carburetor
and its components carefully.
Even slight scratches, bends or
damage to carburetor parts may
prevent the carburetor from
functioning correctly. Carefully
perform all servicing with the ap-
propriate tools and without ap-
plying excessive force.
• When the engine is stopped or
when riding at no load, do not
open and close the throttle un-
necessarily. Otherwise, too
much fuel may be discharged,
starting may become difficult or
the engine may not run well.
• After installing the carburetor,
check that the throttle operates correctly and opens and closes
smoothly.
ATMOSPHERIC CONDITIONS AND
CARBURETOR SETTINGS
The air density (i.e., concentration of
oxygen in the air) determines the rich-
ness or leanness of the air/fuel mix-
ture.
• Higher temperature expands the air
with its resultant reduced density.
• Higher humidity reduces the
amount of oxygen in the air by so
much of the water vapor in the
same air.
• Lower atmospheric pressure (at a
high altitude) reduces the density of
the air.
EFFECT OF SETTING PARTS IN
RELATION TO THROTTLE VALVE
OPENING
A. Closed
B. Fully open
1. Pilot screw/pilot jet
2. Throttle valve cutaway
3. Jet needle
4. Main jetCONSTRUCTION OF
CARBURETOR AND SETTING
PARTS
The FLATCR carburetor has a prima-
ry main jet. This type of main jet is
perfect for racing machines since it
supplies an even flow of fuel, even at
full load. Use the main jet and the jet
needle to set the carburetor.
The FLATCR carburetor is manufac-
tured with a pilot screw. The pilot
screw adjustment ranges from fully
closed throttle to 1/4 open throttle.
1. Jet needle
2. Pilot air jet
3. Needle jet
4. Main jet
5. Pilot jet
6. Pilot screw
ADJUSTING THE MAIN JET
The richness of the air-fuel mixture at
full throttle can be set by changing the
main jet "1".
If the air-fuel mixture is too rich or too
lean, the engine power will drop, re-
sulting in poor acceleration.
Effects of changing the main jet
(reference)
A. Idle
B. Fully open
1. #162
2. #158 Air
tem
p.Hu-
midi-
tyAir
pres-
sure
(alti-
tude)Mix-
tureSet-
ting
High HighLow
(high)Rich-
erLean-
er
Low LowHigh
(low)Lean-
erRich-
er
Standard main jet #160

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. #55
→#50CARBURETOR SETTING PARTS
Standard leak jet #55
Main jet Size Part number
Rich #172 4MX-14943-92
#170 4MX-14943-41
#168 4MX-14943-91
#165 4MX-14943-40
#162 4MX-14943-90
(STD) #160 4MX-14943-39
#158 4MX-14943-89
#155 4MX-14943-38
#152 4MX-14943-88
#150 4MX-14943-37
Lean #148 4MX-14943-87
Pilot jet Size Part number
Rich #52 4MX-14948-08
#50 4MX-14948-07
#48 4MX-14948-06
(STD) #45 4MX-14948-05
#42 4MX-14948-04
#40 4MX-14948-03
Lean #38 4MX-14948-02
Jet needle Size Part number
Rich NFPN 5TA-14916-PN
NFPP 5TA-14916-PP
NFPQ 5TA-14916-P1
NFPR 5TA-14916-PR
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 #45 4JT-1494F-05
#50 4JT-1494F-07
(STD) #55 4JT-1494F-09
#60 4JT-1494F-11
Lean #65 4JT-1494F-13

7-4
ENGINE
SPECIFICATIONS OF JET NEEDLE
EXAMPLES OF CARBURETOR SETTING DEPENDING ON SYMPTOMDiameter of straight portion
RichLean
NPQRS TU
Rich 1 richer NFLN-4 NFLP-4 NFLQ-4 NFLR-4 NFLS-4 NFLT-4 NFLU-4
0.5 richer NFPN-3 NFPP-3 NFPQ-3 NFPR-3 NFPS-3 NFPT-3 NFPU-3
STD NFLN-3 NFLP-3 NFLQ-3 NFLR-3 NFLS-3 NFLT-3 NFLU-3
0.5 leaner NFPN-2 NFPP-2 NFPQ-2 NFPR-2 NFPS-2 NFPT-2 NFPU-2
Lean 1 leaner NFLN-2 NFLP-2 NFLQ-2 NFLR-2 NFLS-2 NFLT-2 NFLU-2
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

7-5
ENGINE
* This should be taken simply for an example. It is necessary to set the carburetor while checking the operating conditions
of the engine.