2001 YAMAHA WR 250F fuel

7 - 3
TUN

SETTING
Effects of the setting parts on the throttle
valve opening

Closed
õ
Fully open
1
Pilot screw/pilot jet
2
Throttle valve cutaway
3
Jet needle
4
Main jet
4/4
3/4
1/2
1/4
1/8
0õ

1/2 3/4 1/4 1/8
1
2
3
4
õ

Main system
The FLATCR carburetor has a primary main
jet. This type of main jet is perfect for racing
motorcycles since it supplies an even flow of
fuel, even at full load. The main air jet has
almost no effect on the air-fuel mixture. Use
the main jet and the jet needle to set the car-
buretor.
Pilot system
The FLATCR carburetor is manufactured with
a pilot screw. The pilot screw adjustment
ranges from fully closed throttle to 1/4 open
throttle.

7 - 4
TUN

SETTING
1
Jet needle
2
Main air jet
3
Pilot air jet
4
Needle jet
5
Main jet
6
Pilot jet
7
Pilot screw
Main jet adjustment
The richness of the air-fuel mixture at full throt-
tle 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, resulting in poor accel-
eration.
Effects of changing the main jet (reference)

Idle
õ
Fully open
Standard main jet #170
1/4 1/2 3/4
+10%
#170
–10%
#180
#160
õ

7 - 5
TUN

SETTING
Pilot screw adjustment
The richness of the air-fuel mixture with the
throttle fully closed to 1/4 open can be set by
turning the pilot screw
1
. Turning in the pilot
screw will make the mixture lean at low
speeds, and turning it out will enrich it.
NOTE:

l
If the engine idling speed fluctuates, turn the
pilot screw only 1/2 of a turn in either direc-
tion.
l
To optimize the fuel flow at a smaller throttle
opening, each machine’s pilot screw has
been individually set at the factory. Before
adjusting the pilot screw, turn it in fully and
count the number of turns. Record this num-
ber as the factory-set number of turns out.
Effects of adjusting the pilot screw (reference)

Idle
õ
Fully open
1
2-1/4 turns out
2
1-1/4 turns out
3
1-3/4 turns out
Standard pilot
screw position1-3/4
(example)
1/4 1/2 3/4
+5%
–5% AB
1
23

Pilot jet adjustment
The richness of the air-fuel mixture with the
throttle open 1/4 or less can be set by adjust-
ing the pilot jet
1
.
Standard pilot jet #42

7 - 6
TUN

SETTING
Effects of adjusting the pilot jet (reference)

Idle
õ
Fully open
1/4 1/2 3/4
+5%
–5%
#45
#40#42

õ

Jet needle groove position adjustment
Adjusting the jet needle
1 position affects the
acceleration when the throttle is 1/8 to 3/4
open.
1. Too rich at intermediate speeds
Rough engine operation is felt and the
engine will not pick up speed smoothly. In
this case, step up the jet needle clip by
one groove and move down the needle to
lean out the mixture.
2. Too lean at intermediate speeds
The engine breathes hard and will not
pick up speed quickly.
Step down the jet needle clip by one
groove and move up the needle to enrich
the mixture.
* For EUROPE
** For CDN, ZA, AUS and NZ
NOTE:
It is recommended that in the summer the clip
position be changed from the 4th (STD) to the
2nd groove for a leaner air-fuel mixture. (For
AUS and NZ only)
Effects of changing the jet needle groove
position (reference)
Idle
õFully open
1No.4 groove
2No.2 groove
3No.3 groove
Standard clip positionNo.3 groove
*, ** No. 4 groove
1/4 1/2 3/4
+10%
–10%
1
23
õ

7 - 7
TUN
Jet needle adjustment
The jet needle is adjusted by changing it.
* For EUROPE
** For CDN, ZA, AUS and NZ
The tapered sections of all jet needles have
the same starting positions, but the needles
are available with different straight-portion
diameters.Standard jet needleOBELP
*OBEGP
**OBEHP

OBELP - 4
Clip position
Diameter a of straight portion
Leak jet adjustment (accelerator pump
adjustment)
The leak jet 1 is a setting part that adjusts the
flow of fuel discharged by the accelerator
pump. Since the accelerator 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 different from other
setting parts that adjust a fuel mixture for each
throttle opening (each engine speed).
1. The engine breathes hard in quick throt-
tle opening.
Select a leak jet having lower calibrating
No. than standard to enrich the mixture.
#105 ® #95
2. Rough engine operation is felt in quick
throttle opening.
Select a leak jet having higher calibrating
No. than standard to lean out the mixture.
#105 ® #115
* For EUROPE
** For CDN, ZA, AUS and NZStandard leak jet#60
*
, ** #105
1
SETTING

7 - 8
TUN
Effects of changing the jet needle (reference)
(Diameter of the straight portion)
Changing the diameter of the straight portion
adjusts the air-fuel mixture when the throttle is
1/8 to 1/4 open.
Idle
õFully open
Relationship with throttle opening
The flow of the fuel through the carburetor
main system is controlled by the main jet and
then, it is further regulated by the area
between the main nozzle and the jet needle.
The fuel flow relates to the diameter of the
straight portion of the jet needle with the throt-
tle 1/8 to 1/4 open and relates to the clip posi-
tion with the throttle 1/8 to 3/4 open.
Therefore, the fuel flow is balanced at each
stage of throttle opening by the combination of
the jet needle straight portion diameter and
clip position.
+10 %
P
–10 %N
Q
1/4 1/2 3/4õ
SETTING

7 - 10
TUN
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.
(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 oper-
ates smoothly.
At full throttle
Speed pick-up stops
Slow speed pick-up
Slow response
Sooty spark plug
Rich mixtureDecrease main jet calibration no.
(Gradually)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 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 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
intermediate 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 oper-
ates smoothly.
Jet needleClip
Groobe 7 Groobe 6
Groobe 5
Groobe 4
Groobe 3
Groobe 2
Groobe 1
Leaner
­
(Standard)
¯
Richer
*
SETTING