KTM 250 SX-F 2005, Repair Manual

Page: 104 / 125

8-13
CARBURETOR – Adjust idling
Idling adjustment of the carburetor strongly affects the engine’s
starting behavior. That is, an engine whose idling speed is adjusted
correctly will be easier to start than one whose idling speed has not
been adjusted correctly.
The idle speed is controlled by means of the adjusting wheel
1
and
the mixture control screw
2
. The adjusting wheel is used to adjust the
basic setting of the slide. The mixture control screw is used to control
the idle mixture which arrives at the engine by way of the idle system.
Clockwise turning reduces the fuel quantity (lean mixture),
counterclockwise turning increases the fuel quantity (rich mixture).
TO ADJUST IDLING CORRECTLY, PROCEED AS FOLLOWS:
1. Turn in mixture control screw
2
up to the stop, and turn it back out
to the basic position (see technical date-engine)
2. Warm up the engine
3. Use the adjusting wheel
1
to set the normal idle speed (1400 -
1500 rpm).
4. Turn mixture control screw
2
slowly clockwise until idling speed
starts to decrease. Memorize this position, and turn mixture control
screw slowly counterclockwise until the idling speed decreases
again. Adjust the point of the highest idling speed between these
two positions. If, in the course of this procedure, the speed
undergoes a relatively high increase, reduce the idle speed to a
normal level and repeat the procedure specified in 4. Serious
competitive racers will choose a setting approx. 1/4 turn (clockwise)
leaner than this ideal value because their engine will heat up more
when used in competitions.
NOTE: If you fail to obtain a satisfying result by following the
procedure described above, an incorrectly dimensioned idling nozzle
may be the cause. If:
a) the mixture control screw has been screwed in up to the stop
without causing any change in rotational speed, a smaller idling jet
has to be installed;
b) the engine dies when the mixture control screw is still open by
2 turns, a larger idling jet needs to be selected.
Naturally, in cases of jet changes, you have to start your adjusting
work from the beginning.
5. Then, use the adjusting wheel to set the desired idle speed.
6. In cases of greater changes in the outside temperature and
extremely different altitudes, the idling speed should be readjusted.
Basic information on carburetor wear
As a result of engine vibrations, the throttle valve, jet needle, and
needle jet are subjected to increased wear. This wear may cause the
carburetor to malfunction (e.g., overly rich mixture). Therefore, these
parts should be replaced after 200 hours.
Adjusting the mixture control screw
Introduce the special tool 590.29.034.000 on the mixture control
screw
2
at the carburetor bottom. Press the tool slightly upward and
turn the adjusting wheel
3
until the tool engages the slot of the
mixture control screw.
Now, you can go about adjusting the screw. Marks were provided on the
adjusting wheel, making it easier to keep track of the turns.
1
2
3