Carburetor - Adjust idling. KTM 250 SX-F, 250 SX-F 2007, 450 SX-F 2007

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MAINTENANCE WORK ON CHASSIS AND ENGINE

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CARBURETOR – Adjust idling (Keihin FCR-MX 39/41) *

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.

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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 approx. 200 hours.

Checking the float level (float height) *

For this purpose, dismount the carburetor and remove the float chamber. Hold the carburetor in a slanted position such that the float will abut the float needle valve but not compress it.

In this position, the edge of the float should be parallel with the float chamber sealing surface (see illustration).

If the float height does not correspond to the desired value, check the float needle valve and, if necessary, replace it.

If the float needle valve is o.k., you can adjust the float height by bending the float lever [3].

Mount the float chamber, install the carburetor, and adjust the idle speed.

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