Idling Range – A; Opening Up – B; Part-Throttle Range – C; Full Throttle Range – D - KTM 400-660 LC4 1998-2005 Repair Manual

8-24E
Idling range – A
Operation with closed throttle valve. This range is influenced by the position of the air control screw
2
screw . 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 throttle stop screw is used to adjust the basic position of the throttle. The mixture control screw is used to control the idle
mixture which arrives at the engine via the idle system. Turning it clockwise will reduce the amount of gasoline (lean mixture),
turning it counterclockwise will increase the amount of gasoline (rich mixture).
TO ADJUST IDLING CORRECTLY, PROCEED AS FOLLOWS:
1 Turn in mixture control screw up to the stop, and turn it back out by 1.5 turns
2 Warm up the engine
3 Use throttle stop screw to adjust normal idling speed (1400-1500 r.p.m.)
4 Turn mixture control screw slowly clockwise until idling speed starts to decrease. Memorize this position, and turn mixture
control screw slowly counterclockwise until the idling speed will decrease again. Adjust the point of the highest idling speed
between these two positions. (The highly competitive user will make his adjustment 1/4 turn leaner because his engine will
heat up more in competitive use).
NOTE: If you fail to obtain a satisfying result by following the procedure described above, an incorrectly dimensioned idling
nozzle may be the cause. In case:
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 Now, use the slide stop screw to adjust the desired idling speed
6 In cases of greater changes in outside temperature and extremely different altitudes, the idling speed should be readjusted.
Opening up – B
Engine behavior when the throttle opens. The idle jet and the shape of the throttle valve influences this range. If, despite good
idling-speed and part-throttle setting, the engine sputters when the throttle is fully opened and develops its full power not
smoothly but suddenly at high engine speeds, the mixture to the carburetor will be too rich, the fuel level too high or the float
needle is leaking.
Part-throttle range – C
Operation with partly open throttle valve. This range is only influenced by the jet needle (shape and position). The optimum part-
throttle setting is controlled by the idling setting in the lower range and by the main jet in the upper range.
Full throttle range – D
Operation with the throttle fully open (flat out). This range is influenced by the main jet and the jet needle. If the porcelain of the
new spark plug is found to have a very bright or white coating after a short distance of riding flat out, a larger main jet is
required. If the porcelain is dark brown or black with soot the main jet must be replaced by a smaller one.
Basic information on carburetor wear
As a result of engine vibrations, throttle valve, jet needle, and needle jet are subjected to increased wear. This wear may cause
carburetor malfunction (e.g., overly rich mixture). Therefore, these parts should be replaced after 10000 kilometers (6000 miles).
main jet
jet needle
jet needle
idling jet
throttle valve
mixture regulating screw
idle adjusting screw
1
and the idle adjusting