KTM 250 SX - Manual (Page 16)

Repair manual KTM 250 / 300 / 380
Art No 3206004 -E
2-2C
main jet
jet needle
jet needle
air control screw
idle adjusting screw
idle jet
throttle valve
Idling range A
Operation with closed throttle valve. This range is influenced by the position of the air control screw 1 and the idle adjusting screw
2 . Only make adjustments when the engine is hot.
To this end, slightly increase the idling speed of the engine by means of the idle adjusting screw. Turning it clockwise produces a
higher idling speed and turning the screw counterclockwise produces a lower idling speed. Create a round and stable engine speed
using the air control screw (basic position of the air control screw = open by 1.5 turns). Then adjust to the normal idling speed by
means of the idle adjusting screw.
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 and smokes 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. If the engine runs
on a four-stroke cycle or with reduced power when it is accelerated with the throttle partly open, the jet needle must be lowered by
one notch. If then the engine pings, especially when accelerating under full power at maximum engine revs, the jet needle should
be raised.
If these faults should occur at the lower end of the part throttle range at a four-stroke running, make the idling range leaner; if the
engine pings, adjust the idling range richer.
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 or if the engine rings, 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.
mixture too rich:
too much fuel in proportion to air
mixture too lean:
not enough fuel in proportion to air
1
2
OPERATING RANGES OF THE CARBURETOR