Carburetor Adjustment - KTM 125 SX Owner's Manual

MAINTENANCE WORK ON CHASSIS AND ENGINE

Carburetor adjustment *

Basic information on the original carburetor setting
The original carburetor setting was adapted for an altitude of approx. 500 meters (1600 ft.) above sea level, and the ambient
temperature of approx. 20° C (68° F), mainly for off-road use and central European premium-grade fuel (95 ROZ).
Mixing ratio (2-stroke motor oil/super fuel):
Basic information on changing the carburetor setting
Always start with the original carburetor setting (the provided factory setting is meant for break-in operation; after the break-in period,
the carburetor needs to be adjusted according to the carburetor datasheet - see appendix). Essential requirements are a clean air fil-
ter system, air-tight exhaust system and an intact carburetor. Experience has shown that adjusting the main jet, the idling jet and the
jet needle is sufficient and that changes of other parts of the carburetor will not greatly affect engine performance.
RULE OF THUMB: high altitude or high temperatures
low altitude or low temperatures
– Only use premium-grade gasoline ROZ 95 (USA = Premium PON 91) mixed with high-grade two-stroke engine oil. Other types of
gasoline can cause engine failure, and will void your warranty.
– Only use high-grade 2-stroke engine oil of known brands (e. g. Motorex Cross Power 2T).
– Not enough oil or low-grade oil can cause erosion of the piston. if you use too much oil the engine can start smoking and foul the
spark plug and the exhaust control system.
– In the case of a leaner adjustment of the carburetor, proceed cautiously. Always reduce the jet size in steps of one number to avoid
overheating and piston seizure.
NOTE: If the engine does not run properly despite a changed adjustment, look for mechanical faults and check the ignition system.
General information on carburetor wear
Engine vibrations subject the throttle slide, jet needle, float needle valve and the needle jet to extreme wear. Wear leads to malfunctio-
ning of the carburetor (e.g. over-enrichment). These parts should be replaced after 100 operating hours. The carburetor body, main jet
holder and the float support should be replaced after 200 operating hours.
Press oil into the system until the fluid runs out of the hole [1] in the master
cylinder without bubbles. Occasionally extract the fluid from the master cylin-
der reservoir to keep it from overflowing.
After you finish bleeding, remove the bleeder syringe, mount the bleeder screw,
correct the level in the reservoir and mount the cover (see above).
1
1:40 (125 SX), 1:60 (250 SX)
> choose leaner carburetor adjustment
> choose richer carburetor adjustment
Definitions
2
Mixture too rich:
Too much fuel in proportion to air.
Mixture too lean:
1
Not enough fuel in proportion to air.
Idling range A
Operation with closed throttle valve. This range is influenced by the position
of the air control screw [1] and the air control screw [2]. Only make adjust-
ments when the engine is hot.
To this end, slightly increase the idling speed of the engine by means of the
main jet
air control screw. Turning it clockwise produces a higher idling speed and turn-
ing the screw counterclockwise produces a lower idling speed. Create a round
jet needle
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 air control screw.
jet needle
Opening up B
Engine behavior when the throttle opens. The idle jet and the shape of the
idle jet
throttle valve influences this range. If, despite good idling-speed and part-
throttle valve
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,
air control screw
the mixture to the carburetor will be too rich, the fuel level too high or the
idle adjusting screw
float needle is leaking.
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