KTM XC-W 250 SX - Manual (Page 44)

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MAINTENANCE WORK ON CHASSIS AND ENGINE
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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)
1:40 – 1:60 (depends on oil quality)
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 > choose leaner carburetor adjustment
low altitude or low temperatures > choose richer carburetor adjustment
– ONLY USE PREMIUM-GRADE GASOLINE ROZ 95 (USA = PREMIUM RON 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 SMOK-
ING 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.
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 carbu-
retor malfunction (e.g. overly rich mixture). Therefore, these parts should be replaced after 10000 kilometers (6000 miles).
Definitions
Mixture too rich:
Too much fuel in proportion to air.
Mixture too lean:
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 [2] and the air control screw [3]. Only make adjust-
ments when the engine is hot.
To this end, slightly increase the idling speed of the engine by means of the
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
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.
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. B
C
D
A
idle jet
throttle valve
air control screw
idle adjusting screw
main jet
jet needle
jet needle
2
3