KTM LC4 400-660LC42003 engine. Manual - part 21

8-24D

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

. 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

mixture regulating screw
idle adjusting screw

idling jet
throttle valve

Repair manual  KTM LC4 

Art.-Nr

.  3.206.006 -E

8-25D

Check float level (Dell’Orto PHM 38ND / 40SD)

Stand the carburettor diagonally at about 60° so that the spring in the
float needle valve is not pressed together. In this position, the edge of
the float should be parallel with the float bowl sealing surface (see
illustration).

Checking axial play of float (Dell’Orto PHM 38ND / 40SD)

For proper functioning of the float, there must be an axial play of 0.8
mm (0,03 in). If necessary, slightly abrade and deburr lateral float guide.

Adjust hot start device (Dell’Orto PHM 40SD)

If the hot start button was removed when cleaning the carburetor,
readjust the hot start device.
Install carburetor and adjust idling as discribed above. Then unlock nut

1

, press in hot start device 

2

and adjust engine rev with adjusting

screw 

3

to 2000-2500 rpm. Tighten locking nut.

Mixture too lean:
Not enough fuel in proportion to air

Mixture too rich:
Too much fuel in proportion to air

60°

0,8 mm

1

2

1

2

3

Adjusting the throttle cable

There must always be a 3-5 mm (0.1-0.2 in) play in the throttle cable.
To  check this, move back the protective cover 

1

on the throttle grip.

You must be able to lift the outer covering of the cable 3-5 mm from
the adjusting screw 

2

, until resistance is felt.

To  adjust, loosen the counter nut 

3

and turn the adjusting screw

accordingly. Finally tighten counter nut and slide the protective cover
back on.

Adjusting the choke cable

At the choke cable, there must always exist a play of approx. 2 mm (0.1
in). To check this, push choke lever fully forward and pull protective
cover 

4

from the adjuster piece 

5

. Now, it must be possible to lift the

outer covering of the cable by approx. 2 mm from the adjuster piece
until feeling a resistance. If necessary, loosen counter nut and readjust
play by turning the adjuster piece. Tighten counter nut, and slide on
protective cover.

Adjusting the choke cable

The choke cable must have a play of approximately 2 mm at all times.
To  check the play turn the knob 

6

into the initial position. Now it

should be possible to lift the exterior case of the choke cable
approximately 2 mm from the supporting surface of the choke knob
before the upward movement is blocked by resistance.

To  adjust, remove the seat and the gastank, push the protective cover
upwards, loosen the counter nut and turn the adjustment screw 

7

accordingly. Turn the adjustment screw clockwise for more play or
anticlockwise for less play.
Tighten the counter nut, replace the protective cover and mount the
gas-tank and the seat.

8-26D

3-5 mm

2 mm

7

4

5

1

2

3

2 mm

6

Repair manual  KTM LC4 

Art.-Nr

.  3.206.006 -E

8-27D

CARBURATOR - KEIHIN FCR 41

8-28D

Disassembling the carburetor

NOTE: Before you start disassembling the carburetor, you should look
for a clean work place. It should offer you enough space to lay out all
individual components of the carburetor in perfect order.

– Dismount the carburetor and remove any coarse dirt.

– Remove the wire clip 

1

and pull the ventilation hoses out of the 

carburetor.

– Disconnect the hose 

2

.

– Loosen the 2 screws 

3

and dismount all ventilation hoses from the

carburetor.

– Remove the 2 screws 

4

and dismount the slide cover together with

its gasket.

– Remove the screw 

5

and take the jet needle out of the throttle

valve.

– Remove the screw 

6

.

1

2

3

3

4

5

6

Repair manual  KTM LC4 

Art.-Nr

.  3.206.006 -E

8-29D

– Now, pull the cable disc approx. 5 mm outward and turn it until the

throttle valve can be lifted out of the carburetor and detach the
rollers 

1

at the throttle valve.

– Take the throttle valve together with the 4 rollers 

2

and the valve

paddle out of the carburetor.

NOTE: When you turn the cable disc, it must not be blocked by the
stop bolt (see photo). Otherwise, pull the shaft further outward.

– Turn the carburetor around, remove the 3 screws and remove the

cover of the accelerator pump.

NOTE: When dismounting the cover, watch out for the spring and the
sealing rings as they may get lost easily.

– Remove the 2 sealing rings, the spring and the diaphragm from the

pump housing.

– Remove the screw and dismount the float chamber.
– Unhitch the push rod 

3

of the accelerator pump and dismount it.

– Take the plastic part 

4

off the needle jet.

– Loosen the screw 

5

, pull out the float hinge pin 

6

and dismount

the float together with the float needle valve.

– Remove the screw 

7

and use pliers to carefully extract the seat of

the float needle valve from the carburetor.

– Turn out the idling jet 

8

, the starting jet 

9

and the needle jet

together with the main jet 

bk

.

– Turn in the mixture control screw 

bl

down to the stop, count the

number of turns and write it down.

– Turn out the mixture control screw and dismount it together with the

spring, the washer, and the O-ring.

NOTE: The spring, the washer, and the O-ring will usually remain in the
bore. These parts can be removed with the help of compressed air.

1

2

3

4

5

6

7

11

8

9

10

8-30D

– Remove the screw 

1

and dismount the throttle-valve sensor 

2

.

When unfastening the screw, be sure to watch out for the 
bushing 

3

.

NOTE: To dismount the throttle-valve sensor, always remove the screw

1

. After the screw 

4

was loosened, the throttle-valve sensor must be

adjusted again.

– Remove the screw and the clip together with the bushing and pull

the connection piece 

5

out of the carburetor.

– Remove the 2 screws and take the intake trumpet together with the

O-ring 

6

off the carburetor.

– Unscrew the idle-air jet 

7

and the main air jet 

8

.

– Thoroughly clean all jets and other parts and blow compressed air

through them.

– Clean the carburetor housing and blow compressed air through all

ducts in the carburetor.

– Check all gaskets for damage and, if necessary, replace them.

5

6

7

8

2

1

3

4

Repair manual  KTM LC4 

Art.-Nr

.  3.206.006 -E

8-31D

Checking the choke slide

It must be easy to actuate the choke slide.
The piston 

1

of the choke slide must not have any pronounced score

marks or deposits.

Checking the accelerator pump

Check the membranes for cracking or brittleness.
Check gaskets for damage.
Check if the bores 

2

are unobstructed.

Checking the jet needle

Check the jet needle for bending and wear.

Checking the float needle valve

Check the sealing surface of the needle valve for notches.
There must not be any dirt between valve seat and float needle.

Checking the throttle valve

Check the throttle valve paddles 

3

for damage.

The rollers 

4

at the throttle valve must be easy to turn and must not

have any flat spots.

1

2

2

3

4