KTM LC4 400-660LC42003 engine. Manual - part 22

8-32D

Assembling the carburetor

– Mount the idle-air jet 

1

and the main air jet 

2

.

– Place the O-ring 

3

in the groove and secure the intake trumpet to

the carburetor by means of the 2 screws.

– Insert the fuel connection 

4

into the carburetor and secure it with

the clip.

NOTE: In the mounted state, the connection piece must be easy to
turn.

– Mount the choke slide 

5

and actuate it several times, checking

whether it can be moved smoothly. Besides, check whether the
choke locks properly.

– Thread the spring, the washer and the O-ring onto the mixture

control screw 

6

and screw the mixture control screw in as far as it

will go.

– Now, unscrew the mixture control screw the number of turns written

down during disassembly.

NOTE: Basic setting see technical spezifications.

– Mount idling jet 

7

, starting jet 

8

and needle jet together with main

jet 

9

.

– Insert the needle jet 

bk

into bore and secure it by means of the screw

bl

.

1

2

3

4

5

6

7

8

9

10

11

Repair manual  KTM LC4 

Art.-Nr

.  3.206.006 -E

8-33D

– Position the float, mount the float hinge pin and secure it by means

of the screw.

– Check the float level.
– Stick the plastic component 

1

on the needle jet.

– Engage the push rod 

2

of the accelerator pump at the lever.

– Mount the float chamber and at first secure it with only 1 screw.

When positioning the float chamber, make sure that the push rod 

2

of the accelerator pump slides into the bore.

– Place the membrane 

3

with the labeling facing upwards and the

spring into the pump housing.

– Place the O-ring 

4

into the groove. Secure the sealing ring 

5

with

some grease in the cover and fasten the cover by means of 3 screws.

– Turn the cable disc and push the throttle valve into the carburetor

such that the rollers 

6

engage the throttle valve (see photo). Push

the throttle valve all the way into the carburetor.

– Turn the cable disc several times and while doing so check whether

the throttle valve moves smoothly.

1

2

2

3

4

5

6

8-34D

– Coat the thread of the screw 

1

with Loctite 243 and mount the

screw, however, do not tighten it yet.

– Push the slide pin 

2

inward. At the same time, push the slide lever

3

to the extreme right and tighten the screw 

1

.

– Now, the distances 

A

on the left and on the right should be

identical. Then, turn the cable disc and check if the throttle valve
moves smoothly.

– Mount the jet needle and secure it with the screw 

4

.

– Position the slide cover together with its gasket and fasten it by

means of the 2 screws 

5

.

– Secure the ventilation hoses with the 2 screws 

6

of the float

chamber and connect the hose 

7

.

– Insert the 2 hose connections into the bores and fasten them with

the retaining clip 

8

.

A

A

1

2

4

5

6

6

3

7

8

Repair manual  KTM LC4 

Art.-Nr

.  3.206.006 -E

8-35D

– Mount the throttle valve sensor such that the flat spot at the 

carburetor engages the groove of the throttle valve sensor and
secure it by means of the screw.

Adjusting the position of the throttle valve sensor

NOTE: Before checking the position of the throttle valve sensor, you
have to adjust the idle speed correctly.

– Disengage the plug-and-socket connection of the throttle valve

sensor.

– Connect a multimeter (measuring range Ω x 1k) to the blue (+) and

the black (-) cable of the throttle valve sensor and measure the
throttle valve resistance.

– Now, multiply this value by 0.15. This yields the adjustment value for

the throttle valve sensor.

Example:
throttle valve sensor resistance (bl/s) = 5 kΩ

throttle valve sensor resistance (ge/s) = 

5 kΩ x 0.15 = 750 Ω ± 50 Ω

– Connect the multimeter (measuring range Ωx100) to the yellow (+)

and the black (-) cable of the throttle valve sensor and measure the
throttle valve sensor resistance with the throttle grip closed.
According to the above example, this value should be 750 Ω ± 50 Ω.

– If the value measured does not correspond to the desired value,

loosen the screw 

1

and turn the throttle valve sensor 

2

until the

instrument displays the desired value.

– Secure the throttle valve sensor in this position by fastening the

screw and check the value once more.

– Connect the throttle valve sensor to the wiring harness.

bl

bl

ge

ge

s

s

1

2

8-36D

Checking the throttle valve sensor

NOTE: The following measurement must be taken at a component 
temperature of approx. 20°C.

– Open the plug-and-socket connection of the throttle valve sensor.
– Connect a multimeter (measuring range Ω x 1k) to the blue (+) and

the black (-) cable of the throttle valve sensor.

throttle valve sensor resistance: 4 - 6 kΩ

– Now, connect the multimeter to the yellow (+) and the black (-)

cable of the throttle valve sensor.

– As you open the throttle grip slowly, the resistance must change

evenly.

throttle valve sensor resistance: 0-5 kΩ ±1 kΩ

(while opening the throttle grip)

Dismounting and installing the throttle valve sensor

– Disconnect the plug-and-socket connection of the throttle valve

sensor and remove the bolt 

1

.

– Take the throttle valve sensor off the carburetor.

– When mounting the throttle valve sensor, make sure that the flat

spot at the throttle valve pin 

2

engages the groove on the throttle

valve sensor.

– Mount the bolt, however, do not yet tighten fully and adjust the

position of the throttle valve sensor. Secure the bolt with 
Loctite 243.

bl

bl

ge

ge

s

s

bl = blue
ge = yellow
s = black

1

2

Repair manual  KTM LC4 

Art.-Nr

.  3.206.006 -E

8-37D

Adjust idling

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 idle speed is controlled by means of the adjusting wheel 

1

and the

mixture control screw 

2

. The adjusting wheel is used to adjust the basic

setting of the slide. The mixture control screw is used to control the idle
mixture which arrives at the engine by way of the idle system.
Clockwise turning reduces the fuel quantity (lean mixture),
counterclockwise turning increases the fuel quantity (rich mixture).

TO ADJUST IDLING CORRECTLY, PROCEED AS FOLLOWS:
1 Turn in mixture control screw 

2

up to the stop, and turn it back out

to the basic position (see technical data engine)

2 Warm up the engine
3 Use the adjusting wheel 

1

to set the normal idle speed (1400 - 1500

rpm).

4 Turn mixture control screw 

2

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. If, in the course of this procedure, the speed
undergoes a relatively high increase, reduce the idle speed to a
normal level and repeat the procedure specified in 4. Serious
competitive racers will choose a setting approx. 1/4 turn (clockwise)
leaner than this ideal value because their engine will heat up more
when used in competitions.

NOTE: If you fail to obtain a satisfying result by following the procedure
described above, an incorrectly dimensioned idling nozzle may be the
cause.

a) If 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) If 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 Then, use the adjusting wheel to set the desired idle speed.
6 In cases of greater changes in outside temperature and extremely 

different altitudes, the idling speed should be readjusted.

1

2

2

8-38D

Adjusting the mixture control screw

The mixture control screw is difficult to reach. For this reasons, we have
created an appropriate special tool 590.29.034.000.
Introduce the special tool into the bore at the carburetor bottom. Press
the tool slightly upward and turn the adjusting wheel 

1 

until the tool

engages the slot of the mixture control screw 

2

.

Now, you can go about adjusting the screw. Marks were provided on
the adjusting wheel, making it easier to keep track of the turns.

Checking the float level (float height) 

For this purpose, dismount the carburetor and remove the float
chamber. Hold the carburetor in a slanted position such that the float
will abut the float needle valve but not compress it (see photo).
Now, use a sliding caliper to measure the distance 

H

between the

casing edge and the float's upper edge.
The float height 

H

is to be 9 mm.

If the float height does not correspond to the desired value, check the
float needle valve and, if necessary, replace it.

If the float needle valve is correct, you can adjust the float height by
bending of the float lever 

3

.

Mount the float chamber, install the carburetor, and adjust the idle
speed.

H

3

1

2

8-39D

Repair manual  KTM LC4 

Art.-Nr

.  3.206.006 -E

CARBURETOR - KEIHIN FCR-MX 41