Stihl FS 40 User manual

STIHL FS 40, 50 Instruction Manual

Original Instruction ManualPrinted on chlorine-free paper
Printing inks contain vegetable oils, paper can be recycled.
© ANDREAS STIHL AG & Co. KG, 2021 0458-546-0121-E. VA0.H21. 0000001049_031_GB
FS 40, FS 40 C, FS 50, FS 50 C
English
1
This instruction manual is protected by copyright. All rights reserved, especially the rights to reproduce, translate and process
with electronic systems.
Contents
Dear Customer,
Thank you for choosing a quality
engineered STIHL product.
It has been built using modern
production techniques and
comprehensive quality assurance.
Every effort has been made to ensure
your satisfaction and trouble-free use of
the product.
Please contact your dealer or our sales
company if you have any queries
concerning this product.
Your
Dr. Nikolas Stihl
Guide to Using this Manual 2
Safety Precautions and Working
Techniques 2
Approved Combinations of Cutting
Attachment, Deflector, Handle and
Harness 9
Mounting the Loop Handle 10
Mounting the Deflector 10
Mounting the Cutting Attachment 11
Fuel 13
Fueling 14
Starting / Stopping the Engine 15
Operating Instructions 17
Cleaning the Air Filter 17
Adjusting the Carburetor 18
Spark Plug 18
Engine Running Behavior 19
Storing the Machine 19
Maintaining the Mowing Head 20
Maintenance and Care 24
Minimize Wear and Avoid Damage 25
Main Parts 26
Specifications 27
Maintenance and Repairs 28
Disposal 28
EC Declaration of Conformity 29
UKCA Declaration of Conformity 29

FS 40, FS 40 C, FS 50, FS 50 C
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2
Pictograms
The meanings of the pictograms
attached to the machine are explained in
this manual.
Depending on the model concerned, the
following pictograms may be attached to
your machine.
Symbols in text
WARNING
Warning where there is a risk of an
accident or personal injury or serious
damage to property.
NOTICE
Caution where there is a risk of
damaging the machine or its individual
components.
Engineering improvements
STIHL's philosophy is to continually
improve all of its products. For this
reason we may modify the design,
engineering and appearance of our
products periodically.
Therefore, some changes, modifications
and improvements may not be covered
in this manual.
Observe all applicable local safety
regulations, e.g. by trade organizations,
social insurance institutions, labor safety
authorities etc.
If you have never used a power tool
before: Have your dealer or other
experienced user show you how to
operate your machine – or attend a
special course to learn how to operate it.
Minors should never be allowed to use
the machine – except for apprentices
over the age of 16 when working under
supervision.
Children, animals and onlookers must
not be allowed near the machine.
When not using the machine, it must be
laid down in such a way that it does not
endanger anyone. Ensure that the
machine cannot be used without
authorization.
Guide to Using this Manual
Fuel tank; fuel mixture of
gasoline and engine oil
Operate decompression
valve
Manual fuel pump
Operate manual fuel
pump
Tube of grease
Intake air: Summer
operation
Intake air: Winter
operation
Handle heating
Safety Precautions and
Working Techniques
Because the machine is a
high-speed fast-cutting
power tool, special safety
precautions must be
observed to reduce the
risk of personal injury.
It is important that you
read and understand the
User Manual before com-
missioning and keep it in
a safe place for future ref-
erence. Non-compliance
with the User Manual
may cause serious or
even fatal injury.

FS 40, FS 40 C, FS 50, FS 50 C
English
3
The user is responsible for accidents or
risks involving third parties or their
property.
Lend or rent your power tool only
together with this User Manual and only
to persons who are familiar with this
model and its operation.
The use of noise-emitting power tools
may be restricted to certain times by
national or local regulations.
Anyone operating the machine must be
well rested, in good physical health and
in good mental condition. If you have
any condition that might be aggravated
by strenuous work, check with your
doctor before operating a machine.
If you have a pacemaker: The ignition
system of your machine produces an
electromagnetic field of very low
intensity. This field may interfere with
some pacemakers. STIHL recommends
that persons with pacemakers consult
their physician and the pacemaker
manufacturer to reduce any health risk.
Anyone who has consumed alcohol or
drugs or medicines affecting their ability
to react must not operate a power tool.
Depending on the cutting attachment
fitted, use your tool only for cutting
grass, undergrowth and similar
materials.
Other uses are not permitted and may
lead to accidents or damage to the
machine. Never attempt to modify your
power tool in any way since this may
result in accidents or damage to the
machine.
Only use cutting attachments and
accessories that are explicitly approved
for this power tool model by STIHL or
are technically identical. If you have any
questions in this respect, consult your
dealer. Use only high-quality parts and
accessories. in order to avoid the risk of
accidents and damage to the machine.
STIHL recommends the use of genuine
STIHL tools and accessories. They are
specifically designed to match the
product and meet your performance
requirements.
The guard provided with your machine
may not protect the operator from all
foreign objects (gravel, glass, wire etc.)
ejected by the revolving cutting
attachment. Ejected objects may also
ricochet and strike the operator.
Clothing and equipment
Wear proper protective clothing and
equipment.
Do not wear clothing which could
become trapped in wood, brush or
moving parts of the machine. Do not
wear a scarf, necktie or jewelry. Tie up
and confine long hair above your
shoulders.
WARNING
Wear face protection and make sure it is
a good fit. Face protection alone is not
sufficient to protect the eyes.
Wear "personal" sound protection, e.g.
ear defenders.
STIHL can supply a comprehensive
range of personal protective equipment.
Transporting the machine
Always stop the engine.
Carry the unit properly balanced by the
shaft or loop handle.
Clothing must be robust
but allow complete free-
dom of movement. Wear
close-fitting clothes such
as a boiler suit, not a
loose jacket.
Wear sturdy shoes with
non-slip soles.
To reduce the risk of eye
injuries, wear close-fit-
ting safety glasses in
accordance with Euro-
pean Standard EN 166.
Make sure the safety
glasses are a snug fit.
Wear sturdy protective
gloves made of a resist-
ant material (e.g.
leather).
546BA001 KN

FS 40, FS 40 C, FS 50, FS 50 C
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4
By vehicle: When transporting in a
vehicle, properly secure your machine to
prevent turnover, damage and fuel
spillage.
Refueling
Always shut off the engine before
refueling.
Do not fuel a hot engine – fuel may spill
and cause a fire.
Open the fuel cap carefully to allow any
pressure build-up in the tank to release
slowly and avoid fuel spillage.
Only refuel the machine in a well
ventilated place. If fuel has been spilled,
immediately clean the machine – do not
allow your clothes to be splashed with
fuel. If that happens, change your
clothes at once.
This helps reduce the risk of unit
vibrations causing an incorrectly
tightened fuel cap to loosen or come off
and spill quantities of fuel.
Check for leaks. Do not start the engine
if there is a fuel leak – serious or fatal
burns could result!
Before starting
Check that your power tool is properly
assembled and in good condition – refer
to appropriate chapters in the User
Manual.
–Check the fuel system for leaks,
especially the visible parts, e. g.,
fuel cap, hose connections, manual
fuel pump (only in machines with a
manual fuel pump). In case of
leakage and damage, do not start
the engine – risk of fire! Have the
machine serviced by a dealer
before using it
–Use only an approved combination
of cutting attachment, deflector,
handle and harness. All parts must
be assembled properly and
securely. To reduce the risk of
injury, never use metal cutting
attachments!
–The stop switch must move easily to
0.
–Check the choke lever, throttle
trigger and throttle trigger lockout
for smooth action - throttle trigger
must return automatically to idle
position. The choke lever must
spring back from the g and <
positions to the run position F when
the throttle trigger lockout and
throttle trigger are squeezed.
–Check that the spark plug boot is
secure – a loose boot may cause
sparking that could ignite
combustible fumes and cause a fire!
–Check cutting attachment for
correct and secure assembly and
good condition
–Check safety devices (e. g.,
deflector for cutting attachments) for
damage and/or wear. Always
replace damaged parts. Never
operate the unit with a damaged
deflector
–Never attempt to modify the controls
or safety devices.
–Keep the handles dry and clean –
free from oil and dirt – this is
important for safe control of the
machine
–Adjust the harness and handle(s) to
suit your height and reach
To reduce the risk of personal injury, do
not operate your power tool if it is
damaged or not properly assembled!
To prepare for emergencies when using
a harness: Practice setting down the
machine quickly. To avoid damage, do
not throw the machine to the ground
when practicing.
Starting the engine
Start the engine at least 3 meters from
the fueling spot, outdoors only.
Place the unit on firm ground in an open
area. Make sure you have a stable and
secure footing. Hold the unit securely.
The cutting attachment must be clear of
the ground and all other obstructions
because it may begin to run when the
engine starts.
Your power tool is a one-person unit. To
reduce the risk of injury from ejected
objects, do not allow other persons
within a radius of 15 meters of your own
position – even when starting.
Gasoline is highly flam-
mable – keep away from
fire or flame – do not spill
any fuel – no smoking.
After refueling, close the
fuel cap as tightly as
possible.

FS 40, FS 40 C, FS 50, FS 50 C
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5
Do not drop-start the engine – start it as
described in the User Manual.
Check engine idling: The cutting
attachment must remain at a standstill
when the engine idles – throttle trigger
released.
To reduce the risk of fire, keep hot
exhaust gases and hot muffler away
from easily combustible materials (e.g.
wood chips, bark, dry grass, fuel).
Holding and guiding the machine
Always hold the unit firmly with both
hands on the handles. Make sure you
always have a firm and secure footing.
Left hand on loop handle, right hand on
control handle, even if you are left-
handed.
While working
In the event of impending danger or in
an emergency, switch off the engine
immediately – move the stop switch in
the direction of 0.
There is a risk of accident from ejected
objects within a wide area around the
working space, so you must ensure that
there is no-one within a 15 m radius of
the machine. This distance must also be
maintained in relation to objects
(vehicles, window panes) – risk of
property damage! Even at distances
beyond 15 m, the danger cannot be
ruled out.
Check that the engine is properly idling
so that the cutting tool will not continue
rotating after you release the throttle
trigger.
Check and correct the idle speed setting
at regular intervals. If the cutting
attachment still rotates at idle speed,
have your dealer make proper
adjustments or repairs. STIHL
recommends you have this work done
by a STIHL servicing dealer.
Take special care in slippery conditions
– damp, snow, ice, on slopes or uneven
ground.
Watch out for obstacles: tree stumps,
roots – risk of tripping or stumbling!
Make sure you always have a firm and
secure footing.
Only work while standing on the ground,
never on a ladder, work platform or other
unstable surface.
More care and attention than usual are
required when wearing ear protection,
as sounds warning you of impending
danger (shouts, beeps, etc.) cannot be
heard properly.
Take breaks when you start getting tired
or feeling fatigue – risk of accidents!
Work calmly and carefully – in daylight
conditions and only when visibility is
good. Proceed with caution, do not put
others in danger.
To reduce the risk of serious or fatal
injury from breathing toxic fumes,
ensure proper ventilation when working
in trenches, hollows or other confined
locations.
Stop work immediately if you start
suffering from nausea, headaches,
impaired vision (e.g. your field of vision
Avoid contact with the
cutting attachment – risk
of injury!
The cutting attachment
runs on for a short while
after releasing the throttle
trigger – coasting effect!
546BA002 KN
15m (50ft)
As soon as the engine is
running, the power
machine generates toxic
exhaust gas.As soon as
the engine is running, the
power machine gener-
ates toxic exhaust gas.
These gases may be
odorless and invisible
and may contain
unburned hydrocarbons
and benzene. Never run
the engine indoors or in
poorly ventilated loca-
tions, even if your model
is equipped with a cata-
lytic converter.

FS 40, FS 40 C, FS 50, FS 50 C
English
6
gets smaller), impaired hearing,
dizziness, or impaired concentration –
these symptoms may possibly be the
result of too-high exhaust gas
concentration – Risk of accidents!
Operate your power tool so that it
produces a minimum of noise and
emissions – do not run the engine
unnecessarily, accelerate the engine
only when working.
To reduce the risk of fire, do not smoke
while operating or standing near your
power tool. Note that combustible fuel
vapor may escape from the fuel system.
Dusts, mist and fumes emissions during
the work may be hazardous to your
health. Wear respiratory protection in
case of heavy dust or smoke emission.
If your power tool is subjected to
unusually high loads for which it was not
designed (e.g. heavy impact or a fall),
always check that it is in good condition
before continuing work – see also
"Before Starting". Check in particular
that the fuel system has no leaks and the
safety equipment is fully operative.
Never use a power tool that is no longer
safe to operate. In case of doubt, contact
a dealer.
Do not operate your power tool with the
choke lever in the warm start position <
– the engine speed cannot be controlled
in this position.
Be particularly careful when working on
difficult, densely grown terrain.
When mowing in tall scrub, under
shrubbery and hedges: Hold the cutting
tool at a working height of at least 15 cm
– this avoids risks to animals.
Always shut off the engine before
leaving the unit unattended.
Check the cutting tool regularly at
frequent intervals, and immediately if
you notice a change in its behavior:
–Stop the engine, hold the machine
securely, allow the cutting
attachment to come to a stop
–Check condition and tightness of
connections, look out for fine cracks
–Immediately replace defective
cutting tools, also in case of minor
hairline cracks
Clean grass and plant residue off the
cutting attachment mounting at regular
intervals – remove any accumulated
material from the cutting attachment and
deflector.
To reduce the risk of injury, shut off the
engine before replacing the cutting
attachment.
Do not continue using or attempt to
repair damaged or cracked cutting
attachments by means of welding,
straightening or modifying the shape
(unbalanced).
Particles or pieces may come off and hit
the operator or a bystander at a high
speed – risk of most severe injuries!
If a rotating cutting attachment touches a
rock or another hard object, sparks may
be generated which may possibly ignite
combustible materials. Also dried-out
plants and brushwood are combustible,
above all in hot and dry weather. If there
is a risk of fire, do not use cutting
attachments in the vicinity of
combustible materials, dried-out plants
or brushwood. It is mandatory that you
ask the responsible forestry office about
current fire hazards.
Using mowing heads
Only use the deflector with a properly
mounted blade that ensures the mowing
line is restricted to the permissible
length.
Always switch off the engine to adjust
the mowing line for manually adjustable
mowing heads – risk of injury!
Misuse with mowing lines that are too
long reduces the working speed of the
engine. The constant slipping of the
clutch causes overheating and damage
to important components (e.g. clutch,
plastic housing parts) – e.g. due to the
cutting attachment rotating during idling
– risk of injury!
Never work with-
out protection
suitable for the
respective
machine and cut-
ting tool – risk of
injury from ejected
objects!
Check the work site –
rocks, metal objects etc.
could get caught up and
ejected – potentially
beyond a distance of
15 m – risk of injury! Such
objects can also damage
the cutting attachment
and other property (e.g.
parked vehicles,
windows).

FS 40, FS 40 C, FS 50, FS 50 C
English
7
Vibrations
Prolonged use of the power tool may
result in vibration-induced circulation
problems in the hands (whitefinger
disease).
No general recommendation can be
given for the length of usage because it
depends on several factors.
The period of usage is prolonged by:
–Hand protection (wearing warm
gloves)
–Work breaks
The period of usage is shortened by:
–Any personal tendency to suffer
from poor circulation (symptoms:
frequently cold fingers, tingling
sensations).
–Low outside temperatures.
–The force with which the handles
are held (a tight grip restricts
circulation).
Continual and regular users should
monitor closely the condition of their
hands and fingers. If any of the above
symptoms appear (e.g. tingling
sensation in fingers), seek medical
advice.
Maintenance and Repairs
Service the machine regularly. Do not
attempt any maintenance or repair work
not described in the instruction manual.
Have all other work performed by a
servicing dealer.
STIHL recommends that you have
servicing and repair work carried out
exclusively by an authorized STIHL
servicing dealer. STIHL dealers are
regularly given the opportunity to attend
training courses and are supplied with
the necessary technical information.
Only use high-quality replacement parts
in order to avoid the risk of accidents
and damage to the machine. If you have
any questions in this respect, consult a
servicing dealer.
STIHL recommends the use of genuine
STIHL replacement parts. These parts
are specifically designed to match your
machine model and meet your
performance requirements.
To reduce the risk of injury from
unintentional engine startup, always
shut off the engine and disconnect the
spark plug boot before performing any
repairs, maintenance or cleaning work. –
Exception: Carburetor and idle speed
adjustments.
Do not turn the engine over on the
starter with the spark plug boot or spark
plug removed since there is otherwise a
risk of fire from uncontained sparking.
To reduce the risk of fire, do not service
or store your machine near open flames.
Check the fuel filler cap for leaks at
regular intervals.
Use only a spark plug of the type
approved by STIHL and make sure it is
in good condition – see "Specifications".
Inspect the ignition lead (insulation in
good condition, secure connection).
Check the condition of the muffler.
To reduce the risk of fire and damage to
hearing, do not operate your machine if
the muffler is damaged or missing.
Do not touch a hot muffler since burn
injury will result.
Symbols on Deflectors
An arrow on the deflector shows the
correct direction of rotation of the cutting
attachments.
Use deflector in
combination with
mowing heads
only. Do not use
metal cutting
attachments.

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English
8
Mowing Head with Nylon Lines
Nylon line achieves a soft cut for edging
and trimming around fence posts, trees,
etc. – less risk of damaging tree bark.
WARNING
To reduce the risk of injury, never use
steel wire in place of the nylon cutting
line.
STIHL PolyCut 6-3 Mowing Head with
Polymer Blades
For mowing unobstructed edges of
meadows (without posts, fences, trees
or similar obstacles).
It is important to follow the maintenance
instructions for the Polycut mowing
head.
Check the wear limit marks!
Wear limit marks are integrated in the
base of the PolyCut.
Do not continue using the PolyCut 6-3 if
one of the circular holes (1; arrow)
becomes visible or if the projecting rim
(2; arrow) has worn away. Install a new
mowing head.
WARNING
If the wear limit marks are ignored, there
is a risk of the cutting tool shattering and
flying parts injuring the operator or
bystanders.
To reduce the risk of accidents from
shattered blades, avoid contact with
stones, metal and similar solid objects.
Check PolyCut blades for cracks at
regular intervals. If a crack is found in
one of the blades, always replace all the
blades.
000BA019 KN
1
002BA073 KN
2
002BA074 KN

FS 40, FS 40 C, FS 50, FS 50 C
English
9
Permissible combinations
Choose the correct combination from
the table depending on the cutting tool!
WARNING
For safety reasons no other
combinations are permitted – risk of
accidents!
Cutting attachments
Mowing heads
1STIHL AutoCut C 6-2
2STIHL DuroCut 5-2
3STIHL PolyCut 7-3
4STIHL PolyCut 6-2
Deflector
5Guard with blade for mowing heads
Handle
6Loop handle
Carrying strap
7Shoulder strap can be used
Approved Combinations of Cutting Attachment, Deflector, Handle and Harness
Cutting attachment Deflector Handle Carrying strap
0000-GXX-0401-A4
1
67
5
2
34

FS 40, FS 40 C, FS 50, FS 50 C
English
10
Your new power tool comes with the
loop handle mounted on the drive tube,
but it must be turned and lined up to suit
your requirements.
Adjusting the loop handle
Versions with screw (FS 40)
NUse a screwdriver or the
combination wrench to loosen the
screw (1) on the handle.
Versions with star knob (FS 50)
NLoosen the star knob (2) on the
handle.
All versions
NTurn the handle to the vertical
position.
The loop handle can be adjusted to suit
the height and reach of the operator and
the application by changing
distance (A).
Recommended distance (A):
about 15 cm
NSlide the handle to the required
position.
NTighten down the screw or star knob
so that the handle cannot be rotated
on the drive tube.
NPush the deflector (1) over the
clamp (2) as far as stop.
NInsert the nut (3) in the hex recess in
the deflector – make sure the holes
line up.
NInsert the screw (4) with washer and
tighten it down firmly.
Mounting the Loop Handle
2
1
546BA003 KN
545BA004 KN
546BA004 KN
A
Mounting the Deflector
1
2
232BA066 KN
4
546BA005 KN
3

FS 40, FS 40 C, FS 50, FS 50 C
English
11
Placing power tool on the ground
NSwitch off the engine.
NLay your trimmer on its back with
the loop handle and shroud pointing
down and the output shaft facing up.
Thrust Plate
The thrust plate (1) is shipped with the
DuroCut 5-2 and PolyCut 6-3. It is only
required for these mowing heads.
STIHL AutoCut 5-2 mowing head,
STIHL AutoCut C 5-2 mowing head
NPull the thrust plate (1), if fitted, off
the shaft (2).
STIHL PolyCut 5-2 mowing head,
STIHL FixCut 6-3 mowing head
NSlip the thrust plate (1) over the
shaft (2) and engage the hex
recess (3) on the external
hexagon (4).
Blocking the Shaft
NTo block the shaft, insert a suitable
tool (5) through the holes (6) in the
deflector and thrust plate – turn the
thrust plate back and forth if
necessary.
Fitting the mowing head with screw
mounting
Keep the supplement sheet for the
mowing head in a safe place.
Mounting the Cutting
Attachment
546BA016 KN
232BA033 KN
1
2
3
4
6
6
546BA017 KN
5

FS 40, FS 40 C, FS 50, FS 50 C
English
12
NFit the thrust plate
NTurn the mowing head
anticlockwise on the shaft (1) as far
as it will go
NRetain the shaft
NTighten the mowing head
NOTICE
Remove the tool that was used to block
the shaft.
Removing the Mowing Head
NRetain the shaft
NTurn the mowing head clockwise
Fitting Mowing Head without Screw
Mounting
Keep the supplement sheet for the
mowing head in a safe place!
STIHL AutoCut C 6-2
NPlace the mowing head (2) on the
shaft (3) while inserting the internal
hexagon (2) on the hexagon (4)
NHold the mowing head (2) with your
hand.
NTurn cap (1) clockwise and tighten it
firmly by hand.
STIHL PolyCut 6-2, PolyCut 7-3
NPush thrust plate onto the shaft
NInsert the nut (10) into the mowing
head
NTurn the mowing head (11)
clockwise until it engages the shaft
(5)
NRetaining the Shaft
NTighten the mowing head
WARNING
Nuts that move too easily must be
replaced.
NOTICE
Remove the tool that was used to block
the shaft.
1
002BA385 KN
0000-GXX-3937-A0
2
1
3
4
11
5
681BA019 KN
10

FS 40, FS 40 C, FS 50, FS 50 C
English
13
Removing the Mowing Head
STIHL AutoCut
NHold the spool housing steady
NUnscrew the cap counterclockwise.
STIHL PolyCut
NRetaining the Shaft
NTurn the mowing head
counterclockwise
Your engine requires a mixture of
gasoline and engine oil.
WARNING
For health reasons, avoid direct skin
contact with gasoline and avoid inhaling
gasoline vapor.
STIHL MotoMix
STIHL recommends the use of STIHL
MotoMix. This ready-to-use fuel mix
contains no benzol or lead, has a high
octane rating and ensures that you
always use the right mix ratio.
STIHL MotoMix uses STIHL HP Ultra
two-stroke engine oil for an extra long
engine life.
MotoMix is not available in all markets.
Mixing Fuel
NOTICE
Unsuitable fuels or lubricants or mix
ratios other than those specified may
result in serious damage to the engine.
Poor quality gasoline or engine oil may
damage the engine, sealing rings, hoses
and the fuel tank.
Gasoline
Use only high-quality brand-name
gasoline with a minimum octane rating
of 90 – leaded or unleaded.
Gasoline with an ethanol content of
more than 10% can cause running
problems in engines with a manually
adjustable carburetor and should not be
used in such engines.
Engines equipped with M-Tronic deliver
full power when run on gasoline with an
ethanol content of up to 25% (E25).
Engine Oil
If you mix the fuel yourself, use only
STIHL two-stroke engine oil or another
high-performance engine oil in
accordance with JASO FB, JASO FC,
JASO FD, ISO-L-EGB, ISO-L-EGC or
ISO-L-EGD.
STIHL specifies STIHL HP Ultra two-
stroke engine oil or an equivalent high-
performance engine oil in order to
maintain emission limits over the
machine’s service life.
Mix Ratio
STIHL 50:1 two-stroke engine oil: 50
parts gasoline to 1 part oil
Examples
NUse a canister approved for storing
fuel. Pour oil into canister first, then
add gasoline and mix thoroughly.
Fuel
Gasoline STIHL engine oil 50:1
Liters Liters (ml)
1 0.02 (20)
5 0.10 (100)
10 0.20 (200)
15 0.30 (300)
20 0.40 (400)
25 0.50 (500)

FS 40, FS 40 C, FS 50, FS 50 C
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14
Storing Fuel
Store fuel only in approved safety-type
fuel canisters in a dry, cool and safe
location protected from light and the sun.
Fuel mix ages – only mix sufficient fuel
for a few weeks work. Do not store fuel
mix for longer than 30 days. Exposure to
light, the sun, low or high temperatures
can quickly make the fuel mix unusable.
STIHL MotoMix may be stored for up to
2 years without any problems.
NThoroughly shake the mixture in the
canister before fueling your
machine.
WARNING
Pressure may build up in the canister –
open it carefully.
NClean the fuel tank and canister
from time to time.
Dispose of remaining fuel and cleaning
fluid properly in accordance with local
regulations and environmental
requirements.
Preparations
NBefore fueling, clean the filler cap
and the area around it to ensure that
no dirt falls into the tank.
NPosition the machine so that the
filler cap is facing up.
Opening screw-type tank cap
NTurn the cap counterclockwise until
it can be removed from the tank
opening.
NRemove the cap.
Filling up with fuel
Take care not to spill fuel while fueling
and do not overfill the tank. STIHL
recommends you use the STIHL filler
nozzle (special accessory).
Closing screw-type tank cap
NPlace the cap in the opening.
NTurn the cap clockwise as far as
stop and tighten it down as firmly as
possible by hand.
Fueling
547BA045 KN
002BA447 KN
002BA448 KN

FS 40, FS 40 C, FS 50, FS 50 C
English
15
Controls
Version with loop handle
1Throttle trigger lockout
2Throttle trigger
3Stop switch with Run and 0= Stop
positions.
Function of stop switch and ignition
system
The stop switch is normally in the Run
position, i.e. when it is not depressed:
The ignition is switched on – the engine
is ready to start. If the stop switch is
moved to the 0 position, the ignition is
switched off. The ignition is switched on
again automatically after the engine
stops.
Starting the Engine
NPress the manual fuel pump
bulb (4) at least five times – even if
the bulb is filled with fuel.
Cold engine (cold start)
NPress in the choke lever (5) and turn
it to g at the same time.
Warm engine (warm start)
NPress in the choke lever (5) and turn
it to < at the same time.
Also use this setting if the engine has
been running but is still cold.
Cranking
NPlace the unit on the ground: It must
rest securely on the engine support
and the deflector. Check that the
cutting attachment is not touching
the ground or any other obstacles.
NMake sure you have a safe and
secure footing.
NHold the unit firmly on the ground
with your left hand and press down
– do not touch the throttle trigger or
lockout lever.
NOTICE
Do not stand or kneel on the drive tube.
Starting / Stopping the
Engine
546BA007 KN
1
3
2
4
547BA015 KN
5
547BA016 KN
5
547BA017 KN
547BA018 KN

FS 40, FS 40 C, FS 50, FS 50 C
English
16
NHold the starter grip with your right
hand.
Version without ErgoStart
NPull the starter grip slowly until you
feel it engage and then give it a brisk
strong pull.
Version with ErgoStart
NPull the starter grip steadily.
NOTICE
Do not pull out the starter rope all the
way – it might otherwise break.
NDo not let the starter grip snap back.
Guide it slowly back into the housing
so that the starter rope can rewind
properly.
NContinue cranking until the engine
runs.
As soon as the engine runs
NPress down the throttle trigger
lockout and open the throttle – the
choke lever moves to the run
position F. After a cold start, warm
up the engine by opening the
throttle several times.
WARNING
Make sure the carburetor is correctly
adjusted. The cutting attachment must
not rotate when the engine is idling.
Your machine is now ready for
operation.
Stopping the Engine
NMove the stop switch in the direction
of 0 – the engine stops – release
the stop switch – it springs back to
the run position.
Other Hints on Starting
Engine stalls in cold start position g or
under acceleration
NMove the choke knob to < and
continue cranking until the engine
runs.
Engine does not start in warm start
position <
NMove the choke knob to g and
continue cranking until the engine
runs.
If the engine does not start
NCheck that all settings are correct.
NCheck that there is fuel in the tank
and refuel if necessary.
NCheck that the spark plug boot is
properly connected.
NRepeat the starting procedure.
Engine is flooded
NMove the choke knob to F and
continue cranking until the engine
runs.
Fuel tank run until completely dry
NAfter refueling, press the manual
fuel pump bulb at least five times –
even if the bulb is filled with fuel.
NSet the choke lever to suit the
engine temperature.
NNow start the engine.
547BA020 KN
547BA021 KN

FS 40, FS 40 C, FS 50, FS 50 C
English
17
During break-in period
A factory-new machine should not be
run at high revs (full throttle off load) for
the first three tank fillings. This avoids
unnecessary high loads during the
break-in period. As all moving parts
have to bed in during the break-in
period, the frictional resistances in the
engine are greater during this period.
The engine develops its maximum
power after about 5 to 15 tank fillings.
During Operation
After a long period of full throttle
operation, allow the engine to run for a
short while at idle speed so that engine
heat can be dissipated by the flow of
cooling air. This protects engine-
mounted components (ignition,
carburetor) from thermal overload.
After Finishing Work
Storing for a short period: Wait for the
engine to cool down. Empty the fuel tank
and keep the machine in a dry place,
well away from sources of ignition, until
you need it again. For longer out-of-
service periods – see "Storing the
Machine".
If there is a noticeable loss of engine
power
NMove the choke lever (1) to <.
NTurn the screw (2) in the filter
cover (3) counterclockwise until the
cover is loose.
NEase the filter cover (3) over the
choke lever and lift it away.
NClean away loose dirt from around
the filter.
NReach into the recess (4) in the filter
housing and take out the felt
filter (5).
NFit a new felt filter element (5). As a
temporary measure you can knock it
out on the palm of your hand or blow
it out with compressed air. Do not
wash.
NOTICE
Replace damaged parts.
NFit the felt filter (5) in the filter
housing, make sure it is properly
seated – the arrow points to the
recess.
NMove the choke lever (1) to <.
NFit the filter cover in position,
making sure the screw is square.
Tighten down the screw.
Operating Instructions Cleaning the Air Filter
1
547BA022 KN
2
3
547BA023 KN
4

FS 40, FS 40 C, FS 50, FS 50 C
English
18
The carburetor has been set at the
factory to provide an optimum fuel-air
mixture under most operating
conditions.
Adjusting Idle Speed
Engine stops while idling
NWarm up the engine for about
3 minutes.
NTurn the idle speed screw (LA)
slowly clockwise until the engine
runs smoothly – the cutting
attachment must not run.
Cutting attachment runs when engine is
idling
NTurn the idle speed screw (LA)
counterclockwise until the cutting
attachment stops moving and then
rotate the screw another 1/2 to 3/4
turn in the same direction.
WARNING
If the cutting attachment continues to run
when the engine is idling, have your
machine checked and repaired by your
servicing dealer.
NIf the engine is down on power,
difficult to start or runs poorly at idle
speed, first check the spark plug.
NFit a new spark plug after about 100
operating hours – or sooner if the
electrodes are badly eroded. Install
only suppressed spark plugs of the
type approved by STIHL – see
"Specifications".
Removing the Spark Plug
NShut off the engine.
NRemove the spark plug boot (1).
NUnscrew the spark plug (2).
Checking the Spark Plug
NClean dirty spark plug.
NCheck electrode gap (A) and
readjust if necessary – see
"Specifications".
NRectify the problems which have
caused fouling of the spark plug.
Possible causes are:
–Too much oil in fuel mix.
–Dirty air filter.
–Unfavorable running conditions.
WARNING
Arcing may occur if the adapter nut (1) is
loose or missing. Working in an easily
combustible or explosive atmosphere
may cause a fire or an explosion. This
can result result in serious injuries or
damage to property.
Adjusting the Carburetor
0815BA006 KN
Spark Plug
1
547BA041 KN
2
000BA039 KN
A
1
000BA045 KN