STIHL Fs 50 C Manual

							FS 40, FS 40 C, FS 50, FS 50 C
English
19 Block the drive shaft.
NTo block the shaft, insert a suitable 
tool (12) through the holes (13) in 
the deflector and thrust plate – turn 
the thrust plate back and forth if 
necessary.STIHL FixCut 5-2
NSlip the thrust plate (2) over the 
shaft.
NPlace the mowing head on the 
thrust plate (2).
Collar (see arrow) must engage the 
mowing head's mounting hole.
NPush the thrust washer (3) over the 
shaft (1) so it is seated on the base.
NBlock the shaft – see "Blocking the 
Shaft".
NUse the combination wrench (5) 
(special accessory) to screw the 
mounting nut (4) on to the output 
shaft and tighten it down firmly.
Remove the locking pin.
Removing the mowing head
STIHL AutoCut
NHold the mowing head steady.
NUnscrew the cap counterclockwise.
STIHL PolyCut
NBlock the shaft – see "Blocking the 
Shaft".
NUnscrew the mowing head 
counterclockwise.
STIHL FixCut
NBlock the shaft – see "Blocking the 
Shaft".
NUse the combination wrench 
(special accessory) to loosen and 
unscrew the mounting nut 
counterclockwise from the output 
shaft.
If the mounting nut turns too freely, fit a 
new one.
3
1
2 4
232BA074 KN
5 
						

							FS 40, FS 40 C, FS 50, FS 50 C
English
20
Adjusting Nylon Line
STIHL AutoCut
NHold the rotating mowing head 
above the ground – tap it on the 
ground once – about 1.2 in. (3 cm) 
fresh line is advanced.
NThe blade (1) on the deflector (2) 
trims surplus line to the correct 
length – avoid tapping the mowing 
head more than once at a time.
Line feed operates only if both lines still 
h a v e  a  m i n i m u m  l e n g t h  o f   1 in. (2.5 cm).
If the mowing line is shorter than 1 in. 
(2.5 cm):
To reduce the risk of injury, always shut 
off the engine before adjusting the 
mowing line by hand.NTurn the trimmer on its back.
NPress down the cap on the spool as 
far as stop.
NPull the ends of the lines out of the 
spool.
If the spool is empty, see "Replacing 
Nylon Line".
All other mowing heads
Refer to the instructions supplied with 
the mowing head.
To reduce the risk of injury, always shut 
off the engine before adjusting the 
mowing line by hand.
Replacing Nylon Line / Polymer Blades 
STIHL AutoCut
See chapter on "Replacing Nylon Line".
STIHL PolyCut, FixCut
Refer to the instructions supplied with 
the mowing head.This engine is certified to operate on 
unleaded gasoline and the STIHL two-
stroke engine oil at a mix ratio of 50:1.
Your engine requires a mixture of high-
quality gasoline and two-stroke air 
cooled engine oil.
Use mid-grade unleaded gasoline with a 
minimum octane rating of 89 (R+M/2) 
and no more than 10% ethanol content.
Fuel with a lower octane rating may 
increase engine temperatures. This, in 
turn, increases the risk of piston seizure 
and damage to the engine.
The chemical composition of the fuel is 
also important. Some fuel additives not 
only detrimentally affect elastomers 
(carburetor diaphragms, oil seals, fuel 
lines, etc.), but magnesium castings and 
catalytic converters as well. This could 
cause running problems or even 
damage the engine. For this reason 
STIHL recommends that you use only 
high-quality unleaded gasoline!
Gasoline with an ethanol content of 
more than 10% can cause running 
problems and major damage in engines 
with a manually adjustable carburetor 
and should not be used in such engines.
The ethanol content in gasoline affects 
engine running speed – it may be 
necessary to readjust the carburetor if 
you use fuels with various ethanol 
contents.
1
232BA007 KN
2
)XHO 
						

							FS 40, FS 40 C, FS 50, FS 50 C
English
21 The idle speed and maximum speed of 
the engine change if you switch from a 
fuel with a certain ethanol content to 
another fuel with a much higher or lower 
ethanol content.
This problem can be avoided by always 
using fuel with the same ethanol 
content.
Use only STIHL two-stroke engine oil or 
equivalent high-quality two-stroke 
engine oils that are designed for use 
only in air cooled two-cycle engines.
W e  r e c o m m e n d  S T I H L  H P  U l t r a  2 - C y c l e  
Engine Oil since it is specially 
formulated for use in STIHL engines.
Do not use BIA or TCW rated (two-
stroke water cooled) mix oils or other 
mix oils that state they are for use in both 
water cooled and air cooled engines 
(e.g., outboard motors, snowmobiles, 
chain saws, mopeds, etc.).
Take care when handling gasoline. 
Avoid direct contact with the skin and 
avoid inhaling fuel vapor. When filling at 
the pump, first remove the container 
from your vehicle and place the 
container on the ground before filling. To 
reduce the risk of sparks from static discharge and resulting fire and/or 
explosion, do not fill fuel containers that 
are sitting in or on a vehicle or trailer.
The container should be kept tightly 
closed in order to limit the amount of 
moisture that gets into the mixture.
The machine‘s fuel tank should be 
cleaned as necessary.
Fuel mix ages
Only mix sufficient fuel for a few days 
work, not to exceed 3 months of storage. 
Store in approved fuel-containers only. 
When mixing, pour oil into the container 
first, and then add gasoline. Close the 
container and shake it vigorously by 
hand to ensure proper mixing of the oil 
with the fuel.
Dispose of empty mixing-oil containers 
only at authorized disposal locations.
Preparations
NBefore fueling, clean the filler cap 
and the area around it to ensure that 
no dirt falls into the tank.
Always thoroughly shake the mixture in 
the canister before fueling your 
machine.
NPosition the machine so that the 
filler cap is facing up.
In order to reduce the risk of fire and 
personal injury from escaping gas vapor 
and fumes, remove the fuel filler cap 
carefully so as to allow any pressure 
build-up in the tank to release slowly.
One of two different filler caps is 
installed as standard at the factory. Warning!
To reduce the risk of personal injury 
from loss of control and / or contact with 
the running cutting tool, do not use your 
unit with incorrect idle adjustment. At 
correct idle speed, the cutting tool 
should not move.
If your power tool shows an incorrect 
idle adjustment, have your STIHL dealer 
check your power tool and make proper 
adjustments and repairs.
Gaso-
lineOil (STIHL 50:1 or equiva-
lent high-quality oils)
US gal. US fl.oz.
12.6
2 1/2 6.4
5 12.8
Fueling
002BA456 KN 
						

							FS 40, FS 40 C, FS 50, FS 50 C
English
22 Toolless filler cap (with folding grip)
Threaded filler cap
Opening the toolless filler cap
NRaise the grip into an upright 
position.NTurn the cap counterclockwise 
(approx. 1/4 turn).
NRemove fuel filler cap.Refueling
Take care not to spill fuel while fueling 
and do not overfill the tank. STIHL 
recommends use of the STIHL filling 
system (special accessory).
Closing the toolless filler cap
With grip in an upright position:NInsert the cap – positioning marks 
on the cap and the fuel tank opening 
must line up.
NThe cap should drop fully into the 
opening in this position.
NWhile pressing the cap down, twist it 
firmly clockwise as far as it will go 
(approx. 1/4 turn).
NFold down the grip.
002BA418 KN
002BA419 KN
001BA218 KN
001BA219 KN
001BA224 KN
001BA220 KN
001BA221 KN
001BA222 KN 
						

							FS 40, FS 40 C, FS 50, FS 50 C
English
23
Checking for proper closure
NThe lug on the grip must engage 
entirely in the recess (arrow), and 
the grip must lie completely flush 
with the top of the cap.
NGrip the cap and check for 
tightness.
NIf the cap can be moved, it is not 
properly installed.
Misalignment of the cap parts
NIf the cap does not drop fully into the 
opening when the positioning marks 
line up and/or if the cap does not 
tighten properly when twisted, the 
base of the cap may be rotated out 
of position vis-à-vis the top.
NSuch misalignment can result from 
handling, cleaning or an improper 
attempt at tightening.NTo correct a misalignment, turn the 
cap (with the grip up) until it drops 
fully into the tank opening.
NTwist the cap counterclockwise as 
far as it will go (approx. 1/4 turn) – 
this will twist the base of the cap into 
the correct position.
NTwist the cap clockwise, closing it 
normally – see the sections 
"Closing" and "Checking for proper 
closure."
NIf your cap still does not tighten 
properly, it may be damaged or 
broken; immediately stop use of the 
unit and take it to your authorized 
STIHL dealer for repair.
Opening the threaded filler cap
NTurn the cap counterclockwise until 
it can be removed from the tank 
opening.
NRemove the filler cap.
Refueling
Take care not to spill fuel while fueling 
and do not overfill the tank. STIHL 
recommends use of the STIHL filling 
system (special accessory).
Closing the threaded filler cap
NPosition cap.
NTurn the cap clockwise as far as it 
will go and tighten it as securely as 
possible by hand.
001BA223 KN
001BA225 KN
/HIW %DVHRILPSURSHUO\DOLJQHG
FDSZLWKRSHQVSDFH
5LJKW %DVHRIFDSFRUUHFWO\SRVL-
tioned for installation
001BA227 KN
001BA226 KN
002BA447 KN
002BA448 KN 
						

							FS 40, FS 40 C, FS 50, FS 50 C
English
24
Controls
Version with loop handle
1Throttle trigger lockout
2Throttle trigger
3Momentary stop switch with Run 
and 0= Stop positions.
Function of momentary stop switch and 
ignition system
The momentary stop switch is normally 
in the Run position: Ignition is on in this 
position – the engine is ready to start 
and may be started. If the momentary 
stop switch is moved to the 0 position, 
the ignition is off. It is automatically 
switched on again after the engine 
comes to a standstill.
Starting the Engine
NPress the fuel pump bulb (4) at least 
five times – even if the bulb is 
already 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.
Starting
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 of any other obstacles.
NMake sure you have a firm footing.
NHold the unit firmly on the ground 
with your left hand and press down 
– do not touch the throttle trigger or 
throttle trigger lockout lever.
Do not stand or kneel on the drive tube.
Starting / Stopping the 
Engine
546BA007 KN
13
2
4
547BA015 KN
5
547BA016 KN
5
547BA017 KN
547BA018 KN 
						

							FS 40, FS 40 C, FS 50, FS 50 C
English
25 NHold the starter grip with your right 
hand.
Models without Easy2Start
NPull the starter grip slowly until you 
feel it engage and then give it a brisk 
strong pull.
Models with Easy2Start
NPull the starter grip steadily.
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 lever and open the throttle – 
the choke lever moves to the run 
positionF . After a cold start, warm 
up the engine by opening the 
throttle several times.
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 momentary stop switch in 
the direction of 0  – the engine 
stops – release the momentary stop 
switch – t springs back to the run 
position.
Other hints on starting
Engine stalls in cold start positiong or 
under acceleration
NMove the choke lever to < and 
continue cranking until the engine 
runs.Engine does not start in warm start 
position<
NMove the choke lever 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 spark plug boot is 
properly connected.
NRepeat the starting procedure.
Fuel tank run until dry
NAfter refueling, press the 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
26
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 
						

							FS 40, FS 40 C, FS 50, FS 50 C
English
27 Exhaust emissions are controlled by the 
design of the fundamental engine 
parameters and components (e.g. 
carburation, ignition, timing and valve or 
port timing) without the addition of any 
major hardware.The carburetor comes from the factory 
with a standard setting.
This setting provides an optimum fuel-air 
mixture under most operating 
conditions.
With this carburetor it is only possible to 
make corrections with the adjusting 
screws within fine limits.
Standard Setting
NShut odd the engine.
NMount a cutting tool – we 
recommend you use a mowing 
head.
NCheck the air filter and replace the 
element if necessary.
NHave the spark arresting screen 
(not all markets) checked – see 
“Inspection and Maintenance by 
Dealer"
NTurn the high speed screw (H) 
counterclockwise as far as stop 
(max. 3/4 turn possible).
NTurn the low speed screw (L) 
counterclockwise as far as stop 
(max. 3/4 turn possible).NStart and warm up the engine if 
necessary.
NAdjust idle speed with the idle speed 
screw (LA) so that the cutting tools 
does not turn.
Fine Tuning for Operation in Mountains 
or at Sea Level
A slight correction of the setting may be 
necessary if engine power is not 
satisfactory when operating at high 
altitude or at sea level.
NWarm up the engine.
At high altitude
NTurn high speed screw (H) 
clockwise (leaner) – no further than 
stop.
At sea level
NTurn high speed screw (H) 
counterclockwise (richer) – no 
further than stop.
Adjusting Idle Speed
NWarm up the engine.
Engine stops while idling
NTurn the idle speed screw (LA) 
slowly clockwise until the engine 
runs smoothly – the cutting 
attachment must not turn.
Engine ManagementAdjusting the Carburetor
547BA024 KN 
						

							FS 40, FS 40 C, FS 50, FS 50 C
English
28 Cutting attachment rotates when engine 
is idling
NTurn the idle speed screw (LA) 
counterclockwise until the tool stops 
moving and then turn the screw 
another full turn in the same 
direction.
If the cutting tool continues to rotate 
when the engine is idling, have your 
machine checked and repaired by your 
servicing dealer.
Erratic idling behavior, engine stops 
even though setting of LA-screw has 
been corrected, poor acceleration
Idle setting is too lean
NTurn the low speed screw (L) 
counterclockwise, no further than 
stop, until the engine runs and 
accelerates smoothly.
Erratic idling behavior, engine speed 
drops when swinging the machine
Idle setting is too rich
NTurn the low speed screw (L) 
clockwise (1/8 of a turn or 45° at a 
time) until the engine runs smoothly 
and accelerates well.
It is usually necessary to change the 
setting of the idle speed screw (LA) after 
corrections to the low speed screw (L).If engine is down on power, difficult to 
start or runs poorly at idling speed, first 
check the spark plug.
Fit a new spark plug after approx. 100 
operating hours or earlier if the 
electrodes are badly eroded.
Wrong fuel mix (too much engine oil in 
the gasoline), a dirty air filter and 
unfavorable running conditions (mostly 
at part throttle etc.) affect the condition 
of the spark plug. These factors cause 
deposits to form on the insulator nose 
which may result in trouble in operation.
Removing the Spark Plug
NShut off the engine
The spark plug boot (2) sits inside the 
cap (1).
The cap (1) protects the spark plug boot 
from damage. Do not operate the 
machine without a cap – replace a 
damaged cap.
NPull off the spark plug boot (2) 
together with the cap (1).
NUnscrew the spark plug (3).If the cap becomes detached while 
pulling off the spark plug boot, see 
"Installing the Spark Plug".
Checking the Spark Plug
NClean dirty spark plug.
NCheck electrode gap (A) and 
readjust if necessary – see 
"Specifications".
NUse only resistor type spark plugs of 
the approved range.
Rectify problems which have caused 
fouling of spark plug:
–Too much oil in fuel mix.
–Dirty air filter.
–Unfavorable running conditions, 
e.g. operating at part load.
Spark Plug
547BA028 KN
1
000BA039 KNA