STIHL Fs 40 C Owners Manual

							FS 40, FS 40 C, FS 50, FS 50 C
English
19 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.
NOTICE
Fuel with an octane rating below 89 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 damage the 
engine. For this reason STIHL 
recommends that you use only quality 
unleaded gasoline!
NOTICE
Gasoline with an ethanol content of 
more than 10% can cause running 
problems and major damage in engines 
and should not be used.
For further details, see 
www.STIHLusa.com/ethanol
The ethanol content in gasoline affects 
engine speed – it may be necessary to 
readjust the carburetor if you use fuels 
with various ethanol contents.
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 an incorrect idle adjustment. At 
correct idle speed, the cutting tool 
should not move.
If your machine’s idle speed is 
incorrectly adjusted, have your 
authorized STIHL servicing dealer 
check your machine and make the 
proper adjustments and repairs.
The idle speed and maximum speed of 
the engine change if you switch from a 
fuel with a certain ethanol content to a 
fuel with a much higher or lower ethanol 
content.
This problem can be avoided by always 
using fuel with the same ethanol 
content.
To ensure the maximum performance of 
your STIHL engine, use a high quality 2-
cycle engine oil. To help your engine run 
cleaner and reduce harmful carbon 
deposits, STIHL recommends using 
STIHL HP Ultra 2-cycle engine oil or ask 
your dealer for an equivalent fully 
synthetic 2-cycle engine oil. 
To meet the requirements of EPA and 
CARB we recommend to use STIHL HP 
Ultra oil.
STIHL MotoMix
STIHL recommends the use of STIHL 
MotoMix. STIHL MotoMix has a high 
octane rating and ensures that you 
always use the right gasoline/oil mix 
ratio.STIHL MotoMix uses STIHL HP Ultra 
two-stroke engine oil suited for high 
performance engines.
For further details, see 
www.STIHLusa.com/ethanol
If not using MotoMix, use only STIHL 
two-stroke engine oil or equivalent high-
quality two-stroke engine oils that are 
designed for use in air cooled two-cycle 
engines.
The use of non-seasonal gasoline 
blends may increase the potential for 
pressure to build in the fuel tank during 
operation. For example, using a winder 
blend during the summer will increase 
pressure in the fuel tank. Always use 
gasoline blends appropriate to the 
season, altitude and other 
environmental factors.
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.).
WARNING
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.
Fuel 
						

							FS 40, FS 40 C, FS 50, FS 50 C
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20 The machine‘s fuel tank should be 
cleaned as necessary.
Fuel mix ages
If not using MotoMix, only mix sufficient 
fuel for a few days of work, not to exceed 
30 days 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 
mix of oil and gasoline.
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.
WARNING
To reduce the risk of fire and personal 
injuries due to escaping fuel and vapor, 
remove the fuel filler cap carefully so 
that any excess pressure in the tank can 
escape slowly. Use the proper seasonal 
gasoline blend, i.e., winter blend in 
winter, summer blend in summer.
One of two different filler caps is 
installed as standard at the factory.Toolless filler cap (with folding grip)
Threaded filler cap
Opening the toolless filler cap
NRaise the grip into an upright 
position. Gaso-
lineOil (STIHL 50:1 or equiva-
lent high-quality oils)
US gal. US fl.oz.
12.6
2 1/2 6.4
512.8
Fueling
547BA045 KN
002BA418 KN
002BA419 KN
001BA218 KN 
						

							FS 40, FS 40 C, FS 50, FS 50 C
English
21 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.
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.
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.
001BA219 KN
001BA224 KN
001BA220 KN
001BA221 KN
001BA222 KN
001BA223 KN
001BA225 KN 
						

							FS 40, FS 40 C, FS 50, FS 50 C
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22 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.
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.
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 the0 position, the ignition is 
switched off. The ignition is switched on 
again automatically after the engine 
stops. Left: Base of improperly aligned 
cap (with open space)
Right: Base of cap correctly posi-
tioned for installation
001BA227 KN
001BA226 KN
002BA447 KN
002BA448 KN
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546BA007 KN
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							FS 40, FS 40 C, FS 50, FS 50 C
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23
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.NHold the starter grip with your right 
hand.
Version without Easy2Start
NPull the starter grip slowly until you 
feel it engage and then give it a brisk 
strong pull.
Version with Easy2Start
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.
4
547BA015 KN
5
547BA016 KN
5
547BA017 KN
547BA018 KN
547BA020 KN 
						

							FS 40, FS 40 C, FS 50, FS 50 C
English
24 As soon as the engine runs
NPress down the throttle trigger 
lockout 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.
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 
of0  – the engine stops – release 
the stop switch – it 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 the spark plug boot is 
properly connected.
NRepeat the starting procedure.
Engine is flooded
NMove the choke lever 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.
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".
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							FS 40, FS 40 C, FS 50, FS 50 C
English
25 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
26 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.If the engine is down on power, difficult 
to start or runs poorly at idling speed, 
first check the spark plug.
Install a new spark plug after approx. 
100 operating hours or earlier if the 
electrodes are badly eroded/corroded.
The 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 degrade performance.
Removing the Spark Plug
NShut off the engine.
NRemove the spark plug boot (1).
NUnscrew the spark plug (2).
Checking the spark plug
NClean the spark plug if it is dirty
NCheck the electrode gap (A) and 
readjust if necessary – see 
"Specifications"
NUse only resistor type spark plugs of 
the approved range
Correct the problems that have caused 
fouling of the spark plug:
–too much oil in fuel mix,
–dirty air filter,
–unfavorable running conditions, e.g. 
operating at part throttle.Adjusting the Carburetor
0815BA006 KN
6SDUN3OXJ
1
547BA041 KN
2
000BA039 KNA 
						

							FS 40, FS 40 C, FS 50, FS 50 C
English
27
WARNING
To reduce the risk of fire and burn injury, 
use only spark plugs authorized by 
STIHL. Always press the spark plug 
boot (1) snugly onto the spark plug 
terminal (2).
Do not use a spark plug with a 
detachable SAE adapter terminal (3). 
Arcing may occur that could ignite 
combustible fumes and cause a fire. 
This can result in serious injuries or 
damage to property. 
NOnly use resistor type spark plugs 
with solid, non-threaded terminals
Installing the Spark Plug
NScrew the spark plug into the 
cylinder.
NPress the boot firmly onto the spark 
plug.If engine running behavior is 
unsatisfactory even though the air filter 
is clean and the carburetor is properly 
adjusted, the cause may be the muffler.
Have the muffler checked for 
contamination (carbonization) by your 
servicing dealer.
STIHL recommends that you have 
servicing and repair work carried out 
exclusively by an authorized STIHL 
servicing dealer.For periods of about 3 months or longer
NDrain and clean the fuel tank in a 
well ventilated area.
NDispose of fuel properly in 
accordance with local 
environmental requirements.
NRun the engine until the carburetor 
is dry – this helps prevent the 
carburetor diaphragms sticking 
together.
NRemove, clean and inspect the 
cutting attachment. Coat metal 
cutting attachments with corrosion 
inhibitor.
NThoroughly clean the machine – pay 
special attention to the cylinder fins 
and air filter.
NStore the machine in a dry and 
secure location – out of the reach of 
children and other unauthorized 
persons.
2
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							FS 40, FS 40 C, FS 50, FS 50 C
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28
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.
Replacing Nylon Line
Always check the mowing head for signs 
of wear before replacing the nylon line.
WARNING
If there are signs of serious wear, 
replace the complete mowing head.
The nylon mowing line is referred to as 
"nylon line" or "line" in the following.
The mowing head is supplied with 
illustrated instructions for replacing the 
nylon line. Keep the instructions for the 
mowing head in a safe place.
NIf necessary, remove the mowing 
head.
Adjusting Nylon Line
STIHL AutoCut
NHold the rotating mowing head 
above the ground – tap it on the 
ground once – about 3 cm (1.2 in) of 
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 
have a minimum length of 2.5 cm (1 in).
If the line is shorter than 2.5 cm (1 in):
WARNING
To reduce the risk of injury, always shut 
off the engine before adjusting the nylon 
mowing line by hand.NTurn the machine over.
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, refill with nylon line.
All other mowing heads
Refer to the leaflet supplied with the 
mowing head.
WARNING
To reduce the risk of injury, always shut 
off the engine before adjusting the nylon 
mowing line by hand.
Replacing Nylon Line
STIHL DuroCut
WARNING
To reduce the risk of injury, always shut 
off the engine before refilling the mowing 
head.
NFit nylon line in the mowing head as 
described in the instructions 
supplied.
STIHL AutoCut C 5-2
Disassembling the mowing head and 
removing remaining nylon line
In normal operation, the supply of nylon 
line in the head is used up almost 
completely.
Maintaining the Mowing 
Head
546BA016 KN
1
232BA007 KN
2