STIHL BR 420
{
Instruction Manual
Notice d’emploi
G Instruction Manual
1 - 29
F Notice d’emploi
31 - 60
Contents
English
Guide to Using this Manual 2
Safety Precautions and Working
Techniques 2
Assembling the Unit 7
Fuel 10
Fueling 11
Information Before You Start 12
Starting / Stopping the Engine 12
Original Instruction ManualPrinted on chlorine-free paper
Operating Instructions 15
Cleaning the Air Filter 15
Engine Management 16
Adjusting the Carburetor 16
Catalytic Converter 17
Checking the Spark Plug 17
Spark Arresting Screen in Muffler 18
Replacing the Starter Rope and
Rewind Spring 19
Storing the Machine 21
Maintenance and Care 22
Main Parts 24
Specifications 25
Special Accessories 26
Printing inks contain vegetable oils, paper can be recycled.
Maintenance and Repairs 27
STIHL Limited Emission Control
Warranty Statement 27
Dear Customer,
Thank you for choosing a quality
engineered STIHL product.
This machine has been built using
modern production techniques and
comprehensive quality assurance.
Every effort has been made to ensure
your satisfaction and troublefree use
of the machine.
Please contact your dealer or our
sales company if you have any
queries concerning your machine.
Your
Hans Peter Stihl
{
© ANDREAS STIHL AG & Co. KG, 2011
0458-373-8221-B. M0-5.A11.MET.
0000000022_001_GB
BR 420
1
English
Guide to Using this Manual
Pictograms
All the pictograms attached to the
machine are shown and explained in this
manual.
Symbols in text
Warning where there is a risk of
an accident or personal injury or
serious damage to property.
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.
Safety Precautions and
Working Techniques
Special safety precautions must be observed
when working with a
power tool.
It is important you read
and understand the
instruction manual before
first use and keep the
manual in a safe place for
future reference. Nonobservance of the safety
precautions may result in
serious or even fatal
injury.
Observe all applicable local safety
regulations, standards and ordinances.
If you have not used this power tool
model before: Have your dealer or other
experienced user show you how to
operate your power tool or attend a
special course in its operation.
Minors should never be allowed to use a
power tool.
Keep bystanders, especially children,
and animals away from the work area.
When the power tool is not in use, shut if
off so that it does not endanger others.
Secure it against unauthorized use.
The user is responsible for avoiding
injury to third parties or damage to their
property.
Do not lend or rent your power tool
without the instruction manual. Be sure
that anyone using your power tool
understands the information contained
in this manual.
The use of noise emitting power tools
may be restricted to certain times by
national or local regulations.
Do not operate your power tool if any of
its components are damaged.
Do not use a pressure washer to clean
the power tool. The solid jet of water
may damage parts of the power tool.
Accessories and Spare Parts
Only use parts and accessories that are
explicity approved for this power tool by
STIHL or are technically identical. If you
have any questions in this respect,
consult a servicing dealer. Use only high
quality parts and accessories in order to
avoid the risk of accidents and damage
to the power tool.
STIHL recommends the use of genuine
STIHL replacement parts. They are
specifically designed to match your
model and meet your performance
requirements.
Never attempt to modify your power tool
in any way since this may increase the
risk of personal injury. STIHL excludes
all liability for personal injury and
damage to property caused while using
unauthorized attachments.
2
BR 420
English
Physical Condition
To operate the power tool you must be
rested, in good physical condition and
mental health.
If you have any condition that might be
aggravated by strenuous work, check
with your doctor before operating a
power tool.
Persons with pacemakers only: The
ignition system of your power tool
produces an electromagnetic field of a
very low intensity. This field may
interfere with some pacemakers. To
reduce health risks, STIHL recommends
that persons with pacemakers consult
their physician and the pacemaker
manufacturer before operating this tool.
Do not operate the power tool if you are
under the influence of any substance
(drugs, alcohol) which might impair
vision, dexterity or judgment.
Applications
The blower is designed for blowsweeping leaves, grass, paper and
similar materials, e.g. in gardens, sports
stadiums, car parks and driveways. It is
also suitable for blow-sweeping forest
paths.
Do not blow-sweep hazardous
materials.
It must not be used for any other
purpose because of the increased risk of
accidents and damage to the power tool
itself. Never attempt to modify the power
tool in any way since this may result in
accidents or damage to the power tool.
Clothing and Equipment
Wear proper protective clothing and
equipment.
Clothing must be sturdy
but allow complete freedom of movement. Wear
snug-fitting clothing, an
overall and jacket combination, do not wear a
work coat.
Avoid any clothing,
scarves, neckties, jewellery or anything that could
get into the air intake. Tie
up and confine long hair
(e.g. with a hair net, cap,
hard hat, etc.).
Wear sturdy shoes with non-slip soles.
Wear safety glasses and
hearing protection, e.g.
earplugs or ear muffs.
STIHL offers a comprehensive range of
personal protective clothing and
equipment.
Transporting the Power Tool
Always stop the engine.
Transporting in a vehicle:
– Properly secure your power tool to
prevent turnover, fuel spillage and
damage.
Fueling
Gasoline is an
extremely flammable
fuel. Keep clear of naked
flames. Do not spill any
fuel – do not smoke.
Always shut off the engine before
refueling.
Do not fuel a hot engine – fuel may spill
and cause a fire.
Always remove the power tool from your
back before refueling. Fuel the unit only
when it is standing on the ground.
Open the fuel cap carefully to allow any
pressure build-up in the tank to release
slowly and avoid fuel spillage.
Fuel your power tool only in wellventilated areas. If you spill fuel, wipe
the machine immediately – if fuel gets on
your clothing, change immediately.
Check for leakage. To
reduce the risk of seri-
ous of fatal burn
injuries, do not start or
run the engine until leak
is fixed.
After fueling, tighten
down the screw-type fuel
cap as securely as
possible.
This reduces the risk of unit vibrations
causing the fuel cap to loosen or come
off and spill quantities of fuel.
BR 420
3
English
373BA005 KN
Before Starting
Check that your power tool is properly
assembled and in good condition – refer
to appropriate chapters in the instruction
manual.
– Throttle trigger must move freely
and spring back to the idle position
when released.
– The setting lever must move easily
to STOP or 0
– The blower tubes must be properly
assembled.
– Keep the handles dry and clean –
free from oil and dirt – for safe
control of the power tool.
– Check that the spark plug boot is
secure – a loose boot may cause
arcing that could ignite combustible
fumes and cause a fire.
– Never attempt to modify the controls
or the safety devices in any way.
– Check condition of impeller
housing.
A worn impeller housing (cracks, nicks,
chips) may result in an increased risk of
injury from thrown foreign objects. If the
impeller housing is damaged, consult
your dealer – STIHL recommends you
contact a STIHL servicing dealer.
To reduce the risk of accidents and
personal injury, do not operate your
power tool if it is not properly assembled
and in good condition.
For emergencies: Practise quickly
opening the fastener on the waist belt,
loosening the shoulder straps and
setting down the unit.
Start the engine.
Start the engine at least 3 meters from
the fueling spot, outdoors only.
Your power tool is designed to be
operated by one person only. Do not
allow other persons in the work area –
even when starting.
Do not drop start your machine – the
correct starting procedure is described
in the instruction manual.
Place the unit on level ground, make
sure you have secure footing, hold the
unit securely.
As soon as the engine starts, the air flow
may throw small objects (e.g. stones) in
your direction.
Holding and Controlling the Power
Tool
The unit s carried as a backpack. Hold
and control the blower tube with your
right hand on the control handle – even
if you are left-handed.
Walk slowly forwards as you work –
observe the nozzle outlet at all times –
do not walk backwards – risk of
stumbling.
Always shut off the engine before taking
the power tool off your back.
During Operation
In the event of impending danger or in
an emergency, switch off the engine
immediately by moving the setting lever
to STOP or 0.
4
BR 420
English
To reduce the risk of personal injury ,
do not allow any other persons within a
radius of 15 meters of your own position.
To reduce the risk of damage to
property, also maintain this distance
from other objects (vehicles, windows).
Do not direct the air blast towards
bystanders or animals since the air
flow can blow small objects at great
speed.
When blow-sweeping (in open ground
and gardens), watch out for small
animals to avoid harming them.
Never leave a running machine
unattended.
Take special care in slippery conditions,
on slopes or uneven ground.
Watch out for obstacles: Be careful of
refuse, tree stumps, roots and ditches
which could cause you to trip or stumble.
Never work on a ladder or any other
insecure support.
Be particularly alert and cautious when
wearing hearing protection because
your ability to hear warnings (shouts,
alarms, etc.) is restricted.
Work calmly and carefully – in daylight
conditions and only when visibility is
good. Stay alert so as not to endanger
others.
To reduce the risk of accidents, take a
break in good time to avoid tiredness or
exhaustion.
Your power tool produces
toxic exhaust fumes as
soon as the engine is
running. These fumes
may be colorless and
odorless and contain
unburned hydrocarbons
and benzol. Never run
the engine indoors or in
poorly ventilated locations, even if your model
is equipped with a catalytic converter.
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.
To reduce the risk of accidents, stop
work immediately in the event of
nausea, headache, visual disturbances
(e.g. reduced field of vision), problems
with hearing, dizziness, deterioration in
ability to concentrate. Apart from other
possibilities, these symptoms may be
caused by an excessively high
concentration of exhaust gases in the
work area.
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.
If dust levels are very high, always wear
a suitable respirator.
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.
After finishing work, put the unit down on
a level, non-flammable surface. To
reduce the risk of fire, do not put it
down near easily combustible materials
(e.g. wood chips, bark, dry grass, fuel).
Mufflers with a catalytic converter can
become particularly hot.
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 Work". Check the fuel
system in particular for leaks and make
sure the safety devices are working
properly. Do not continue operating your
power tool if it is damaged. In case of
doubt, have the machine checked by
your servicing dealer.
Using the vacuum attachment
*
To reduce the risk of
personal injury from
fire, never attempt to pick
up hot or burning substances (e.g. smoldering
ashes, glowing
cigarettes).
To reduce the risk of
fatal injury from fire or
explosion, never attempt
to pick up combustible
fluids (e.g. gasoline) or
materials soaked in combustible fluids.
Watch out for small animals when
vacuuming in open areas, yards and
gardens.
BR 420
*
Special accessory
5
English
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. They are
specifically designed to match your
model and meet your performance
requirements.
To reduce the risk of injury, always shut
off the engine before carrying out any
maintenance or repairs or cleaning the
machine. – Exception: Carburetor and
idle speed adjustments.
Do not turn the engine over on the
starter with the spark plug boot or spark
plug removed unless the slide control /
stop switch is on STOP or 0 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.
Vibration behavior is influenced by the
condition of the AV elements – check the
AV elements at regular intervals.
Maintenance, replacement, or repair
of the emission control devices and
systems may be performed by any
nonroad engine repair establishment
or individual. However, if you make a
warranty claim for a component
which has not been serviced or
maintained properly or if
nonapproved replacement parts were
used, STIHL may deny coverage.
For any maintenance please refer to the
maintenance chart and to the warranty
statement near the end of the instruction
manual.
6
BR 420
Assembling the Unit
353BA002 KN
2
3
1
Fitting the Elbow
A combination wrench and carburetor
screwdriver are stowed on the underside
of the machine.
The throttle cable is already
connected and must not be kinked
during assembly.
English
N Line up the stops on the elbow and
stub and push the elbow (1) into the
stub (2) as far as it will go.
N Fit the nut (3) in the hexagon recess
in the stub
N Insert the screw in the nut from the
other side and tighten moderately –
the elbow must still turn.
BR 420
7
English
373BA033 KN
7
4
8
10
6
11
5
9
a
b
Mounting Blower Tube with Straight
*
Nozzle
Operate the machine only with a
properly mounted blower tube
and nozzle.
N Secure the throttle cable (4) in
position with the retainer (5).
N Push the blower tube (6) into the
pleated hose (7).
N Push the nozzle (8) onto the blower
tube (1) as far as lug (9),
position (a) (long) or position (b)
(short), and rotate the nozzle in the
direction of the arrow to it lock in
position.
*
Depending on model
8
N Rotate the pleated hose (7)
counterclockwise as far as stop and
leave it in that position.
N Turn the control handle (10)
counterclockwise to the horizontal
position and tighten down the clamp
screw (11).
BR 420
Mounting Blower Tube with Curved
373BA039 KN
7
12
10
6
11
9
a
b
13
*
Nozzle
English
Operate the machine only with a
properly mounted blower tube
and nozzle.
N Push the blower tube (6) into the
pleated hose (7).
N Push the blower tube (12) onto the
blower tube (6) as far as lug (9),
position (a) (long) or position (b)
(short), and rotate the nozzle in the
direction of the arrow to it lock in
position.
N Fit the nozzle (13) on the end of the
blower tube (12) and turn it
clockwise to engage the bayonet
coupling.
*
Depending on model
BR 420
N Rotate the pleated hose (7)
counterclockwise as far as stop and
leave it in that position.
N Rotate the complete blower tube
assembly until the nozzle discharge
opening points in the same direction
as the control handle.
N Turn the control handle (10)
counterclockwise to the horizontal
position.
N Tighten down the clamp screw (11).
9
English
11
10
373BA002 KN
373BA003 KN
373BA004 KN
Adjusting the Control Handle
N Put the machine on your back.
N Loosen the clamp screw (11).
N Move the control handle (10) along
the tube to the most comfortable
position.
N Tighten down the clamp screw (11).
Adjusting the Harness Straps
N Pull the ends of the straps
downwards to tighten the harness.
Loosening the Harness Straps
N Lift the tabs of the sliding adjusters.
N Adjust the harness straps so that
the backplate fits snugly and
securely against your back.
Fuel
This engine is certified to operate on
unleaded gasoline and with the mix ratio
50:1.
Your engine requires a mixture of highquality premium gasoline and highquality two-stroke air-cooled engine oil.
Use premium branded unleaded
gasoline with a minimum octane rating
of 89 RON.
Note: Models equipped with a catalytic
converter require unleaded gasoline. A
few tankfuls of leaded gasoline can
reduce the efficiency of the catalytic
converter by more than 50%.
Fuel with a lower octane rating may
result in preignition (causing "pinging")
which is accompanied by an increase in
engine temperature. This, in turn,
increases the risk of the 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 as
well. This could cause running problems
or even damage the engine. For this
reason it is essential that you use only
high-quality fuels!
Fuels with different percentages of
ethanol are being offered. Ethanol can
affect the running behaviour of the
engine and increase the risk of lean
seizure.
10
BR 420
English
373BA006 KN
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.
Engines equipped with M-Tronic can be
run on gasoline with an ethanol content
of up to 25% (E25).
Use only STIHL two-stroke engine oil or
equivalent high-quality two-stroke aircooled engine oils for mixing.
We recommend STIHL 50:1 two-stroke
engine oil since it is specially formulated
for use in STIHL engines.
Do not use BIA or TCW (two-stroke
water cooled) mix oils!
Use only STIHL 50:1 heavy-duty
engine oil or an equivalent quality twostroke engine oil for the fuel mix in
models equipped with a catalytic
converter.
Take care when handling gasoline.
Avoid direct contact with the skin and
avoid inhaling fuel vapour.
The canister should be kept tightly
closed in order to avoid any moisture
getting into the mixture.
The fuel tank and the canister in which
fuel mix is stored should be cleaned
from time to time.
Fuel mix ratio
Only mix sufficient fuel for a few days
work, not to exceed 3 months of storage.
Store in approved safety fuel-canisters
only. When mixing, pour oil into the
canister first, and then add gasoline.
Examples
Gasoline Oil (STIHL 50:1 or equiva-
lent high-quality oils)
liters liters (ml)
10.02(20)
5 0.10 (100)
10 0.20 (200)
15 0.30 (300)
20 0.40 (400)
25 0.50 (500)
Dispose of empty mixing-oil canisters
only at authorized disposal locations.
Fueling
Preparations
N Before fueling, clean the filler cap
and the area around it to ensure that
no dirt falls into the tank.
N Position the machine so that the
filler cap is facing up.
STIHL recommends you use the STIHL
filler nozzle for fuel (special accessory).
Filling up with fuel
Take care not to spill fuel while fueling
and do not overfill the tank.
BR 420
11
English
373BA037 KN
373BA008 KN
1
373BA009 KN
2
N Open the filler cap.
N Fill up with fuel.
N Close the filler cap.
After fueling, tighten down the
filler cap as securely as possible
by hand.
Information Before You
Start
With the engine stopped and
before starting, check the air
intake grille between the
backplate and powerhead for
blockages and clean if necessary.
A protective screen is available as
a special accessory to keep the
air intake clear.
Only models with catalytic converter
N Move the setting lever to the idle
position.
The throttle cable disconnects itself from
the throttle trigger if the engine is not
shut off in the idle position.
The throttle cable reconnects itself
automatically when the setting lever is
moved to the idle position.
Starting / Stopping the
Engine
Starting the engine
N Observe safety precautions.
N Slide the stop switch (1) to #
N Move setting lever (2) to center
position – starting throttle.
The setting lever enables you to select
and hold any throttle position between
(lower stop) idle and full throttle (upper
stop).
Set the lever to the idle position before
switching off the engine.
12
BR 420
English
373BA010 KN
373BA011 KN
373BA012 KN
373BA013 KN
If the engine is cold
N Turn the choke knob to g.
If the engine is warm
N Turn the choke knob to e.
N Also use this setting if the engine
has been running but is still cold.
Starting
N Place the unit securely on the
ground and make sure that
bystanders are well clear of the
nozzle outlet.
N Make sure you have a firm footing:
Hold the unit with your left hand on
the housing and put one foot against
the base plate to prevent it slipping.
N Hold the starter grip with your right
hand.
N Pull the starter grip slowly until you
feel it engage and then give it a brisk
strong pull.
Do not pull out the starter rope all
the way – it might otherwise
break.
N Do not let the starter grip snap back.
Guide it slowly back into the housing
so that the starter rope can rewind
properly.
When the engine begins to fire:
If the engine is cold:
N Move the choke knob to
e and
continue cranking until the engine
runs.
If the engine is warm:
N Continue cranking until the engine
runs.
BR 420
13
English
373BA014 KN
2
1
373BA015 KN
373BA016 KN
As soon as the engine runs
N Move setting lever (2) to lower stop
– the engine settles down to idle
speed.
To shut down the engine
N Slide the stop switch (1) to $ .
Other hints on starting
At very low outside temperatures
N Warm up the engine.
As soon as the engine runs:
N Move setting lever to lower stop –
the engine settles down to idle
speed.
N Open throttle slightly – warm up the
engine for a short period.
14
If the engine does not start
If you did not turn the choke knob to e
quickly enough after the engine began to
fire, the combustion chamber is flooded.
N Pull off the spark plug boot.
N Unscrew and dry off the spark plug.
N Move the stop switch to 0.
N Open the throttle wide.
N Crank the engine several times with
the starter to clear the combustion
chamber.
N Refit the spark plug and connect the
spark plug boot.
N Slide the stop switch to
# and set
the choke knob to e – even if the
engine is cold.
N Now start the engine.
If fuel tank has been run completely
dry and then refueled
N Pull the starter rope several times to
prime the fuel system.
BR 420
Operating Instructions Cleaning the Air Filter
1
373BA017 KN
2
1
373BA018 KN
3
4
5
English
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 enginemounted 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-ofservice periods – see "Storing the
Machine".
If there is a noticeable loss of engine
power
N Turn the choke knob to g.
N Take out the screw (1) and remove
the filter cover (2).
N Remove the main filter (3) from the
cover and inspect it – if it is dirty or
damaged, fit a new one.
Always install a new prefilter together
with the new main filter.
N Take the prefilter (4) out of the filter
cover.
N If prefilter is wet, dry it – then knock
it out on the palm of your hand or
blow it out with compressed air.
Always replace a damaged prefilter.
N Clean loose dirt from the filter cover
and filter chamber.
N Install the main filter (3) and
prefilter (4) in the filter cover.
N Fit the cover on the filter base (5)
and tighten it down firmly.
BR 420
15
English
H
L
373BA043 KN
LA
L
373BA044 KN
Engine Management Adjusting the Carburetor
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
adjust the high speed and low speed
screws within fine limits.
Standard Setting
N Stopping the engine
N Check the air filter and clean or
replace as necessary.
N Check the spark arresting screen
(not in all versions) in the muffler
and clean or replace as necessary.
N Carefully turn both adjusting screws
counterclockwise as far as stop.
N The high speed screw (H) is 1/4 turn
open.
N The low speed screw (L) is 1/4 turn
open.
N Start and warm up the engine.
Adjusting Idle Speed
Engine stops while idling
N Turn the idle speed screw (LA)
slowly clockwise until the engine
runs smoothly.
Erratic idling behavior, engine stops
even though setting of LA screw has
been corrected, poor acceleration
Idle setting is too lean:
N Turn the low speed screw (L)
counterclockwise, no further than
stop, until the engine runs and
accelerates smoothly.
Erratic idling behavior
Idle setting is too rich
N Turn the low speed screw (L)
clockwise, no further than stop, until
the engine runs and accelerates
smoothly.
It is usually necessary to change the
setting of the idle speed screw (LA) after
every correction to the low speed
screw (L).
16
BR 420
English
000BA002 KN
Fine tuning for operation at high
altitude
A slight correction of the setting may be
necessary if the engine does not run
satisfactorily:
N Check the standard setting.
N Warm up the engine.
N Turn high speed screw (H)
clockwise (leaner) – no further than
stop.
If the setting is too lean there is a
risk of engine damage due to
insufficient lubrication and
overheating.
Catalytic Converter Checking the Spark Plug
Power tools with a catalytic converter
(depending on model) must be run on a
50 : 1 mixture of unleaded gasoline
and STIHL two-stroke engine oil or
equivalent two-stroke engine oil – see
chapter on "Fuel".
The catayltic converter integrated in the
muffler reduces the harmful emissions in
the exhaust gas.
The correct carburetor setting (if
adjustable) and strict observance of the
prescribed mix ratio of gasoline and twostroke engine oil are essential to
minimize harmful emissions and ensure
a long catalytic converter service life.
If the engine is down on power, difficult
to start or runs poorly at idle speed, first
check the spark plug.
N Remove the spark plug – see
"Starting / Stopping the Engine".
N Clean dirty spark plug.
N Check electrode gap (A) and
readjust if necessary – see
"Specifications".
N Rectify 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.
BR 420
17
English
1
000BA045 KN
3
000BA050 KN
2
373BA031 KN
1
2
N Install 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".
To reduce the risk of arcing and fire
If the spark plug comes with a
detachable adapter nut:
N Screw the adapter nut (1) onto the
thread and tighten it down firmly.
Spark Arresting Screen in
Muffler
On all spark plugs
N Always press the boot (2) firmly on
to the spark plug (3).
If the engine is down on power or does
not run smoothly at maximum RPM,
check the spark arresting screen (not
fitted in all versions) in the muffler.
Wait for the engine to cool
down completely before
performing this work.
N Use suitable tool to squeeze ends of
clip (1) together and then lift the clip
away.
N Pull the spark arresting screen (2)
out of the muffler.
N Clean the spark arresting screen if
necessary.
N If the spark arresting screen is
damaged or heavily carbonized, fit a
new one.
18
BR 420
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