Stihl Br 420 C Manual

							20 English / USA
BR 340, BR 340 L, BR 420, BR 420 C :Clean the filter cover and filter 
chamber.
:Install the main filter and prefilter 
elements in the filter cover.
:Fit the cover on the filter base (5) 
and tighten it down firmly.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.
General InformationThe 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 
correct the adjustment of the high speed 
screw within fine limits.
If the setting is too lean, there is a 
risk of engine damage due to 
insufficient lubrication and 
overheating.
Motor Management Adjusting the Carburetor 
						

							21 BR 340, BR 340 L, BR 420, BR 420 CEnglish / USA
Standard Setting:Shut off the engine.
:Turn the high speed screw (H) and 
low speed screw (L) counter-
clockwise as far as stop (max. 
1/4 
turn).
Adjusting Idle SpeedEngine stops while idling
:Carry out standard setting on the 
low speed screw.
:Turn idle speed screw (LA) 
clockwise until the engine runs 
smoothly.
Erratic idling behavior, poor 
acceleration
:Carry out standard setting on the 
low speed screw. 
:Turn low speed screw (L) 
counterclockwise 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).
Fine Tuning for Operation at High 
Altitude or at Sea LevelA slight correction of the setting may be 
necessary if engine power is not 
satisfactory when operating at high 
altitude or at sea level:
:Check standard setting.
:Warm up the engine.
:Turn the high speed screw (H) 
slightly clockwise (leaner) – on 
models with limiter caps (H = 1/4),  
no further than stop.
If the setting is too lean, there is a 
risk of engine damage due to 
insufficient lubrication and 
overheating.
HL
373BA043 KN
LA L
373BA044 KN 
						

							22 English / USA
BR 340, BR 340 L, BR 420, BR 420 C Units with a catalytic converter* may 
only be operated with unleaded 
gasoline and STIHL two cycle engine 
oil or equivalent quality two cycle engine 
oils in a mix ratio of 50:1 (see chapter 
“Fuel”).
The catalylic converter in the muffler 
reduces noxious emissions in the 
exhaust gas.
Correct adjustment of the carburetor (if 
adjustable) and observance of the 
specified mix ratio of gasoline and two 
cycle engine oil are essential to 
minimize harmful exhaust emissions 
and ensure a long catalyst service life.
* see “Guide to Using this Manual”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.If engine is down on power, difficult to 
start or runs poorly at idling speed, first 
check the spark plug.
:Remove spark plug – see Starting / 
Stopping the Engine.
:Clean dirty spark plug.
:Check electrode gap (A) and 
readjust if necessary – see 
Specifications.
:Use 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.
Fit a new spark plug after approx. 
100 operating hours 
or earlier if the electrodes are badly 
eroded.Catalytic Converter* Checking the Spark Plug
000BA002 KN 
						

							23 BR 340, BR 340 L, BR 420, BR 420 CEnglish / USA
To reduce the risk of fire and burn 
injury, use only spark plugs 
authorized by STIHL. Always press 
spark plug boot (2) snugly onto 
spark plug terminal (1) of the proper 
size. (Note: If terminal has 
detachable SAE adapter nut, it must 
be attached.) 
A loose connection between spark 
plug boot and ignition wire 
connector in the boot may create 
arcing that could ignite combustible 
fumes and cause a fire.If the engine is down on power, check 
the spark arresting screen* in the 
muffler.
Wait until engine has cooled down 
completely before performing the 
following operations.
:Use suitable tool to squeeze ends of 
clip (1) together and then lift the clip 
away.
:Pull the spark arresting screen (2) 
out of the muffler.
* see “Guide to Using this Manual”:Clean the spark arresting screen.
:If the screen is damaged or heavily 
carbonized, fit a new one.
:Install the spark arresting 
screen (2).
:Refit the clip (1). 
000BA036 TR
1
Spark Arresting Screen*
in Muffler
373BA031 KN
12 
						

							24 English / USA
BR 340, BR 340 L, BR 420, BR 420 C Replacing the starter rope
:Remove the screws (1).
:Take the starter cover (2) off the 
engine.:Remove the spring clip (3). 
:Remove the rope rotor with washer 
(4) and pawl (5).:Ease the cap (6) out of the starter 
grip. 
:Remove remaining rope from the 
rotor and grip.
:Tie a simple overhand knot in the 
end of the new starter rope (see 
Specifications) and then thread the 
rope through the top of the grip and 
the rope bushing (7).
:Refit the cap in the grip.Replacing Starter Rope and 
Rewind Spring
11
1
373BA025 KN
2
1
353BA034 KN
3 5
4
353BA041 KN
76 
						

							25 BR 340, BR 340 L, BR 420, BR 420 CEnglish / USA
:Thread the rope through the rotor 
and secure it in the rotor with a 
simple overhand knot.:Fit the pawl (5) in the rotor and slip 
the washer (4) over the starter post.
:Use a screwdriver or suitable pliers 
to install the spring clip (3) on starter 
post and over the peg on the pawl  – 
the spring clip must point clockwise 
– see illustration. 
Go to “Tensioning rewind spring”.Replacing a broken rewind spring
:Lubricate the new spring with a few 
drops of non-resinous oil. 
:Remove the rope rotor as described 
in “Replacing the starter rope”.
:Remove parts of old spring.
373BA026 KN
353BA034 KN
3 5
4 
						

							26 English / USA
BR 340, BR 340 L, BR 420, BR 420 C :Fit new spring housing – bottom 
plate must face downward. Engage 
outer spring loop over the lug.
:Refit the rope rotor.
Go to “Tensioning rewind spring”.
If the spring pops out and uncoils 
during installation: Refit it in the 
spring housing in the counter-
clockwise direction – start outside 
and work inward.Tensioning rewind spring
:Make a loop in the unwound starter 
rope and use it to turn the rope rotor 
six full revolutions in the direction of 
the arrow (see illustration).
:Hold the rotor steady – straighten 
the twisted rope.
:Release the rotor and let go of rope 
slowly so that it winds onto the rotor. 
The starter grip must sit firmly in the 
rope guide bushing. If the grip 
droops to one side: Increase spring 
tension by one additional turn.When the starter rope is fully 
extended it must be possible to 
rotate the rotor at least another half 
turn. If this is not possible, the spring 
is overtensioned and could break. 
Take one turn of the rope off the 
rotor.
:Fit the starter cover on the engine.
:Tighten down the screws firmly.
:Move the stop switch to $ 
353BA029 KN
353BA030 KN 
						

							27 BR 340, BR 340 L, BR 420, BR 420 CEnglish / USA
For periods of about 3 months or longer:
:Drain and clean the fuel tank in a 
well ventilated area.
:Dispose of remaining fuel and 
cleaning solution properly in 
accordance with local 
environmental requirements.
:Run engine until carburetor is dry – 
this helps prevent carburetor 
diaphragms sticking together.
:Thoroughly clean the machine – pay 
special attention to the cylinder fins 
and air filter.
:Store the machine in a dry, high or 
locked location – out of the reach of 
children and other unauthorized 
persons.
Storing the Machine 
						

							28 English / USA
BR 340, BR 340 L, BR 420, BR 420 C 1) STIHL recommends that this work 
be done by a STIHL servicing dealerMaintenance ChartThe following maintenance intervals apply to normal operating conditions only.
If your daily working time is longer than normal or operating conditions are 
difficult (very dusty work area etc.), shorten the specified intervals accordingly.
before 
starting work
after finishing
work or daily
after each
refueling stop
weekly
monthly
every 12 months
if problem
if damaged
if required
Complete machineVisual inspection (conditon, leaks)xx
Cleanx
Control handle Check operationxx
Air filterCleanx
Replacex
Pick up body in fuel tankCheckx
Replacexx
Fuel tank Cleanx
CarburetorCheck idle settingxx
Readjust idlex
Spark plugReadjust electrode gapx
Replace after about 
100 operating hours
Cooling air intakes Cleanx
Spark arresting screen in mufflerCheckx
Clean or replacex
All accessible screws and nuts (not 
adjusting screws)Tightenx
Anti-vibration elementsCheckx
Have replaced by servicing 
dealer
1)
xx
Blower air intake screenCheckxx
Cleanx
Safety label Replacex 
						

							29 BR 340, BR 340 L, BR 420, BR 420 CEnglish / USA
    1 Nozzle, straight* 
  2 Nozzle, curved* 
  3 Blower tube* 
  4 Blower tube 
  5 Control handle 
  6 Throttle trigger 
  7 Setting lever 
  8 Stop switch 
  9 Pleated hose 
10 Harness 
11 Back plate 
12 Back padding
13 Air filter
14 Antivibration elements
15 Spark plug boot
16 Muffler (with spark arresting 
screen)*
17 Carburetor adjusting screws
18 Choke knob
19 Starter grip
20 Fuel filler cap
21  Fuel tank
# Serial number
* see “Guide to Using this Manual”
Parts and Controls
1112 10
5 67
1
4
9 8
13
373BA040 KN
2
3
14 14
373BA041 KN
#
2019181716 15
373BA042 KN
21