Stihl BT 45 User manual

STIH)
STIHL Components 4140
BT 45 2009-07

1BT 45
q
© ANDREAS STIHL AG & Co. KG, 2009
Contents
1. Introduction 2
2. Safety Precautions 3
3. Specifications 4
3.1 Fuel System 4
3.2 Gearbox 4
3.3 Tightening Torques 5
4. Troubleshooting 7
4.1 Clutch 7
4.2 Rewind Starter 8
4.3 Fuel System 9
4.4 Engine 11
5. Shroud/Housing
Parts 12
5.1 Handle Housing 12
5.2 Clutch 13
5.2.1 Engine 14
5.3 Muffler/Spark
Arresting Screen 14
6. Rewind Starter 15
6.1 General 15
6.2 Removing and
Installing 15
6.3 Starter Cup 16
6.3.1 Rope Rotor 16
6.4 Replacing the
Rewind Spring 17
6.4.1 Tensioning the Rewind
Spring 18
6.5 Starter Rope 18
6.6 Starter Rope
Guide Bush 19
6.7 Pawl 20
7. Throttle Control 20
7.1 Throttle Trigger 20
7.2 Replacing the Throttle
Cable 21
7.3 Stop Switch 21
8. Fuel System 22
8.1 Air Filter 22
8.1.1 Removing and
Installing 22
8.1.2 Choke Shutter 22
8.2 Carburetor 23
8.2.1 Leakage Test 23
8.2.2 Removing and
Installing the
Carburetor 24
8.3 Spacer Flange 25
8.4 Servicing 25
8.4.1 Manual Fuel Pump 25
8.4.2 Metering Diaphragm 26
8.4.3 Inlet Needle 27
8.4.4 Fixed Jet 27
8.4.5 Pump Diaphragm
and Fuel Strainer 28
8.4.6 Adjusting 28
8.4.7 Basic Setting 30
8.5 Tank Vent 30
8.5.1 General 30
8.5.2 Replacing 31
8.6 Fuel Tank 31
8.6.1 Pickup Body 31
8.6.2 Hoses 32
9. Gearbox 32
9.1 Removing and
Installing 32
9.2 Diassembling and
Assembling 33
10. Special Servicing
Tools 40
11. Service Accessories 41

2BT 45
This service manual contains
detailed descriptions of all the
typical repair and servicing
procedures for the BT 45 which is
based on the series 4140
powerhead.
You will find detailed descriptions of
procedures for servicing and
repairing engine components in the
service manual for the “Series 4140
Powerhead“.
You should make use of the
illustrated parts lists while carrying
out repair work. They show the
installed positions of the individual
components and assemblies.
Refer to the latest edition of the
relevant parts list to check the part
numbers of any replacement parts.
A fault on the machine may have
several causes. To help locate the
fault, consult the chapter on
"Troubleshooting" in this manual
and the "STIHL Service Training
System" for all assemblies.
Refer to the "Technical Information"
bulletins for engineering changes
which have been introduced since
publication of this service manual.
Technical information bulletins also
supplement the parts list until a
revised edition is issued.
The special tools mentioned in the
descriptions are listed in chapter
"Special Servicing Tools" of this
manual. Use the part numbers to
identify the tools in the "STIHL
Special Tools" manual.
The manual lists all special
servicing tools currently available
from STIHL.
Symbolsareincludedinthetextand
pictures for greater clarity.
The meanings are as follows:
In the descriptions:
:= Action to be taken as
shown in the illustration
(above the text)
– = Action to be taken that is
not shown in the illustration
(above the text)
b4.2
Reference to another chapter, i.e.
chapter 4.2 in this example.
In the illustrations:
Pointer
 Direction of movement
Service manuals and technical
information bulletins are intended
exclusively for the use of properly
equipped repair shops. They must
not be passed to third parties.
Always use original STIHL
replacement parts.
They can be identified by the
STIHL part number,
the STIH) logo
and the
STIHL parts symbol (
This symbol may appear alone on
small parts.
1. Introduction

3BT 45
If the engine is started up in the
course of repairs or maintenance
work, observe all local and country-
specific safety regulations as well
as the safety precautions and
warnings in the instruction manual.
Gasolineisanextremelyflammable
fuel and can be explosive in certain
conditions.
Improper handling may result in
burns or other serious injuries.
Warning!
Do not smoke or bring any fire,
flame or other source of heat near
the fuel. All work with fuel must be
performed outdoors only. Spilled
fuel must be wiped away
immediately.
2. Safety Precautions

4BT 45
3. Specifications
3.1 Fuel System Carburetor: Diaphragm carburetor
Carburetor leakage test
at gauge pressure: 0.8 bar (11.6 psi)
Checking operation of tank vent
at gauge pressure: 0.3 bar (4.4 psi)
under vacuum: 0.05 bar (0.725 psi)
Fuel: see instruction manual
Octane rating: min. 90 RON
Fuel mixture: Regular brand-name gasoline and
STIHL 50:1 two-stroke engine oil or
brand-name two-stroke engine oil
Mix ratio: 50:1 with STIHL two-stroke engine oil
Fuel mix for units with catalytic
converter: Use only STIHL 50:1 two-
stroke engine oil with lead-free
gasoline!
3.2 Gearbox Type: Multi-stage spur gear drive with
straight teeth
Gear ratios: 1 = 11.3:1
2 = 3.8:1
R = 12.7:1
Bearings: Deep-groove ball bearings
Lubrication: STIHL gear lubricant for brushcutters
(total capacity 20 g)

5BT 45
3.3 Tightening Torques
DG and P (Plastoform) screws are used in polymer and lightmetal components. These screws form a
permanent thread when they are installed for the first time. They can be removed and installed as often as
necessary without impairing the strength of the screwed assembly, providing the specified tightening torque is
observed.
For this reason it is essential to use a torque wrench.
Fastener Thread size For component Torque Remarks
lbf.ft Nm
M14x7 Stub/muffler 7.5 10.0
Screw IS-DG5x24 Spacer flange/cylinder 4.4 6.0
Screw IS-DG5x20Z Fan housing/spiral housing 4.4 6.0
Screw IS-DG5x24 Spiral housing/crankcase 4.4 6.0
Screw IS-DG5x24 Spiral housing/engine pan 4.4 6.0
Screw IS-DG5x20Z Fan housing/spiral housing 4.4 6.0
Screw M4x8 Guide plate/gear housing 3.0 4.0
Screw IS-DG5x60 Muffler/cylinder 6.6 9.0
Screw M6x25LH Lock screw, chuck/output shaft 7.5 10.0
1/2“-20 Chuck/output shaft 26 35.0 1)
Screw IS-P4x14 Rotary knob/driver, gearbox 0.75 1.0
Fastening nut M5 Filter cover/handle housing/screw 1.5 2.0
Screw IS-DG5x20 Gear housing, right/left 5.9 8.0
Screw IS-DG5x24 Gear housing/crankcase 5.9 8.0
Screw IS-DG5x24 Gear housing/engine pan 5.9 8.0
Screw IS-DG5x24 Handle housing, left/right 2.6 3.5
Screw IS-DG5x16 Shroud/gearbox 2.6 3.5
Nut M5 Manifold/carburetor/flange/screw 2.6 3.5
M8x1.25L Carrier/crankshaft 12.5 17.0
Nut M12x1.25 Friction clutch 10.5 14.0
M14x1.25 Spark plug 15 20.0
Remarks:
1) Install screw with medium-strength Loctite 243

6BT 45
Use the following procedure when refitting a DG or P screw in an existing thread:
– Place the screw in the hole and rotate it counterclockwise until it drops down slightly.
– Tighten the screw clockwise to the specified torque.
This procedure ensures that the screw engages properly in the existing thread and does not form a new thread
and weaken the assembly.
Power screwdriver settings for polymer: Plastoform screws max. 600 rpm
DG screws max. 500 rpm
Important:
Do not mix up screws with and without binding head

7BT 45
4. Troubleshooting
4.1 Clutch
Condition Cause Remedy
Tool stops at full throttle
under load Clutch shoes badly worn Install new clutch shoes or a new clutch
Tool runs at idle speed Idle speed too high Readjust idle speed screw
(counterclockwise)
Clutch springs stretched or
fatigued Fit new clutch springs
Spring hooks broken Fit new clutch springs
Loud noises Clutch springs stretched or fatigued Replace all clutch springs
Clutch shoe retainer (carrier)
broken Fit new retainer (carrier) or clutch
Clutch shoes and carrier worn Install a new clutch

8BT 45
4.2 Rewind Starter
Condition Cause Remedy
Starter rope broken Rope pulled out too vigorously as
far as stop or over edge, i.e. not
vertically
Fit new starter rope
Normal wear Fit new starter rope
Rewind spring broken Spring overtensioned – no reserve
when rope is fully extended Fit new rewind spring
Very dirty or corroded Fit new rewind spring
Starter rope can be pulled out
almost without resistance
(crankshaft does not turn)
Guide peg on pawl or pawl itself
is worn Fit new pawl
Spring clip fatigued Fit new spring clip
Starter rope is difficult to pull and
rewinds very slowly Starter mechanism very dirty (dusty conditions) Thoroughly clean complete starter mechanism
Lubricating oil on rewind spring
becomes viscous at very low
outside temperatures (spring
windings stick together)
Coat rewind spring with a standard
solvent-based degreasant
(containing no chlorinated or
halogenated hydrocarbons). Then
pull rope carefully several times
until normal action is restored

9BT 45
4.3 Fuel System
Condition Cause Remedy
Engine stalls at idle speed Idle jet bores or ports blocked Clean jet bores and ports and blow
clear with compressed air
Idle jet too rich Screw down low speed screw
slightly (see Adjusting the
Carburetor)
Setting of idle speed screw
incorrect – throttle shutter
completely closed
Reset idle speed screw correctly
Engine speed drops quickly under
load – low power Air filter plugged Clean the air filter or replace if necessary
Tank vent faulty Clean the tank vent or replace if
necessary
Leak in fuel line between tank and
fuel pump Seal or renew connections and fuel line
Pump diaphragm damaged or
fatigued Fit new pump diaphragm
Main jet bores or ports blocked Clean the bores and ports
Fuel pickup body dirty Install new pickup body
Engine will not idle,
idle speed too high Throttle shutter opened too wide by idle speed screw Reset idle speed screw correctly

10 BT 45
Condition Cause Remedy
Poor acceleration Idle jet too lean Back off the low speed screw
slightly
(see Adjusting the Carburetor)
Main jet too lean Back off the high speed screw
slightly
(see Adjusting the Carburetor)
Inlet control lever too low
(relative to correct installed
position)
Replace the inlet control lever
Inlet needle sticking to valve seat Remove inlet needle, clean and
refit
Connecting bore to atmosphere
blocked Clean bore
Diaphragm gasket leaking Fit new diaphragm gasket
Metering diaphragm damaged or
shrunk Fit new metering diaphragm
Carburetor floods,
engine stalls Inlet needle not sealing. Foreign matter in valve seat or cone
damaged
Remove and clean or replace inlet
needle, clean fuel tank, pickup
body and fuel line if necessary
Inlet control lever sticking on
spindle Free off inlet control lever
Helical spring not located on nipple
of inlet control lever Remove inlet control lever and refit correctly
Perforated disc on diaphragm is
deformed and presses constantly
against inlet control lever
Fit new metering diaphragm
Inlet control lever too low
(relative to correct installed
position)
Fit new inlet control lever

11BT 45
4.4 Engine
Always check and, if necessary,
repair the following parts before
looking for faults on the engine:
– Air filter
– Fuel system
– Carburetor
– Ignition system1)
Condition Cause Remedy
Engine does not start easily, stalls
at idle speed, but operates normally
at full throttle
Oil seals in crankcase faulty Install new oil seals 1)
Crankcase leaking / damaged
(cracks) Seal / replace the crankcase 1)
Muffler leaking Seal / replace the muffler
Engine does not deliver full power
or runs erratically Piston rings worn or broken Install new piston rings 1)
Muffler / spark arresting screen
carbonized Clean muffler (inlet and exhaust openings), replace
spark arresting screen (stub)
Air filter element dirty Fit new air filter element
Fuel / impulse line kinked or
cracked Fit new lines or position without kinks
Engine overheating Insufficient cylinder cooling.
Air inlets in fan housing
blocked or cooling fins on
cylinder very dirty
Thoroughly clean all cooling air
passages and cooling fins
1) see "Series 4140 Powerhead" service manual

12 BT 45
– Remove the air filter - b8.1
:Unscrew the nuts (1) and pull the
manifold with flange off the
carburetor.
– Remove the fan housing with
rewind starter - b6.1
1
VA
932RA238
:Take out the screws (arrows).
VA
932RA240
:Take the handle hose (1) out of
the guides (arrows) and pull it off
the handle housing.
– Remove the right-hand handle
housing.
VA
932RA241
1
– Lift the left-hand handle housing
and swing it just a little to one
side.
:Pull the fuel fuel hoses (1) off the
carburetor connectors.
:Disconnect the throttle cable (2)
1 2
VA
932RA242
fromtheslotted pin on the throttle
lever and pull it out of the spacer
flange (arrow). Do not kink the
throttle cable during this
operation.
– Remove the left-hand handle
housing.
Install in the reverse sequence.
– When reinstalling, first remove
the rubber buffers (1) from the
engine and fit them in the left-
hand and right-hand handle
housings.
932RA243
VA
1 1
5. Shroud/Housing Parts
5.1 Handle Housing

13BT 45
Troubleshooting - b4.1
Removing
– Remove the gearbox - b9.1
– Inspect the clutch drum. There
should be no scores or signs of
excessive wear.
:If there are noticeable wear
marks on the inside diameter of
theclutchdrum (1),checkits wall
thickness. If it is less than about
VA
148RA101
80%
100%
1
!
80%ofthe original thickness,fita
new clutch drum.
:It is necessary to disassemble
the gearbox (b9.2) to replace
the clutch drum (1).
932RA114
VA
1
– Unscrew the spark plug.
:Use the locking strip (1)
4221 893 5903 to block the
piston.
:Unscrew the clutch (2).
VA
932RA152
2
1
Note that clutch carrier has a left-
hand thread.
:If necessary, remove the
washer (1).
932RA157
1
VA
Disassembling
:Use special hook (2)
5910 890 2800 to detach the
clutch springs (1).
VA
932RA153
2
1
1
1
:Take the clutch shoes off the
carrier and clean the individual
parts - b11
– Replace any damaged parts.
VA
932RA154
5.2 Clutch

14 BT 45
Assembling
:Slide the clutch shoes (1) over
the arms of the carrier (2).
VA
932RA155
2
1
:Hook one end of each spring (1)
to the clutch shoes.
– Usespecial hook 5910 890 2800
to attach the other ends of the
springs to the clutch shoes.
Installing
VA
932RA156
1
1
1
Install in the reverse sequence
while observing the specified
tightening torques -
b3.3.
– Remove the fan housing with
rewind starter - b6.1
– Remove the handle housing
- b5.1
:Pull off the carburetor along with
the gasket.
– Remove the gearbox - b9.1
Install in the reverse sequence.
932RA255
VA
Troubleshooting - b4.4
– Remove the fan housing with
rewind starter - b6.1
– Remove the right-hand handle
housing - b5.1
Muffler
:Take out the screws (1) and
remove the muffler (2).
1
2
VA
932RA202
:Remove the exhaust gasket (1).
932RA203
1
VA
5.2.1 Engine 5.3 Muffler/Spark Arresting
Screen

15BT 45
Install in the reverse sequence.
– Fit a new exhaust gasket.
– Apply sealant to the underside of
the screw head - b11.
– Tighten down the screws -
b3.3
Spark arresting screen (stub)
:Use a 15 mm wrench to unscrew
the stub (1).
– Clean the spark arresting screen
or install a new stub if necessary.
1
VA
932RA204
– Clean the thread of the stub and
coat it with sealing compound -
b11
Follow the manufacturer’s
instructions.
– Screw home the stub and tighten
it down firmly - b3.3
Troubleshooting - b4.2
If the action of the starter rope
becomes very stiff and the rope
rewinds very slowly or not
completely, it can be assumed that
the starter mechanism is in order
but plugged with dirt. At very low
outsidetemperaturesthelubricating
oil on the rewind spring may thicken
and cause the spring windings to
stick together. This has a
detrimental effect on the function of
the starter mechanism. In such a
case it is sufficient to apply a few
drops of a standard solvent-based
degreasant (containing no
chlorinated or halogenated
hydrocarbons) to the rewind spring.
Carefully pull out the starter rope
several times and allow it to rewind
until its normal smooth action is
restored.
If clogged with dirt or pitch, the
entire starter mechanism, including
the rewind spring, must be removed
and disassembled. Take particular
care when removing the spring.
Clean all parts with a standard
commercial solvent-based
degreasant containing no
chlorinated or halogenated
hydrocarbons - b11.
Lubricate the rewind spring and
starter post with STIHL special
lubricant, see b11, before
installing.
:Take out the screws (1) and lift
away the fan housing with rewind
starter.
Install in the reverse sequence.
1
1
1
VA
932RA160
6. Rewind Starter
6.1 General 6.2 Removing and Installing

16 BT 45
The machine features a starter cup
integrated in the flywheel. For
servicing information refer to
"Series 4140 Powerhead" service
manual.
– Remove the rewind starter -
b6.1
Relieving tension of rewind
spring
The rewind spring will not be under
tension if the starter rope is broken.
– Pull out the starter rope about
10 cm and hold the rope rotor
steady.
– While still holding the rope rotor
steady,takethreefullturnsoffthe
rope rotor.
– Pull out the rope with the starter
grip and slowly release the rope
rotor.
The rope rotor spins back and
relieves the tension on the rewind
spring.
Removing
The rewind spring must not be
under tension.
:Remove the spring clip (1) from
the starter post.
VA
932RA181
1
3
2
:Remove the washer (2) and
pawl (3).
– Carefully pull the rope rotor off
the starter post.
– Replace the broken or worn
starter rope - b6.5
Installing
– Coat bore in rope rotor with
STIHL special lubricant -
b11
:Fit the rotor on the starter post
so that the driver (1) on the rope
rotor slips behind the inner spring
loop (2).
2
1
VA
932RA182
6.3 Starter Cup 6.3.1 Rope Rotor

17BT 45
– Check that the spring loop has
engaged byturning theroperotor
a little and letting it go. It must
spring back.
– Coat pawl with special lubricant
and install - b6.7 and 11
:Fit the washer and engage the
spring clip (1) in the groove in the
starter post.
:Make sure the spring clip (1)
engages the guide peg (arrow)
on the pawl (2) and points
clockwise.
VA
932RA183
1
2
Handle the spring clip with care.
The rewind starter may not function
properly if the spring clip is
deformed.
– Tension the rewind spring -
b6.4.1
– Install the rewind starter -
b6.1
– Remove the rope rotor - b6.3.1
– Remove the pieces of broken
spring from the rope rotor and
starter cover.
– Before installing, lubricate the
new spring with a few drops of
STIHL special lubricant - b11
:Position the rewind spring in the
starter andpress the outer spring
loop over the lug (arrow).
The rewind spring may pop out and
uncoil if rope rotor is not installed
very carefully..
VA
932RA184
If the rewind spring has popped out,
refit it as follows:
– Hold the spring it your hand and
wind from the inside outwards
and tension it to a diameter of
55 mm.
:Then grip the rewind spring with
pointed nose pliers about 10 mm
fromtheendoftheouterloopand
place it in the starter.
:Check the distance of the inner
spring loop from the hub and
correct if necessary. Dimension
"A" must not be more than 2 mm.
– Install the rope rotor - b6.3.1
– Tension the rewind spring -
b6.4.1
6.4 Replacing the
Rewind Spring

18 BT 45
:Make a loop in the unwound
starter rope. Grip the rope close
to the rotor and use it to turn the
rotor six full turns clockwise.
VA
932RA187
:Hold the rope rotor steady.
:Pull out the rope with the starter
grip and straighten it out.
– Holdthestartergripfirmlyto keep
the rope tensioned.
VA
932RA188
– Let go of the rope rotor and
slowly release the starter grip so
that the rope winds itself onto the
rotor.
The starter grip must sit firmly in the
rope guide bush without drooping to
one side. If this is not the case,
tension the spring by one additional
turn.
When the starter rope is fully
extended, it must still be possible to
VA
932RA189
rotatetheroperotor at leastanother
half turn before maximum spring
tension is reached. If this is not the
case, pull the rope out, hold the
rope rotor steady and take off one
turn of the rope.
Do not overtension the rewind
spring as this will cause it to
break,
– Place the rewind starter in
position, insert the screws and
tighten them down firmly - b3.3
– Remove the rewind starter -
b6.1
– If the starter rope is broken,
remove the remaining rope from
the rope rotor and starter grip.
– If the starter rope is worn, relieve
tension of rewind spring,
b6.3.1. Pull the end of the rope
outoftherotorand undotheknot.
Then pullthewornropeoutof the
starter grip, starter housing and
rotor.
:Thread one end of the new rope
through the rotor and starter
housing. Tie a simple overhand
knot in the end of the rope.
– Pull the rope back into the rotor
until the knot locates in the
recess.
VA
932RA190
– Thread the other end of the rope
through the starter grip. Tie a
simple overhand knot in the end
of the rope.
6.4.1 Tensioning the Rewind
Spring 6.5 Starter Rope

19BT 45
:On machines with "ElastoStart":
Thread end of new starter rope
through the grip (1) and the
spring element (2). Tie a simple
overhand knot in the end of the
rope. Pull the rope and spring
element into the grip and then fit
the cap (3).
VA
380RA065
3
1 2
– Tension the rewind spring -
b6.4.1
– Install the rewind starter -
b6.1
Wear on the guide bush is acceler-
ated by the starter rope being pulled
sideways. The wall of the guide
bush eventually wears through and
the bush becomes loose.
– Removeany remainingropefrom
the starter mechanism.
– Remove the rope rotor - b6.3.1
:Pull the knot (1) out of recess (2)
in rope rotor.
– Undo the knot.
:Pull the starter rope out of the
rotor and guide bush.
VA
176RA187
2 1
– Use a suitable tool to pry the
damaged bush out of the fan
cover.
Installing the rope bush
– Place the new bush in its seat in
the fan housing/starter cover.
:Insert the screw spindle (1) of
installing tool 0000 890 2201
through the bush from inside the
housing.
– Fit the thrust sleeve (1), tapered
end first, and fit the hex nut.
:Tightendownthehexnutuntilthe
bush is firmly seated.
The installing tool flares the lower
end of the rope bush.
– Remove the installing tool.
6.6 Starter Rope Guide Bush