Stihl MS 251 Instruction Manual - Page 21

20

MS 231, MS 231 C, MS 251, MS 251 C

5.

Chain Brake

5.1

Checking Operation

The chain brake is one of the most

important safety devices on the

chain saw. Its efficiency is

measured in terms of the chain

braking time, i.e. the time that

elapses between activating the

brake and the saw chain coming to

a complete standstill.

Contamination (with chain oil,

chips, fine particles of abrasion,

etc.) and smoothing of the friction

surfaces of the brake band and

clutch drum impair the coefficient of

friction, which prolongs the braking

time. A fatigued or stretched brake

spring has the same negative effect.

–

Starting the engine

–

With the chain brake

activated (locked), open the

throttle wide for a brief

period (max. 3 seconds) – the

chain must not rotate.

–

With the chain brake released,

open the throttle wide and

activate the brake manually – the

chain must come to an abrupt

stop.

Machines with QuickStop Super

The clutch drum must rotate freely

when the lockout lever is

depressed.

With the coasting brake

disengaged, open the throttle wide

and let go of the lockout lever on the

rear handle – the chain must come

to an abrupt stop.

All models

The braking time is in order if

deceleration of the saw chain (less

than a second) is imperceptible to

the eye.

If the chain brake does not operate

properly, refer to troubleshooting,

b

3.2.

5.2

Brake Band

–

Remove the clutch drum,

b

4.1

– Troubleshooting,

b

3.2

:

Take out the screws (1).

:

Remove the cover (2).

–

Engage the chain brake.

:

Remove the screw (1) from the

underside of the machine.

TOP

2310RA006 TG

1

1

2

2

2310RA007 TG

1

:

Pry the brake band (2) out of its

seat (arrow).

–

Remove the brake band without

overstretching it.

–

Disengage the chain brake

:

Turn the brake band (1) to one

side and disconnect it from the

brake lever (2).

Install a new brake band if there are

noticeable signs of wear (large

areas on inside diameter and/or

parts of outside diameter – arrows)

and its remaining thickness is less

than 0.6 mm.

OP

2

2310RA008 TG

1

219RA049 TG

Section	Page
Contents	2
1. Introduction and Safety Precautions	4
1.1 Introduction	4
1.2 Safety Precautions	5
2. Specifications	6
2.1 Engine	6
2.2 Fuel System	6
2.3 Ignition System	6
2.4 Chain Lubrication	6
2.5 Tightening Torques	7
3. Troubleshooting	9
3.1 Clutch	9
3.2 Chain Drive, Chain Brake, Chain Tensioner	10
3.3 Chain Lubrication	12
3.4 Rewind Starter	13
3.5 Ignition System	15
3.6 Carburetor	16
3.7 Engine	19
4. Clutch	20
4.1 Clutch Drum	20
4.2 Clutch	20
5. Chain Brake	21
5.1 Checking Operation	21
5.2 Brake Band	21
5.3 Brake Lever	22
5.4 Brake Lever on Machines with QuickStop Super	25
5.4.1 Adjusting the brake cable	27
5.4.2 Brake cable Removing and Installing	28
5.5 Chain Tensioner	30
5.5.1 Quick Chain Tensioner	31
5.5.2 Chain Catcher	31
5.6 Bar Mounting Stud	32
6. Engine	33
6.1 Muffler	33
6.2 Leakage Test	34
6.2.1 Preparations	34
6.2.2 Vacuum Test	35
6.2.3 Pressure Test	35
6.3 Oil Seals	36
6.3.1 Ignition side	36
6.3.2 Clutch side	37
6.4 Shroud	37
6.5 Engine	38
6.6 Cylinder / Crankshaft	39
6.7 Bearings / Crankshaft	42
6.8 Piston	43
6.9 Piston Rings	45
7. Ignition System	46
7.1 Ignition Timing	46
7.2 Preseparator	46
7.3 Install new ignition module	46
7.4 Testing the Ignition Module	48
7.5 Spark Plug Boot / Ignition Lead	49
7.6 Flywheel	50
7.7 Short Circuit Wire	51
7.7.1 Testing	51
7.7.2 Removing and Installing	51
7.7.3 Ground Wire	54
7.7.4 Contact Spring	54
7.8 Ignition System Troubleshooting	56
8. Rewind Starter	59
8.1 General	59
8.2 Fan housing	59
8.3 Pawls	60
8.4 ErgoStart	61
8.5 Rope Rotor	62
8.6 Starter Rope / Grip	62
8.7 Tensioning the Rewind Spring	63
8.8 Replacing the Rewind Spring	64
9. Servicing the AV System	67
9.1 AV Spring on Oil Tank	67
9.2 AV Spring on Fuel Tank	67
9.3 AV Spring on Handlebar	68
9.3.1 Stop Buffers	69
9.3.2 Buffers on Filter Base	70
9.4 Handlebar	70
10. Control Levers	72
10.1 Master Control Lever	72
10.1.1 Removing and Installing	72
10.2 Throttle Trigger/Lockout Lever	73
10.3 Throttle Trigger/Lockout Lever - QuickStop Super	74
10.3.1 Switch Lever QuickStop Super	75
10.3.2 Lockout Lever - QuickStop Super	76
10.3.3 Choke Rod	77
10.3.4 Throttle Rod	77
11. Chain Lubrication	79
11.1 Pickup Body	79
11.2 Oil Suction Hose	79
11.3 Oil Pump	79
11.4 Valve	81
12. Fuel System	82
12.1 Air Filter	82
12.2 Baffle	82
12.3 Filter Base	82
12.4 Air Guide Shroud	84
12.4.1 Air Guide Shroud - Models with Manual Fuel Pump	86
12.5 Carburetor	89
12.5.1 Leakage Test	90
12.6 Servicing the Carburetor	91
12.6.1 Metering Diaphragm	91
12.6.2 Inlet Needle	91
12.6.3 Pump Diaphragm	92
12.6.4 Lever on Throttle Shaft	93
12.6.5 Adjusting Screws	94
12.7 Adjusting the Carburetor	96
12.7.1 Basic Setting	96
12.7.2 Standard setting	97
12.8 Carburetor Carrier	98
12.9 Intake Manifold	99
12.10 Tank Vent	100
12.10.1 Testing	100
12.10.2 Removing and Installing	101
12.11 Fuel Intake	102
12.11.1 Pickup Body	102
12.11.2 Fuel Hose	102
12.11.3 Fuel Hoses - Manual Fuel Pump	106
12.11.4 Manual Fuel Pump	108
12.11.5 Tank Housing	109
13. Special Servicing Tools	111

Stihl MS 251 Instruction Manual - Page 21

Chain Brake, 5.1 Checking Operation, 5.2 Brake Band, Remove the clutch drum - starting

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