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Stihl MS 251 Instruction Manual - Page 18

Clean the carburetor, Pump diaphragm damaged or - air filter

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Condition Cause Engine speed drops quickly under Air filter dirty load - low power Remedy Clean air filter or replace if necessary Throttle shutter not opened fully Tank vent faulty Check throttle rod Replace tank vent Fuel pickup body dirty Install new pickup body Fuel strainer dirty Clean fuel strainer in carburetor, replace if necessary Leak on fuel hose from tank to carburetor Seal connections or install new fuel hose Setting of high speed screw H too Check basic carburetor setting, rich correct if necessary Main jet bores or ports blocked Clean the carburetor Pump diaphragm damaged or fatigued Fit new pump diaphragm Ignition timing wrong, flywheel out of adjustment - key in flywheel has sheared off Install new flywheel Engine running extremely rich, has Choke shutter does not open no power and a very low maximum speed Check carburetor and choke shaft, service or replace if necessary MS 231, MS 231 C, MS 251, MS 251 C 17

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