Stihl MS 391 Factory Service & Work Shop Manual

STIH) STIHL MS 311, 391 2009-06

1 MS 311, MS 391 q © ANDREAS STIHL AG & Co. KG, 2009 RA_542_00_01_01 1. Introduction and Safety Precautions 3 1.1 Introduction 3 1.2 Safety Precautions 4 2. Specifications 5 2.1 Engine 5 2.2 Fuel System 5 2.3 Ignition System 5 2.4 Chain Lubrication 5 2.5 Tightening Torques 6 3. Troubleshooting 8 3.1 Clutch 8 3.2 Chain Drive, Chain Brake, Chain Tensioner 9 3.3 Chain Lubrication 10 3.4 Rewind Starter 11 3.5 Ignition System 12 3.6 Carburetor 13 3.7 Engine 16 4. Clutch 17 4.1 Clutch Drum 19 5. Chain Brake 20 5.1 Checking Operation 20 5.2 Brake Band 20 5.3 Brake Lever 22 5.4 Cam Lever 24 5.5 Pins 25 5.6 Chain Tensioner 25 5.7 Bar Mounting Studs 26 6. Engine 27 6.1 Muffler 27 6.2 Leakage Test 28 6.2.1 Preparations 28 6.2.2 Vacuum Test 29 6.2.3 Pressure Test 29 6.3 Oil Seals 30 6.4 Shroud 31 6.5 Cylinder / Crankshaft 32 6.6 Bearings / Crankshaft 35 6.7 Piston 36 6.8 Piston Rings 38 6.9 Decompression Valve 38 7. Ignition System 39 7.1 Ignition Timing 39 7.2 Preseparator 39 7.3 Ignition Module 39 7.4 Testing the Ignition Module 41 7.5 Spark Plug Boot / Ignition Lead 42 7.6 Flywheel 43 7.7 Short Circuit Wire 44 7.7.1 Testing 44 7.7.2 Removing and Installing 44 7.7.3 Ground Wire 46 7.7.4 Contact Spring 46 7.8 Ignition System Troubleshooting 48 8. Rewind Starter 51 8.1 General 51 8.2 Fan housing 51 8.2.1 Segment 51 8.3 Pawls 51 8.4 Rope Rotor 52 8.5 Starter Rope / Grip 53 8.6 Tensioning the Rewind Spring 54 8.7 Replacing the Rewind Spring 55 9. Servicing the AV System 57 9.1 Buffer on Oil Tank 57 9.2 AV Spring on Oil Tank 57 9.3 AV Spring on Fuel Tank 57 9.4 AV Spring on Front Handle 58 9.5 Stop Buffer at Clutch Side 59 9.6 Annular Buffer at Ignition Side 59 9.7 Handlebar 59 10. Control Levers 61 10.1 Master Control Lever 61 10.2 Throttle Trigger/Interlock Lever 61 10.2.1 Choke Rod 63 10.2.2 Throttle Rod 64 11. Chain Lubrication 65 11.1 Pickup Body 65 11.2 Oil Suction Hose 65 11.3 Oil Pump 66 11.4 Valve 67 Contents

2 MS 311, MS 391 Contents 12. Fuel System 68 12.1 Air Filter 68 12.2 Baffle 68 12.3 Filter Base 68 12.4 Air Guide Shroud 69 12.5 Carburetor 71 12.5.1 Leakage Test 71 12.6 Servicing the Carburetor 72 12.6.1 Metering Diaphragm 72 12.6.2 Inlet Needle 73 12.6.3 Pump Diaphragm 74 12.6.4 Air Valve 76 12.6.5 Levers on Throttle Shaft 76 12.6.6 Adjusting Screws 77 12.7 Carburetor Adjustment 79 12.7.1 Basic Setting 79 12.7.2 Standard Setting 80 12.8 Carburetor Carrier 81 12.9 Intake Manifold 82 12.10 Tank Vent 83 12.10.1 Testing 83 12.10.2 Removing and Installing 84 12.11 Fuel Intake 84 12.11.1 Pickup Body 84 12.11.2 Fuel Hose 85 12.11.3 Fuel Suction Hose 86 12.11.4 Tank Housing 87 13. Special Servicing Tools 88 14. Servicing Aids 90

3 MS 311, MS 391 1. Introduction and Safety Precautions 1.1 Introduction This service manual contains detailed descriptions of all the repair and servicing procedures specific to this power tool. 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" 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 the chapter on "Special Servicing Tools" in 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. Symbols are included in the text and pictures for greater clarity. The meanings are as follows: In the descriptions: N 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 In the illustrations: A Pointer a Direction of movement @ 4.2 =Reference to another chapter, i.e. chapter 4.2 in this example. Service manuals and all technical information bulletins are intended exclusively for the use of properly equipped repair shops. They must not be passed to third parties. Servicing and repairs are made considerably easier if the machine is mounted to assembly stand (3) 5910 890 3100. To do this, secure the mounting plate (2) 5910 850 1650 to the assembly stand with two screws (1) and washers. The screws must not project since they, depending on the machine, may damage housings when the machine is clamped in position. The above operation is not necessary with the new assembly stand 5910 890 3101 since the mounting plate is already fitted. 1 2 3 219RA000 TG

4 MS 311, MS 391 Engage the bar mounting studs in the outer bores in the mounting plate and secure the machine in position with the nuts (arrows). Preparations for servicing Remove the chain sprocket cover, saw chain and guide bar before carrying out repairs or mounting the machine to the assembly stand. Always use original STIHL replacement parts. They can be identified by the STIHL part number, the { logo and the STIHL parts symbol K This symbol may appear alone on small parts. Storing and disposing of oils and fuels Collect fuel or lubricating oil in a clean container and dispose of it properly in accordance with local environmental regulations. 216RA000 TG 1.2 Safety Precautions If the machine 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. Gasoline is an extremely flammable fuel and can be explosive in certain conditions. Always wear suitable protective gloves for operations in which components are heated for assembly or disassembly. Improper handling may result in burns or other serious injuries. 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. Always perform leakage test after working on the fuel system and the engine. Always replace damaged parts. Check disassembled parts for wear or damage before re-installing – replace as necessary. Run the machine only with the shroud mounted in position – there is otherwise a risk of injury from the fanwheel and a risk of engine damage due to overheating. The chapter on tightening torques lists all machine components that have to be tightened to a specific torque or coated with threadlocking adhesive. The specifications must be maintained when tightening down screws, nuts and other fasteners in all the procedures described in this service manual. Fuel system – hose barb connectors Pull off or push on fuel hoses in line with the connector, preferably by hand, to ensure the tightness of the fuel system. Avoid damaging the hose barb – do not use sharp-edged pliers, screwdrivers, etc. Do not cut open fuel hoses with a knife or similar tool. Do not re-use fuel hoses after removal. Always install new hoses – fuel hoses can be overstretched during removal. Install new fuel hoses either dry or with the aid of STIHL press fluid, b 14. Other press fluids are not approved and may result in damage to the fuel hoses. Coat the ends of the hoses and the connectors with STIHL press fluid and then push the new hoses on to the hose barbs, b 14.

5 MS 311, MS 391 2. Specifications 2.1 Engine MS 311 MS 391 Displacement: 59 cm 3 64.1 cm 3 Bore: 47 mm 49 mm Stroke: 34 mm 34 mm Engine power to ISO 7293: 3 kW (4 bhp) at 9,500 rpm 3.2 kW (4.4 bhp) at 9,500 rpm Maximum permissible engine speed with guide bar and chain: 13,000 rpm 13,000 rpm Idle speed: 2,800 rpm 2,800 rpm Clutch: Centrifugal clutch without linings Centrifugal clutch without linings Clutch engages at: 3,500 rpm 3,500 rpm Engine housing leakage test at gauge pressure: 0.5 bar under vacuum: 0.5 bar 2.2 Fuel System Carburetor leakage test at gauge pressure: 0.8 bar Operation of tank vent at gauge pressure: 0.5 bar Fuel: as specified in instruction manual 2.3 Ignition System Air gap between ignition module and fanwheel: 0.20...0.30 mm Spark plug (resistor type): NGK BPMR 7 A BOSCH WSR6F Electrode gap: 0.5 mm 2.4 Chain Lubrication Speed-controlled oil pump with reciprocating piston and manual flow control Oil delivery rate: Ematic oil pump 11.5 (+2.5) cm 3 /min at 10,000 rpm Oil delivery rate: Adjustable oil pump 6.0 (+/-2.0)...17.5 (+3.0 / -4.5) cm 3 /min at 10,000 rpm

6 MS 311, MS 391 2.5 Tightening Torques DG and P (Plastoform) screws are used in polymer and light metal 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 Nm Remarks Screw P 4x14 Chain tensioner cover/engine housing 1.5 Screw P 4x12 Brake band/engine housing 2.0 Collar screw M 10 Collar stud for bar / engine pan 30.0 1) Collar screw D 8x24 Collar stud for bar / engine housing 16.0 Screw P 5x16 Cover, chain brake / engine housing 4.0 M 10x1 Decompression valve 14.0 Screw P 5x16 Handlebar/plug, AV spring 3.0 Screw P 6x26.5 Handlebar / tank housing, right (polymer) 7.0 Screw P 6x26.5 Handlebar / tank housing, bottom (polymer) 7.0 Screw P 6x25 Hand guard / fan housing 8.0 Screw M 6x30 Shroud / engine housing 10.0 Screw P 6x38 Chain catcher / bearing plug 6.0 Screw P 5x16 Spiked bumper / engine housing 4.0 Screw M 4x12 Manifold/cylinder 4.0 3) Screw P 6x21.5 Bearing plug / engine housing 6.0 Screw M 5x16 Bearing plug/cylinder 10.0 4) Screw P 5x20 Fan housing / engine housing 4.0 Screw P 4x12 Air baffle / engine housing 2.0 Carrier M 12x1 LH Carrier / crankshaft 50.0 Screw M 6x25 Engine housing / cylinder, stage 1 4.0 3) Screw M 6x25 Engine housing / cylinder, stage 2 12.0 3) Screw D 4x18 Oil pump 4.0 3) Screw M 5x16 Muffler / cylinder 10.0 1) 3) Screw M 8x1 Flywheel/crankshaft 33.0 5) Nut M 5 Carburetor/collar stud 3.5 Screw P 4x14 Pre-separator / engine housing 2.0 Screw P 4x12 Cover plate/fan housing 2.0 Spark plug M 14x1.25 Spark plug 25.0 Screw D 4x18 Ignition module / engine pan 4.0 3)

7 MS 311, MS 391 Remarks: 1) Loctite 242 or 243, medium strength 2) Loctite 648, high strength 3) Screws with antifricton coated binding head 4) Screws with micro-encapsulated binding head 5) Degrease crankshaft/flywheel and mount oil-free 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. Coat micro-encapsulated screws with medium-strength threadlocking adhesive before re-installing. 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 setting for polymer: DG and P screws max. 500 rpm Do not use an impact wrench for releasing or tightening screws. Do not mix up screws with and without binding heads.

8 MS 311, MS 391 3. Troubleshooting 3.1 Clutch Condition Cause Remedy Saw chain stops under load at full throttle Clutch shoes badly worn Install new clutch Clutch drum badly worn Install new clutch drum Saw chain rotates at idle speed Engine idle speed too high Readjust with idle speed screw LA (counterclockwise) Clutch springs stretched or fatigued Replace the clutch springs or install new clutch Clutch spring hooks broken Replace the clutch springs or install new clutch Loud noises Clutch springs stretched or fatigued Replace the clutch springs or install new clutch Needle cage damaged Fit new needle cage Clutch shoe retainer broken Install new retainer or clutch Clutch shoes and carrier worn Install new clutch

9 MS 311, MS 391 3.2 Chain Drive, Chain Brake, Chain Tensioner Condition Cause Remedy Chain sprocket wears rapidly Chain not properly tensioned Tension chain as specified Wrong chain pitch Fit chain of correct pitch Insufficient chain lubrication Check chain lubrication Chain sprocket worn Fit new chain sprocket Saw chain stops under load at full throttle Clutch shoes badly worn Install new clutch Clutch drum badly worn Install new clutch drum Brake band blocked Check freedom of movement and operation of brake band Saw chain rotates at idle speed Engine idle speed too high Readjust with idle speed screw LA (counterclockwise) Clutch springs stretched or fatigued Replace the clutch springs or install new clutch Clutch spring hooks broken Replace the clutch springs Saw chain does not stop immediately when brake is activated Brake spring stretched or broken Fit new brake spring Brake band stretched or worn Fit new brake band Clutch drum worn Install new clutch drum

Stihl MS 391 Factory Service & Work Shop Manual

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