Stihl MS 271, MS 291 Brushcutters Service Repair Manual INSTANT

STIH) STIHL MS 271, 291 2010-06

1 MS 271 C, MS 291 C q © ANDREAS STIHL AG & Co. KG, 2010 RA_574_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 11 3.4 Rewind Starter 12 3.5 Ignition System 14 3.6 Carburetor 15 3.7 Engine 18 4. Clutch 19 4.1 Clutch Drum 19 4.2 Clutch 19 5. Chain Brake 20 5.1 Checking Operation 20 5.2 Brake Band 20 5.3 Brake Lever 22 5.4 Brake lever on machines with QuickStop Super 24 5.4.1 Adjusting the brake cable 26 5.4.2 Brake cable Removing and Installing 27 5.5 Cam Lever 29 5.6 Pins 30 5.7 Chain Tensioner 31 5.7.1 Quick chain tensioner 31 5.8 Bar Mounting Studs 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 6.4 Shroud 37 6.5 Cylinder / Crankshaft 38 6.6 Crankshaft / Bearings 41 6.7 Piston 42 6.8 Piston Rings 44 7. Ignition System 45 7.1 Ignition Timing 45 7.2 Preseparator 45 7.3 Install new ignition module 45 7.4 Testing the Ignition Module 47 7.5 Spark Plug Boot / Ignition Lead 48 7.6 Flywheel 48 7.7 Short Circuit Wire 49 7.7.1 Testing 49 7.7.2 Removing and Installing 50 7.7.3 Ground Wire 52 7.7.4 Contact Spring 52 7.8 Ignition System Troubleshooting 54 8. Rewind Starter 57 8.1 General 57 8.2 Fan housing 57 8.3 Pawls 58 8.4 ErgoStart 59 8.5 Rope Rotor 60 8.6 Starter Rope / Grip 60 8.7 Tensioning the Rewind Spring 62 8.8 Replacing the Rewind Spring 62 9. Servicing the AV System 65 9.1 Annular Buffer and AV Spring on Oil Tank 65 9.2 AV Spring on Fuel Tank 65 9.3 AV Spring on Handlebar 66 9.4 Stop Buffer at Clutch Side 67 9.4.1 Stop Buffer at Ignition Side 67 9.4.2 Buffers on Filter Base 68 9.5 Install new handlebar 68 10. Control Levers 70 10.1 Master Control Lever 70 10.1.1 Removing and Installing 70 10.2 Throttle Trigger/ Lockout Lever 70 10.3 Throttle Trigger/ Lockout Lever – QuickStop Super 72 10.3.1 Switch Lever – QuickStop Super 73 10.3.2 Lockout Lever – QuickStop Super 74 10.3.3 Choke Rod 75 10.3.4 Throttle Rod 75 11. Chain Lubrication 77 11.1 Pickup Body 77 11.2 Oil Suction Hose 77 11.3 Oil Pump 77 11.4 Valve 79 Contents

2 MS 271 C, MS 291 C Contents 12. Fuel System 80 12.1 Air Filter 80 12.2 Baffle 80 12.3 Filter Base 80 12.4 Air Guide Shroud 82 12.4.1 Air Guide Shroud – Models with Manual Fuel Pump 82 12.5 Carburetor 84 12.5.1 Leakage Test 85 12.6 Servicing the Carburetor 86 12.6.1 Metering Diaphragm 86 12.6.2 Inlet Needle 86 12.6.3 Pump Diaphragm 87 12.6.4 Levers on Throttle Shaft 88 12.6.5 Adjusting Screws 89 12.7 Adjusting the Carburetor 90 12.7.1 Basic Setting 90 12.7.2 Standard setting 92 12.8 Carburetor Carrier 92 12.9 Intake Manifold 94 12.10 Tank Vent 95 12.10.1 Testing 95 12.10.2 Removing and Installing 95 12.11 Fuel Intake 96 12.11.1 Pickup Body 96 12.11.2 Fuel Hose 96 12.11.3 Fuel Hose – Models with Manual Fuel Pump 99 12.11.4 Manual Fuel Pump 102 12.11.5 Tank Housing 103 13. Special Servicing Tools 106 14. Servicing Aids 108

3 MS 271 C, MS 291 C 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 3101. 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. Engage the bar mounting studs in the outer bores in the mounting plate and secure the machine in position with the M 8 nuts (arrows). 2710RA320 TG 2 3 1 2710RA000 TG

4 MS 271 C, MS 291 C Machines with Quick Chain Tensioner Engage the bar mounting stud in the upper bore in the mounting plate and secure the machine in position with the M 8 nut (arrow). The machine is held in position on the mounting plate by the screw heads on the engine housing. 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. 2710RA001 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. 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. Exercise extreme caution while carrying out maintenance and repair work on the ignition system. The high voltages which occur can cause serious or fatal accidents. 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. 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 271 C, MS 291 C 2. Specifications 2.1 Engine MS 271 MS 291 Displacement: 50.2 cm 3 55.5 cm 3 Bore: 44.7 mm 47.0 mm Stroke: 32.0 mm 32.0 mm Engine power to ISO 7293: 2.6 kW (3.5 bhp) at 9,500 rpm 2.8 kW (3.8 bhp) at 9,500 rpm Maximum permissible engine speed with 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: 3,600 rpm 3,600 rpm Crankcase 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 (+ 0.1/- 0.05) mm Spark plug (resistor type): NGK BPMR 7 A Electrode gap: 0.5 mm 2.4 Chain Lubrication Speed-controlled oil pump with reciprocating piston Oil delivery rate: 8.0 (+/2.0) cm 3 /min at 10,000 rpm

6 MS 271 C, MS 291 C 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 4x12 Cover plate/chain sprocket cover 2.5 B Screw P 4x12 Chain tensioner cover/engine housing 1.5 Screw P 4x14 Antivibration spring/engine housing 2.0 Screw P 5x16 Antivibration spring/tank housing 3.0 Screw P 4x12 Brake band/engine housing 2.0 Screw P 4x10 Brake cable retainer/ tank housing 2.0 Q Collar screw M 8 Collar screw/collar bushing 30.0 1) Collar screw M 8/M 10 Collar stud for bar / engine housing 16.0 3) Collar screw M 8/ D 8 Collar screw/engine housing 16.0 B Collar screw M 8/ D 9 Collar stud for bar / engine housing (repair solution) 16.0 Screw P 4x12 Cover, chain brake / engine housing 1.5 Screw M 4x9.6 Spark arresting screen/muffler 1.0 Screw P 6x32.5 Handlebar/tank housing 7.0 Screw P 5x20 Hand guard/fan housing/engine housing 4.0 Screw M 5x20 Hand guard/fan housing/engine housing 8.0 Screw M 5x14 Shroud / engine housing 5.0 Screw P 6x28 Chain catcher/engine housing/bearing plug 6.0 Screw P 5x20 Spiked bumper / engine housing, top 4.0 Screw P 5x20 Spiked bumper / engine housing, bottom 4.0 Screw M 4x12 Manifold/cylinder 3.0 2) Screw D 5x18 Bearing plug/cylinder 9.0 2) Screw P 5x20 Fan housing / engine housing 4.0 Screw P 4x12 Air baffle / engine housing 2.0 Carrier M 12x1 L Carrier / crankshaft 50.0 Screw D 4x18 Oil pump/engine pan 4.0 2) Screw D 5x18 Muffler / cylinder 9.0 2) Nut M 8x1 Flywheel/crankshaft 28.0 4) Screw P 4x12 Side plate/engine housing 2.0 Collar nut M 5 Carburetor / collar screw, 1st stage, fan side 2.0 Collar nut M 5 Carburetor / collar screw, 2nd stage, sprocket side 3.5 Collar nut M 5 Carburetor / collar screw, 3rd stage, fan side 3.5

7 MS 271 C, MS 291 C Fastener Thread size For component Torque Nm Remarks Screw P 5x16 Pre-separator / engine housing 3.0 M 14x1.25 Spark plug/cylinder 25.0 Screw D 4x18 Ignition module/engine pan 4.0 2) Screw D 6x50 Cylinder/engine pan 11.0 Screw D 6x35 Cylinder/engine pan 11.0 Remarks: 1) Loctite 270, high strength 2) Screws with binding head 3) Micro-encapsulated screws 4) Degrease crankshaft/flywheel and mount oil-free Q) QuickStop Super B) Quick chain adjuster 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. Coat micro-encapsulated screws with medium strength Loctite 242 or 243 before reinstalling. 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 271 C, MS 291 C 3. Troubleshooting 3.1 Clutch Condition Cause Remedy Saw chain stops under full load 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 idle speed screw LA Clutch springs stretched Replace the clutch springs or install new clutch Clutch springs broken Replace the clutch springs Loud noises Clutch springs stretched Replace all clutch springs 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 271 C, MS 291 C 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 Saw chain stops under full load 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 idle speed screw LA Clutch springs stretched Replace the clutch springs or install new clutch Clutch springs 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 271, MS 291 Brushcutters Service Repair Manual INSTANT

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