STIHL MS 291 Repair Manual

The STIHL MS 291 is a professional-grade chainsaw featuring a powerful engine, STIHL Quickstop® inertia chain brake, Ematic™ lubrication system, and anti-vibration system. This repair manual provides detailed instructions for maintenance, troubleshooting, and repair procedures. Below are key sections covering safety, specifications, disassembly, engine service, fuel system, ignition, lubrication, chain and bar, troubleshooting, and reassembly.

1. Safety Precautions 2. Specifications & Technical Data 3. Tools Required 4. Disassembly Procedure 5. Engine Service 6. Fuel System 7. Ignition System 8. Lubrication System 9. Chain & Bar Assembly 10. Troubleshooting Guide 11. Reassembly & Testing

Safety Precautions

Before performing any repair, ensure the chainsaw is turned off, spark plug disconnected, and all safety guards are in place.

Precaution	Description
Personal Protective Equipment	Wear safety glasses, gloves, and hearing protection.
Work Area	Work in a well-ventilated, clean, and dry area.
Fuel Safety	Drain fuel tank before service; no smoking or open flames.
Chain Brake	Engage chain brake before starting disassembly.
Sharp Edges	Handle cutting components with care to avoid injury.

Specifications & Technical Data

Engine Type: STIHL 2-MIX engine, single cylinder.
Displacement: 55.5 cc.
Power Output: 3.4 kW (4.6 bhp).
Fuel Capacity: 550 ml.
Oil Capacity: 260 ml.
Chain Pitch: 3/8" or .325".
Bar Length: 16" to 20" (recommended).
Weight (powerhead): 6.2 kg.
Ignition: Electronic, capacitor discharge.

Tools Required

Standard and specialized tools needed for repair.

STIHL combination wrench.
Torx screwdrivers (T27, T20).
Spark plug wrench.
Compression tester.
Feeler gauge set.
Carburetor adjustment tool.
Chain breaker and rivet spinner.
Torque wrench.

NOTE: Use genuine STIHL tools for optimal results.

Disassembly Procedure

Step-by-step guide to disassemble the MS 291.

Remove bar and chain: Loosen bar nuts, remove cover, detach chain.
Remove top cover: Unscrew fasteners, lift off cover.
Remove fuel tank: Drain fuel, disconnect lines, remove screws.
Remove carburetor: Disconnect linkages, remove mounting nuts.
Separate crankcase halves: Mark alignment, remove all screws evenly.

WARNING! Keep screws organized; note orientation of components.

Engine Service

Servicing cylinder, piston, crankshaft, and bearings.

Component	Service Procedure
Cylinder & Piston	Inspect for scoring; measure ring end gap; replace if worn.
Crankshaft	Check runout; replace bearings if noisy or loose.
Spark Plug	Gap at 0.5 mm; replace if electrodes are worn.
Muffler	Clean carbon deposits; ensure screen is not clogged.
Compression	Should be above 120 psi; low compression indicates wear.

Fuel System

Carburetor adjustment, fuel lines, filter, and tank.

Carburetor: Adjust L (low) and H (high) screws to factory settings (initial: L-1 turn, H-1 turn).
Fuel filter: Replace if dirty; ensure pickup line is intact.
Fuel lines: Check for cracks; replace if brittle or leaking.
Impulse line: Inspect for leaks; connects crankcase to carburetor.
Tank vent: Clean to allow proper fuel flow.

Tip: Use STIHL MotoMix or fresh fuel with proper oil mix (50:1).

Ignition System

Testing and replacing ignition components.

Spark Plug: NGK BPMR7A, gap 0.5 mm.
Ignition Module: Test with multimeter; resistance specs: primary 0.5-1.5 Ω, secondary 8-15 kΩ.
Stop Switch: Ensure circuit opens when engaged.
Flywheel: Check keyway; tighten nut to 40 Nm.

CAUTION! Disconnect spark plug wire before testing.

Lubrication System

Ematic™ system service and oil pump adjustment.

Oil pump: Remove cover, check gear and drive mechanism.
Oil lines: Inspect for blockages; replace if cracked.
Adjustment: Turn screw clockwise to increase oil flow.
Filter screen: Clean debris from oil tank pickup.

NOTE: Use STIHL chain oil for best performance.

Chain & Bar Assembly

Maintenance, sharpening, and replacement.

Item	Procedure
Chain Sharpening	File at 30° angle; maintain consistent depth gauges.
Bar Inspection	Check for wear, straightness, and groove condition.
Sprocket	Replace if teeth are hooked or worn.
Tensioning	Adjust so chain snaps back when lifted from bar.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine won't start	No spark, fuel issue, flooded	Check spark, fuel lines, carburetor; dry plug if flooded.
Engine runs poorly	Dirty air filter, carb adjustment	Clean filter; adjust L and H screws.
Overheating	Lean mixture, clogged muffler	Enrich fuel mixture; clean muffler screen.
Oil not flowing	Clogged line, pump failure	Clean oil passages; check pump drive.
Excessive vibration	Loose components, bent crankshaft	Tighten mounts; inspect crankshaft runout.

Reassembly & Testing

Final assembly and operational check.

Reassemble in reverse order of disassembly.
Torque fasteners to specification (crankcase screws: 10 Nm).
Refill with fuel and oil; prime carburetor.
Start engine; check idle and high-speed operation.
Test chain brake and oiler function.

Final Check: Ensure all safety features work before use.