MS 391 Stihl Chainsaw Manual

The Stihl MS 391 is a professional-grade chainsaw featuring a powerful 64.1 cc engine, STIHL Quickstop® inertia chain brake, Ematic™ lubrication system, and anti-vibration system. Designed for demanding cutting tasks, it includes features like decompression valve for easy starting, side-access chain tensioner, and a tool-free air filter cover. Below are key sections for safety, features, assembly, operation, maintenance, and troubleshooting.

Features at a Glance

Key components: 64.1 cc engine, STIHL Quickstop® inertia chain brake, Ematic™ lubrication system, anti-vibration handles, side-access chain tensioner, decompression valve.

Assembly & Preparation

Unpack and inspect all parts. Ensure you have proper personal protective equipment (PPE).

1. Attach guide bar and chain: Place bar on studs, loop chain around bar with cutters facing forward, tension chain using side-access knob.
2. Fill fuel tank: Use STIHL MotoMix® or mix 50:1 with 89+ octane gasoline and STIHL HP Ultra oil.
3. Fill chain oil tank: Use STIHL chain oil or approved substitute.
4. Check chain tension: Chain should snap back when pulled from guide bar.

WARNING! Always wear chainsaw chaps, gloves, eye and ear protection, and helmet with face shield. Never operate without proper PPE.

First-Time Operation

Perform pre-start checks before each use.

1. Check chain brake: Ensure brake is disengaged (handle pulled back).
2. Prime fuel system: Press fuel pump bulb 5-10 times until fuel is visible.
3. Set master control lever: Move to cold start position (choke).
4. Start engine: Place on ground, hold front handle with left hand, step on rear handle, pull starter rope briskly.
5. Warm up: After starting, move master control to fast idle, let run for 30 seconds.

CAUTION! Clear area of bystanders, especially children and pets. Maintain secure footing.

Specifications

• Engine: 64.1 cc single-cylinder 2-stroke.
• Power Output: 4.0 bhp (3.0 kW).
• Fuel System: STIHL diaphragm carburetor.
• Ignition: Electronic, adjustable.
• Chain Pitch: 3/8" or .325".
• Chain Gauge: 0.050" (1.3 mm).
• Sound Pressure: Typically 106 dB(A).
• Vibration Levels: Front/rear handles meet ISO standards.
• Warranty: Limited warranty for personal/commercial use.

Controls and Components

Master Control Lever: Combines choke, throttle lock, stop switch. Throttle Trigger: Controls engine speed. Throttle Lockout: Must be depressed to engage throttle. Chain Brake Handle: Front hand guard activates brake. Decompression Valve: Button to reduce cylinder pressure. Side-Access Chain Tensioner: Knob for adjusting chain tension.

Maintenance Points: Air filter cover, spark plug, fuel and oil caps, muffler.

Fuel & Lubrication

Use fresh 89+ octane gasoline mixed 50:1 with STIHL HP Ultra oil, or STIHL MotoMix® premixed fuel.

1. Fuel Mixing: Pour oil into approved container, add gasoline, shake well.
2. Filling: Clean area around caps, fill tanks to 80% capacity.
3. Chain Oil: Use STIHL chain oil for optimal bar/chain life.
4. Check Oil Flow: Run saw at high idle over light surface; oil line should appear.

Tip: Never use fuel older than 30 days. Store fuel in approved containers away from heat sources.

Chain & Bar Maintenance

Regular maintenance ensures optimal performance and safety.

1. Chain Sharpening: Use round file matching chain pitch (typically 5/32" or 7/32"). Maintain 30° filing angle.
2. Depth Gauge Setting: Use flat file to maintain 0.025" (0.65 mm) below cutter top plate.
3. Bar Maintenance: Clean bar groove, flip bar regularly for even wear.
4. Tension Adjustment: Chain should be snug against bar but pull freely.
5. Inspection: Check for damaged/worn cutters, cracked rivets, bar burrs.

WARNING! Always wear cut-resistant gloves when handling chain. Ensure engine is off and cool.

Safety Features

STIHL Quickstop®: Inertia-activated chain brake stops chain within milliseconds if kickback occurs. Throttle Lockout: Prevents accidental throttle engagement. Chain Catcher: Helps contain broken chain. Right-Hand Guard: Protects from rear kickback. Muffler: Deflects exhaust away from operator.

Care & Storage

Clean saw after each use. For storage over 30 days: Drain fuel tank, run engine until it stops, remove spark plug, add 1 tsp oil through spark plug hole, pull starter 2-3 times, reinstall plug. Store in dry, secure location.

CAUTION! Never store with fuel in tank for extended periods. Keep away from open flames and sparks.

Regulatory Information

EPA/CARB compliant. Meets ANSI B175.1 safety standards. Contains parts that may cause cancer (California Prop 65). Manufactured by STIHL Incorporated. For support, visit www.stihlusa.com or contact 1-800-467-8445.

Troubleshooting

Service: For repairs beyond basic maintenance, contact authorized STIHL dealer.

Emergency: 1-800-467-8445; www.stihlusa.com