STIHL MS 391 Manual

The STIHL MS 391 is a professional-grade chainsaw featuring a 64.1 cc engine, STIHL Quickstop® inertia chain brake, Ematic™ lubrication system, side-access chain tensioner, decompression valve, and anti-vibration system. It is designed for demanding cutting tasks including felling, limbing, and bucking. Below are key sections for safety, features, assembly, operation, maintenance, and troubleshooting.

Features at a Glance

Key components: Engine with decompression valve, Quickstop® inertia chain brake, side-access chain tensioner, Ematic™ lubrication system, anti-vibration handles, and fuel/oil caps with retaining straps.

Feature	Description
Engine displacement	64.1 cc, 2-stroke engine
Guide bar lengths	Compatible with 16" to 25" bars (varies by model)
Chain brake	STIHL Quickstop® inertia-activated for safety
Chain lubrication	Ematic™ system for reduced oil consumption
Anti-vibration system	Reduces operator fatigue
Decompression valve	Easier starting
Side-access chain tensioner	Allows adjustment without removing side cover
Fuel and oil caps	Equipped with retaining straps
Muffler	Spark-arresting screen (meets USDA Forest Service standards)
Throttle trigger interlock	Prevents accidental throttle activation

Assembly & Preparation

Unpack and inspect contents. Assemble on a clean, stable surface.

1. Attach guide bar and chain: Place bar on studs, fit chain with cutters facing forward, tension correctly.
2. Fill fuel tank: Use STIHL MotoMix® or mix 50:1 with 89+ octane gasoline and STIHL HP Ultra oil.
3. Fill oil tank: Use STIHL chain oil or equivalent.
4. Check chain lubrication: Manually press oil pump primer until oil appears at bar tip.

WARNING! Always wear appropriate PPE: safety glasses, hearing protection, gloves, chainsaw chaps, and sturdy boots. Never operate without proper training.

First-Time Operation

Perform pre-start checks before each use.

1. Check chain tension: Chain should be snug but pull freely around bar.
2. Check chain sharpness: Dull chains increase kickback risk and reduce efficiency.
3. Start the saw: Place on ground, engage chain brake, set master control lever to cold start position, pull starter rope until engine fires, then set to run position and restart.
4. Warm-up: Allow engine to idle for 30 seconds before cutting.

CAUTION! Ensure the chain brake is engaged when starting. Keep bystanders at least 15 feet away.

Specifications

• Engine Displacement: 64.1 cc.
• Power Output: 4.0 bhp (2.9 kW).
• Fuel Tank Capacity: 23.3 fl. oz. (0.69 L).
• Oil Tank Capacity: 11.8 fl. oz. (0.35 L).
• Weight (powerhead): 13.9 lbs (6.3 kg).
• Sound Pressure Level: 106 dB(A).
• Vibration Levels: Left/Right 4.5 m/s², Front/Rear 2.5 m/s².
• Ignition System: STIHL Electronic Ignition (CDI).
• Recommended Bar Lengths: 16", 18", 20", 25".
• Chain Pitch/Gauge: 3/8" PM / .050" (1.3 mm).

Controls and Functions

Master Control Lever: Combines choke, throttle lock, and stop functions. Front Handle: Contains throttle trigger with interlock. Rear Handle: Contains master control lever and starter grip.

Key Controls: Master Control Lever (Stop/Choke/Run), Throttle Trigger, Throttle Trigger Interlock, Chain Brake Lever, Decompression Valve Button, Chain Tensioner, Primer Bulb (if equipped).

Operation: Set master control to cold start (choke) for cold engine, to run position for warm start or operation. Engage chain brake by pushing lever forward; disengage by pulling back toward rear handle.

Safe Operation Procedures

Always follow safe cutting practices to avoid kickback and injury.

1. Maintain secure footing and balance.
2. Hold saw firmly with both hands, thumbs wrapped around handles.
3. Do not cut above shoulder height.
4. Be aware of kickback zone (upper quadrant of bar tip).
5. Use proper felling and limbing techniques.
6. Shut off engine or engage chain brake when carrying saw more than a few steps.

Tip: Plan your cut and escape path before starting. Never operate when tired or under the influence.

Maintenance Schedule

Regular maintenance ensures optimal performance and longevity.

1. Daily/Before Use: Check chain tension and sharpness, check fuel/oil levels, inspect for loose fasteners, clean air filter if dirty.
2. Weekly/Every 10 Hours: Clean cooling fins, check spark plug, clean spark arrestor screen.
3. Monthly/Every 20 Hours: Replace air filter, check fuel filter, inspect anti-vibration elements.
4. Annually/End of Season: Thorough cleaning, inspect all components, replace worn parts (sprocket, bar, chain).

WARNING! Always disconnect spark plug wire before performing maintenance. Use only STIHL-approved parts and lubricants.

Chain & Bar Care

Proper care extends the life of the cutting system.

• Sharpen chain regularly with correct file size and guide.
• Rotate bar periodically to ensure even wear.
• Clean bar groove and oil holes.
• Dress bar rails with a flat file if burred.
• Ensure proper chain tension – a loose chain can derail; an overtightened chain can damage the engine.

CAUTION! Always wear heavy gloves when handling the chain. The cutters are extremely sharp.

Storage Guidelines

Prepare the saw for storage to prevent fuel system issues.

1. Drain fuel tank and run engine until it stops to clear carburetor (for storage over 30 days).
2. Remove chain, clean it and the bar, and coat with oil to prevent rust.
3. Clean entire saw, especially cooling fins and air filter area.
4. Store in a dry, secure place, out of reach of children.

CAUTION! Never store a chainsaw with fuel in the tank for extended periods. Use fuel stabilizer if short-term storage is necessary.

Regulatory Information

This chainsaw meets applicable ANSI/OPEI standards. It is equipped with a spark-arresting muffler certified to USDA Forest Service 5100-1c. The STIHL M-Tronic™ engine management system (on some models) optimizes performance. Copyright and trademarks owned by STIHL Incorporated. For genuine parts, accessories, and service, contact an authorized STIHL servicing dealer. For support, visit www.stihlusa.com or call 1-800-467-8445.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	Flooded, no spark, stale fuel	Set master control to run, hold throttle open and pull starter; check spark plug; use fresh fuel mix.
Engine starts then dies	Clogged fuel filter, dirty air filter	Replace fuel filter; clean or replace air filter.
Chain does not turn	Chain brake engaged, clutch worn, sprocket damaged	Disengage chain brake; inspect clutch and sprocket; replace if necessary.
Excessive vibration	Loose components, damaged anti-vibration mounts, unbalanced chain	Tighten all fasteners; inspect mounts; sharpen chain evenly.
Poor cutting performance	Dull chain, improper filing angle, low chain speed	Sharpen chain correctly; check engine RPM; ensure chain is properly tensioned.
Chain throws oil	Overfilled oil tank, incorrect oil viscosity	Check oil level; use recommended STIHL chain oil.

Service: For repairs beyond basic maintenance, take your STIHL MS 391 to an authorized STIHL servicing dealer.

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