MS 391 Stihl Chainsaw Manual

The Stihl MS 391 is a professional-grade gasoline-powered chainsaw featuring a 64.1 cc engine, STIHL Quickstop® inertia chain brake, anti-vibration system, decompression valve, side-access chain tensioner, and electronic ignition. This manual covers essential safety procedures, features, assembly, operation, maintenance, troubleshooting, and specifications for the MS 391 model. Always read and understand the manual before use.

1. Features at a Glance 2. Safety Information 3. Assembly & Preparation 4. Specifications 5. Controls and Components 6. Starting & Stopping 7. Operation & Cutting Techniques 8. Maintenance Schedule 9. Chain Sharpening & Tensioning 10. Storage 11. Troubleshooting

Features at a Glance

Key components: 64.1 cc STIHL engine, guide bar, saw chain, master control lever, chain brake, anti-vibration system, fuel and oil caps, decompression valve, side-access chain tensioner.

Feature	Description
Engine Displacement	64.1 cc, 2-stroke gasoline engine
Guide Bar Lengths	Typically 16" to 25" (40 cm to 63 cm)
Chain Brake	STIHL Quickstop® inertia-activated chain brake
Anti-Vibration System	Reduces operator fatigue
Decompression Valve	Easier starting
Chain Tensioning	Side-access for easy adjustment without tools
Ignition System	Electronic ignition (CDI)
Fuel Tank Capacity	Approx. 0.71 US pt (0.34 l)
Oil Tank Capacity	Approx. 0.42 US pt (0.20 l)
Weight (powerhead)	Approx. 13.9 lbs (6.3 kg)

Safety Information

Always wear proper PPE: helmet, eye protection, hearing protection, gloves, chainsaw chaps, and sturdy boots.

Clear work area of obstacles and bystanders.
Inspect chainsaw before each use: chain sharpness, tension, and all controls.
Never operate with a loose, damaged, or dull chain.
Always hold chainsaw firmly with both hands.
Be aware of kickback zone; do not let bar nose contact objects.

WARNING! Gasoline is highly flammable. Refuel in a well-ventilated area with engine off and cool. Do not smoke.

Assembly & Preparation

Unpack and inspect all parts. Assemble guide bar and chain.

Attach guide bar: Place bar on studs, align chain drive links with bar sprocket.
Install chain: Ensure cutting teeth face forward on top of bar.
Tension chain: Use side-access wheel to adjust until snug against bar (lift slightly in middle).
Fill fuel: Use STIHL 50:1 2-cycle engine oil mixed with unleaded gasoline (min. 89 octane).
Fill bar & chain oil: Use STIHL chain oil or equivalent.

CAUTION! Ensure chain brake is disengaged (pulled back toward rear handle) before starting.

Specifications

Engine: 64.1 cc single-cylinder 2-stroke.
Power Output: 4.0 bhp (3.0 kW).
Fuel/Oil Ratio: 50:1 (STIHL 2-cycle oil).
Idle Speed: 2,800 rpm.
Maximum Power Speed: 13,000 rpm.
Sound Pressure Level: Approx. 106 dB(A).
Vibration Level (front/rear): Approx. 4.5/2.5 m/s².
Ignition: Electronic, air gap 0.30 mm.
Spark Plug: Bosch WSR6F or equivalent, gap 0.020" (0.5 mm).
Warranty: Limited warranty, see dealer for details.

Controls and Components

Master Control Lever: Combines stop switch, choke, and fast idle. Front Handle: With hand guard. Rear Handle: With throttle trigger and throttle trigger lockout. Chain Brake Lever: Manual activation; inertia-activated. Decompression Valve: Reduces cylinder pressure for easier starting. Side-Access Chain Tensioner: Wheel for adjusting chain tension.

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

Starting & Stopping

Cold Start Procedure:

Engage chain brake (push forward).
Set master control lever to choke position (full down).
Press decompression valve (if equipped).
Place saw on firm ground, hold rear handle with right foot.
Pull starter rope sharply until engine fires (usually 1-3 pulls).
Move master control lever to half-throttle position (mid-notch).
Pull starter rope again until engine starts.
Blip throttle to disengage fast idle; release chain brake.

Stopping: Release throttle, move master control lever up to STOP position.

Operation & Cutting Techniques

Basic Cutting:

Use full throttle when cutting.
Let chain do the work; avoid forcing.
For felling: Plan escape path, make notch cut, then felling cut.
For limbing/bucking: Support log; cut from top side.
Avoid cutting with bar tip to prevent kickback.

Kickback Prevention: Hold saw firmly, maintain proper chain tension and sharpness, use reduced-kickback chain, be aware of bar nose contact.

Maintenance Schedule

Interval	Task
Before each use	Check chain tension, sharpness, all fasteners, controls
Daily	Clean air filter, check fuel and oil systems
Every 3 months	Clean spark arrestor screen (if equipped)
Every 6 months	Replace spark plug, clean cooling fins
Annually	Inspect fuel lines, filters, and anti-vibration elements

Note: Service more frequently under heavy use or dusty conditions.

Chain Sharpening & Tensioning

Sharpening: Use round file matching chain pitch (typically 3/8" or .325"). Maintain 30° filing angle. Keep all cutters same length. Depth gauges must be filed to proper height (use depth gauge tool).

Tensioning: Chain should be snug against bar but pull freely by hand. Adjust when cold. Re-check after first few minutes of operation.

CAUTION! Always wear gloves when handling chain.

Storage

Long-term storage (over 30 days):

Drain fuel tank and run engine until it stops.
Remove chain, clean and oil it.
Clean entire saw, especially cooling fins and air filter.
Store in dry, secure place, away from children.

WARNING! Never store with fuel in tank for extended periods; fuel may degrade and damage engine.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine won't start	No fuel, flooded, spark plug	Check fuel; move control to half-throttle; check/replace spark plug.
Engine runs poorly	Dirty air filter, fuel filter, carb adjustment	Clean/replace air filter; replace fuel filter; see dealer for carb adjustment.
Chain doesn't turn	Chain brake engaged, clutch, sprocket	Disengage chain brake; check clutch drum and sprocket for wear.
Excessive vibration	Loose parts, unbalanced chain	Tighten all fasteners; sharpen chain evenly.
Chain throws oil	Oil output too high	Adjust oiler (if adjustable) or check for clog.

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

Support: STIHL Incorporated, 536 Viking Drive, Virginia Beach, VA 23452. Customer Service: 1-800-467-8445. www.stihlusa.com