Owners Manual on MS391 Stihl Chain Saw

The STIHL MS391 is a professional-grade, mid-range chainsaw featuring a powerful 64.1cc engine with STIHL's 2-MIX technology for reduced fuel consumption and emissions. It includes an anti-vibration system, Quickstop® inertia chain brake, side-access chain tensioner, decompression valve for easier starting, and a tool-free fuel and oil cap design. This manual covers safety, assembly, operation, maintenance, and troubleshooting for the MS391 chainsaw.

Features at a Glance

Key components: 64.1cc 2-MIX engine, anti-vibration system, Quickstop® inertia chain brake, side-access chain tensioner, decompression valve, tool-free fuel and oil caps, and STIHL Ematic™ lubrication system.

Safety Information

Read and understand all safety warnings before operation. Always wear appropriate PPE.

1. Personal Protective Equipment (PPE): Wear safety glasses, hearing protection, gloves, chainsaw chaps/pants, and sturdy boots.
2. Pre-Operation Check: Inspect saw for damage, ensure chain is sharp and properly tensioned, check all controls function correctly.
3. Work Area Safety: Clear area of obstacles and bystanders. Ensure secure footing. Never operate on a ladder or unstable surface.
4. Kickback Awareness: Understand kickback zone (upper quadrant of bar tip). Use reduced-kickback chain and maintain a firm grip.

WARNING! Failure to follow safety instructions can result in serious injury or death. The chain brake is a safety device, not a substitute for safe operating practices.

Assembly & Preparation

Assemble the chainsaw from the box and prepare for first use.

1. Attach Guide Bar and Chain: Remove bar cover. Fit guide bar onto studs, ensuring chain drive links engage with sprocket. Place chain over bar, ensuring cutting teeth face forward on top. Install bar nut and tensioning screw cover.
2. Add Fuel and Oil: Use fresh, high-quality unleaded gasoline (min. 89 octane) mixed with STIHL HP Ultra 2-cycle oil at a 50:1 ratio. Fill chain oil reservoir with STIHL chain oil or equivalent.
3. Check Chain Tension: Chain should be snug against the bar but pull freely by hand. Adjust using side tensioner. Re-check when cold chain warms up.

CAUTION! Fuel is highly flammable. Mix and fill in a well-ventilated area away from sparks or flames. Never smoke during refueling.

Specifications

• Engine Displacement: 64.1 cc (3.9 cu. in.)
• Power Output: 4.0 bhp (3.0 kW)
• Fuel/Oil Ratio: 50:1 (STIHL HP Ultra oil)
• Guide Bar Lengths (Recommended): 16", 18", 20"
• Chain Pitch/Gauge: .325" / .063" (typically)
• Dry Weight (w/o bar & chain): Approx. 13.2 lbs (6.0 kg)
• Sound Power Level: 112 dB(A)
• Vibration Levels (Front/Rear Handle): 4.5/2.5 m/s²
• Ignition System: Electronic, capacitor discharge
• Lubrication System: STIHL Ematic™ with adjustable oiler

Controls and Components

Familiarize yourself with all controls before starting the engine.

Main Controls: - Master Control Lever: Combines stop switch, choke, and start positions. - Throttle Trigger & Interlock: Must depress interlock to activate throttle. - Throttle Trigger Lockout: Prevents accidental throttle engagement. - Chain Brake Lever (Front Hand Guard): Manual activation. Inertia-activated during kickback. - Decompression Valve Button: Reduces cylinder pressure for starting. - Fuel Pump (Primer Bulb): Press several times to fill carburetor with fuel for cold starts. - Adjustable Automatic Oiler: Dial to control chain oil flow rate.

Maintenance Points: Air filter cover, spark plug, chain tensioner, muffler, clutch cover.

Starting & Stopping Procedure

Cold Start:

1. Place saw on firm ground. Engage chain brake.
2. Press decompression valve (if equipped).
3. Set Master Control Lever to choke position (full down).
4. Press fuel primer bulb 5-10 times until fuel is visible.
5. Pull starter handle sharply until engine 'fires' (usually 2-5 pulls).
6. Immediately move Master Control Lever to half-throttle position (mid-notch).
7. Pull starter handle again until engine starts. Let run briefly.
8. Squeeze throttle trigger to disengage fast-idle and return to normal idle.
9. Disengage chain brake before cutting.

Warm Start: Master Control Lever in half-throttle position. No choke or primer needed.

Stopping: Release throttle, allow engine to idle. Move Master Control Lever up to STOP position.

Operating Instructions

Proper technique is critical for safety and efficiency.

1. Grip: Hold saw firmly with both hands, right on rear handle/trigger, left on front handle. Thumbs wrapped underneath.
2. Stance: Stand with feet shoulder-width apart, left foot slightly forward for balance.
3. Starting a Cut: Let the chain reach full speed before contacting wood. Use the bumper spike (dogs) to pivot into the wood.
4. Basic Cutting: Use full throttle during the cut. Let the saw do the work; avoid forcing it. Beware of pinch points when cutting logs under tension.
5. Kickback Prevention: Never cut with the upper tip of the bar. Be especially cautious during boring cuts and re-entering a previous cut.
6. Refueling: Stop engine and allow to cool. Clean area around caps. Refuel and oil outdoors.

WARNING! Always plan an escape path from falling wood. Never operate the saw above shoulder height or with one hand.

Maintenance Schedule

Note: Service intervals vary with operating conditions. Heavy use requires more frequent maintenance.

Chain Sharpening & Tensioning

A sharp, properly tensioned chain is essential for safety and performance.

Sharpening: 1. Secure bar in a vise. Identify cutters that are shorter (the "guide" cutters). File all others to match this length. 2. Use correct round file size (.325" chain typically uses a 4.0mm or 5/32" file). Hold file at specified angle (usually 30°). 3. File from inside cutter to outside with smooth strokes. File all cutters on one side, then turn saw around and file the other side. 4. File depth gauges (rakers) with a flat file and gauge to maintain proper height difference (approx. 0.025").

Tensioning: 1. Loosen bar nuts. Ensure chain brake is disengaged. 2. Using the side tensioning screw, adjust until chain is snug against the bar but can still be pulled around by hand. 3. Tighten bar nuts while lifting the tip of the bar. Re-check tension. A cold chain will require re-tensioning after a few minutes of operation.

Storage Guidelines

Proper storage prevents fuel system issues and corrosion.

1. Short-Term (under 30 days): Run carburetor dry or add fuel stabilizer. Store in a dry, secure location.
2. Long-Term: a. Drain all fuel from tank and run engine until it stops to clear carburetor. b. Drain chain oil reservoir. c. Remove, clean, and oil the chain. Store separately. d. Clean entire saw, especially cooling fins and air filter area. e. Remove spark plug, add a teaspoon of engine oil into cylinder, pull starter slowly to coat interior, reinstall plug. f. Store in a dry, dust-free place, covered.

CAUTION! Never store a chainsaw with fuel in the tank for extended periods. Old fuel can gum up the carburetor.

Troubleshooting

For Service: Contact an authorized STIHL servicing dealer for repairs beyond basic maintenance. Use only genuine STIHL replacement parts.

STIHL Customer Service: 1-800-467-8445 | www.stihlusa.com