STIHL MS 391 Service Manual

The STIHL MS 391 is a professional-grade chainsaw featuring a powerful 64.1 cc engine with STIHL anti-vibration system, Quickstop® inertia chain brake, Ematic™ lubrication system, side-access chain tensioner, and decompression valve for easy starting. This manual provides essential information for maintenance, troubleshooting, repair, and safe operation. Below are key sections covering safety, specifications, maintenance procedures, fuel/oil mixture, carburetor adjustment, and common repairs.

Safety Instructions & Symbols

Critical safety warnings to prevent injury. Understand all symbols on the saw.

Symbol	Meaning	Action Required
⚠️	General Warning	Read manual before operation
🔥	Fire Hazard	Keep away from flammable materials
⛔	Do Not Operate	Do not use if damaged or leaking
👂	Hearing Protection	Wear ear defenders
🛡️	Chain Brake	Test before each use

WARNING! Always wear protective equipment: helmet, goggles, gloves, chainsaw chaps, and steel-toe boots. Never operate alone.

Specifications & Technical Data

• Engine Displacement: 64.1 cc
• Power Output: 3.6 kW (4.9 hp)
• Fuel Tank Capacity: 750 ml
• Oil Tank Capacity: 330 ml
• Guide Bar Lengths: 16" to 25" (40 cm to 63 cm)
• Chain Pitch: 3/8"
• Chain Gauge: 0.050" (1.3 mm)
• Weight (powerhead): 6.2 kg (13.7 lbs)
• Ignition System: Electronic, capacitor discharge
• Idle Speed: 2,800 rpm
• Maximum Power Speed: 13,000 rpm

Maintenance Schedule

Regular maintenance ensures optimal performance and longevity.

1. Before Each Use: Check chain tension, sharpness, and lubrication. Inspect for loose fasteners and damage. Test chain brake and throttle trigger.
2. After Each Use: Clean air filter, cooling fins, and guide bar groove. Wipe down saw.
3. Every 10 Hours: Clean spark arrestor screen (muffler). Check fuel filter and spark plug.
4. Every 20 Hours: Replace air filter. Inspect clutch, sprocket, and anti-vibration elements.
5. Every 50 Hours: Replace spark plug. Clean carburetor air passage. Check all gaskets and seals.
6. Annually/Seasonally: Replace fuel filter and line if cracked. Pressure test crankcase for leaks.

CAUTION! Always disconnect spark plug wire before performing maintenance.

Fuel & Lubrication System

Proper fuel/oil mixture and chain oil are critical.

Fuel Mix: Use 89+ octane unleaded gasoline mixed with STIHL HP Ultra or equivalent ISO-L-EGD certified 2-stroke oil at a 50:1 ratio (2% oil). Example: 5 liters gasoline + 100 ml oil.

Chain Oil: Use STIHL chain oil or a biodegradable alternative with good adhesion properties. Check oiler output daily.

1. Fill fuel tank with correct mixture.
2. Fill oil tank with approved chain oil.
3. Adjust oiler (located near throttle) for desired output (more for hardwoods, less for softwoods).
4. To test oiler: Run saw at high idle over light-colored surface; oil line should appear on ground.

WARNING! Never use automotive oil, diesel, or ethanol blends above E10. Store fuel mix for max 30 days.

Engine & Carburetor Service

Carburetor adjustment procedure for optimal performance.

Adjustment Screws (Walbro or Zama carburetor):

• L: Low-speed mixture (affects idle and acceleration)
• H: High-speed mixture (affects top-end power)
• LA or T: Idle speed screw

1. Start engine and warm up for 5 minutes.
2. Set idle speed (LA/T): Turn clockwise to increase, counterclockwise to decrease until chain just stops moving.
3. Adjust L screw: Turn clockwise until engine runs smoothly at idle, then 1/4 turn counterclockwise.
4. Adjust H screw: Only with tachometer. At full throttle, adjust to achieve 13,000 rpm ± 200 rpm. Do not lean out (over-rev).
5. Re-check idle and acceleration. Chain must not move at idle.

CAUTION! Incorrect H adjustment can cause engine seizure. Seek professional help if unsure.

Ignition System

Testing and replacing ignition components.

Component	Test Procedure	Specification
Spark Plug (BR7EC)	Check gap (0.5 mm / 0.020"), clean electrode	Replace if worn or fouled
Ignition Coil	Measure primary resistance (0.2-0.4 Ω) and secondary (8-12 kΩ)	Replace if out of spec
Stop Switch	Check continuity; should be open when OFF	Replace if shorted
Flywheel & Magneto	Inspect for damage, clean magnets, check key	Air gap: 0.3 mm (0.012")

Note: Use only STIHL-approved replacement parts.

Chain & Bar Maintenance

Proper chain care ensures cutting efficiency and safety.

1. Sharpening: Use 4.0 mm (5/32") round file at 30° angle. Maintain consistent depth gauge height (0.65 mm / 0.025") with flat file.
2. Tensioning: Loosen bar nuts, adjust side tensioner until chain snaps back when pulled from bar. Tighten nuts evenly.
3. Bar Maintenance: Flip bar weekly for even wear. Clean bar groove and oil holes. Dress rails with flat file if burred.
4. Sprocket: Replace rim sprocket or drum when worn (teeth hooked). Lubricate bearing.

Tip: A sharp chain reduces kickback risk and operator fatigue.

Cooling & Exhaust System

Prevent overheating for engine longevity.

• Cooling Fins: Clean weekly with brush and compressed air. Blocked fins cause overheating.
• Muffler/Spark Arrestor: Remove screen every 10 hours, burn off carbon with torch, or replace if damaged.
• Cylinder & Piston: Inspect through exhaust port for scoring. Light vertical lines are normal; deep scratches indicate seizure.

WARNING! Muffler gets extremely hot. Allow to cool before servicing.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine won't start	No spark, flooded, fuel issue	Check spark, dry plug, fresh fuel, decomp valve
Engine starts then dies	Clogged fuel filter, carb adjustment	Replace filter, adjust L screw, check tank vent
Loss of power	Dirty air filter, clogged spark screen, bad fuel	Clean/replace filter, clean muffler, drain fuel
Chain won't oil	Clogged oiler, wrong oil, pump failure	Clean oil ports, use correct oil, replace pump
Excessive vibration	Loose parts, worn AV mounts, bent crankshaft	Tighten fasteners, replace mounts, inspect crank
Chain rotates at idle	Idle speed too high, clutch springs worn	Adjust LA screw, replace clutch springs

Reset: For electronic issues, disconnect battery (if equipped) for 10 minutes.

Torque Specifications

Critical fastener torque values (use calibrated torque wrench).

• Cylinder Head Nuts: 20 Nm (15 ft-lbs)
• Spark Plug: 25 Nm (18 ft-lbs)
• Clutch: 40 Nm (30 ft-lbs) – left-hand thread
• Flywheel Nut: 50 Nm (37 ft-lbs)
• Guide Bar Nuts: 10 Nm (7.5 ft-lbs)
• Muffler Bolts: 12 Nm (9 ft-lbs)
• Anti-Vibration Mounts: 8 Nm (6 ft-lbs)

CAUTION! Over-torquing can strip threads; under-torquing can cause failure.

Exploded Views & Parts List

Refer to official STIHL parts catalog (MS 391) for detailed diagrams. Common replacement part numbers:

Description	STIHL Part Number
Air Filter	0000 120 1620
Spark Plug	0000 400 1300 (BR7EC)
Fuel Filter	0000 350 3501
Carburetor Kit	0000 120 0650 (Walbro)
Clutch Drum Sprocket	0000 642 2003 (Rim)
Chain Brake Band	0000 957 0801
Ignition Module	0000 400 1305

Service: Authorized STIHL dealers only. For support, visit www.stihlusa.com or contact 1-800-467-8445.