STIHL MS 391 Repair Manual

The STIHL MS 391 is a professional-grade chainsaw featuring a powerful engine, advanced anti-vibration system, and STIHL Quickstop® inertia chain brake. This repair manual provides comprehensive instructions for troubleshooting, maintenance, disassembly, and reassembly of the MS 391. Below are key sections covering safety, specifications, components, maintenance procedures, troubleshooting, and parts diagrams.

Safety Precautions

Before performing any repair or maintenance, ensure the chainsaw is turned off, the spark plug is disconnected, and all safety gear is worn.

Safety Item	Description
Personal Protective Equipment	Wear safety glasses, gloves, hearing protection, and chainsaw chaps.
Work Area	Work in a well-ventilated, clean area free of flammable materials.
Fuel Safety	Drain fuel before repairs; no smoking or open flames nearby.
Chain Brake	Verify Quickstop® chain brake functions before and after repairs.
Sharp Tools	Use proper tools; keep hands clear of sharp chain and bar edges.

Specifications

• Engine Type: 2-stroke, single-cylinder, air-cooled.
• Displacement: 64.1 cc.
• Power Output: 4.4 bhp (3.3 kW).
• Fuel Tank Capacity: 0.71 US pt (0.34 L).
• Oil Tank Capacity: 0.42 US pt (0.20 L).
• Chain Pitch: 3/8" or .325".
• Bar Length Options: 16" to 25".
• Weight (powerhead): 13.2 lbs (6.0 kg).
• Ignition System: Electronic, capacitor discharge.
• Carburetor: STIHL HD-12 or Zama C1Q-S199.

Component Identification

Key components: Engine assembly, carburetor, air filter, spark plug, fuel tank, oil tank, chain brake, throttle trigger, master control lever, chain tensioner, bar, and chain.

1. Remove top cover to access air filter and spark plug.
2. Remove side cover to access chain, bar, and tensioner.
3. Remove clutch cover and clutch to access drive sprocket.
4. Identify all fasteners and their locations before disassembly.

WARNING! Label parts and take photos during disassembly to ensure correct reassembly.

Maintenance Schedule

Regular maintenance ensures optimal performance and longevity.

1. Daily: Check chain tension, sharpness, and lubrication; clean air filter.
2. Weekly: Inspect spark plug, fuel filter, and bar for wear.
3. Monthly: Clean cooling fins, check anti-vibration elements, inspect clutch and sprocket.
4. Annually: Replace fuel lines, inspect carburetor diaphragm, check ignition system.

CAUTION! Use only STIHL-approved parts and lubricants.

Engine Repair

Common engine repairs include piston/cylinder replacement, crankshaft bearing service, and decompression valve maintenance.

1. Remove engine cover and disconnect all linkages.
2. Remove cylinder head and inspect piston and cylinder for scoring.
3. Replace piston rings if compression is below 110 psi.
4. Check crankshaft seals for leaks; replace if damaged.
5. Reassemble with new gaskets; torque cylinder nuts to 10 Nm (88 in-lb).

Tip: Use a piston stop tool when removing the clutch or flywheel.

Fuel System

Fuel system components: tank, lines, filter, carburetor, and primer bulb.

1. Drain fuel and remove tank cover.
2. Replace fuel filter and lines if cracked or brittle.
3. Clean carburetor: disassemble, soak in cleaner, replace diaphragm and gaskets.
4. Check fuel cap vent for blockage.
5. Reassemble and test for leaks.

WARNING! Do not use compressed air on carburetor diaphragms.

Ignition System

Components: spark plug, ignition coil, flywheel, and stop switch.

1. Check spark plug gap: 0.020" (0.5 mm).
2. Test ignition coil resistance: primary 0.2-0.3 Ω, secondary 8-10 kΩ.
3. Inspect flywheel key for shearing; replace if damaged.
4. Verify stop switch circuit continuity.
5. Set air gap between coil and flywheel: 0.3 mm (0.012").

Note: Use a spark tester to verify ignition output.

Chain & Bar Assembly

Proper chain maintenance ensures safe and efficient cutting.

1. Remove side cover and loosen chain tensioner.
2. Inspect bar for wear, straightness, and groove condition.
3. Sharpen chain with round file matching chain pitch.
4. Lubricate bar groove and sprocket tip.
5. Reinstall and tension chain so it snaps back when pulled from bar.

CAUTION! Always wear gloves when handling chain.

Carburetor Adjustment

Adjust carburetor for optimal performance at different altitudes and temperatures.

1. Start engine and warm up for 5 minutes.
2. Adjust idle speed screw (LA) so chain does not move.
3. Adjust low-speed screw (L) for smooth acceleration.
4. Adjust high-speed screw (H) for maximum RPM (13,500-14,500).
5. Recheck idle after high-speed adjustment.

WARNING! Excessive high-speed RPM can cause engine damage.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine won't start	No spark, fuel issue, flooded	Check spark, fuel lines, carburetor; clear flood.
Engine runs poorly	Clogged air filter, bad fuel, carb adjustment	Clean/replace filter; use fresh fuel; adjust carb.
Chain doesn't turn	Clutch worn, sprocket damaged, brake engaged	Inspect clutch springs; replace sprocket; disengage brake.
Excessive vibration	AV mounts damaged, unbalanced parts	Replace anti-vibration buffers; check clutch balance.
Oil not lubricating	Clogged oil ports, pump failure	Clean oil passages; test oil pump output.

Reset: After repairs, reset chain brake and check all safety functions.

Service: For technical support, contact STIHL at 1-800-467-8445 or visit www.stihlusa.com.

Parts Diagrams

Refer to official STIHL parts diagrams for part numbers and assembly order. Common part groups:

• Cylinder & Piston Assembly (Part # 1140 020 2005)
• Carburetor Kit (Part # 1140 007 1060)
• Ignition Module (Part # 1140 400 1305)
• Clutch Assembly (Part # 1140 160 2010)
• Chain Brake Band (Part # 1140 650 7701)

Note: Always verify part numbers with your serial number.