STIHL MS 261 Repair Manual

The STIHL MS 261 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 maintenance, troubleshooting, and repair procedures. Below are key sections covering safety, specifications, maintenance schedules, disassembly, component repair, and reassembly.

Features at a Glance

Key components: Engine cylinder, carburetor, ignition module, fuel system, chain brake, anti-vibration system, and cutting attachment.

Feature	Description
Engine Type	2-stroke, single cylinder, air-cooled
Displacement	50.2 cm³ (3.06 cu. in.)
Power Output	3.0 kW (4.0 bhp)
Fuel Tank Capacity	0.5 L (16.9 fl. oz.)
Oil Tank Capacity	0.26 L (8.8 fl. oz.)
Chain Pitch	3/8" PM3 or .325"
Weight (powerhead)	5.2 kg (11.5 lbs)
Ignition System	Electronic, capacitor discharge
Anti-Vibration	STIHL Advanced AV system
Chain Brake	STIHL Quickstop® inertia activated

Safety Precautions

Always wear protective equipment and follow safety guidelines.

1. Personal Protective Equipment: Wear safety glasses, hearing protection, gloves, chainsaw chaps, and sturdy boots.
2. Work Area Safety: Ensure clear work area, secure footing, and no bystanders within 15 feet.
3. Tool Inspection: Check chain tension, sharpness, and brake function before each use.
4. Fuel Safety: Refuel in well-ventilated area, away from ignition sources. Use STIHL MotoMix® or equivalent.

WARNING! Never operate a chainsaw with damaged or missing safety features. Disconnect spark plug before performing repairs.

Specifications

• Engine: 2-stroke, 50.2 cm³ displacement.
• Fuel/Oil Ratio: 50:1 (STIHL HP Ultra or equivalent).
• Idle Speed: 2,800 RPM (± 150 RPM).
• Maximum Power Speed: 14,000 RPM.
• Spark Plug: NGK BPMR 7A (gap 0.5 mm / 0.020").
• Chain Oil: STIHL HP Ultra or biodegradable chain oil.
• Bar Lengths: 16" (40 cm) to 20" (50 cm) recommended.
• Sound Pressure: 106 dB(A) typical.
• Vibration Levels: Left/Right 3.2 m/s², Front/Rear 2.8 m/s².

Maintenance Schedule

Regular maintenance ensures optimal performance and longevity.

Daily/Before Use: Check chain tension and sharpness, inspect chain brake, clean air filter, check fuel and oil levels.

Weekly/10 Hours: Clean cooling fins, check spark plug, inspect anti-vibration elements, clean spark arrestor screen.

Monthly/50 Hours: Replace fuel filter, clean carburetor, check clutch and sprocket, inspect bar for wear.

Annually/100 Hours: Replace spark plug, clean combustion chamber, check piston and cylinder, replace fuel lines if cracked.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine won't start	No spark, fuel issue, flooded	Check spark plug, fuel quality, and carburetor. Use choke correctly.
Engine runs poorly	Clogged air filter, carb adjustment	Clean/replace air filter. Adjust H/L screws per manual.
Chain doesn't turn	Clutch worn, sprocket damaged	Inspect clutch shoes and drum. Replace if worn.
Excessive vibration	AV mounts damaged, imbalance	Replace anti-vibration buffers. Balance bar and chain.
Oil not feeding	Clogged oil passages, pump failure	Clean oil channels. Test oil pump output.
Overheating	Dirty cooling fins, lean mixture	Clean fins. Check carburetor settings and fuel mix.

Engine Disassembly

Step-by-step engine teardown for major repairs.

1. Remove top cover and air filter assembly.
2. Disconnect fuel lines and remove carburetor.
3. Remove muffler and spark plug.
4. Separate crankcase halves after removing all screws.
5. Extract crankshaft, connecting rod, and piston assembly.
6. Inspect cylinder for scoring; measure piston clearance.

CAUTION! Mark all components for reassembly. Use proper torque values during disassembly/reassembly.

Carburetor Adjustment

Adjusting Walbro or Zama carburetor for optimal performance.

1. Start engine and warm up for 5 minutes.
2. Adjust idle speed screw (LA) to achieve 2,800 RPM.
3. Adjust low-speed screw (L) for smooth acceleration from idle.
4. Adjust high-speed screw (H) for maximum power without leaning.
5. Re-check idle speed and chain brake engagement.

Tip: Final settings: L = 1 turn open, H = 1 turn open (baseline). Fine-tune for altitude and temperature.

Chain & Bar Maintenance

Proper care extends chain and bar life.

1. Sharpening: Use round file matching chain pitch (typically 3.5 mm). Maintain 30° cutting angle.
2. Tensioning: Adjust so chain snaps back when pulled from bar.
3. Bar Care: Clean bar groove weekly. Flip bar monthly for even wear.
4. Sprocket: Replace drive sprocket every 2-3 chains.

WARNING! Always stop engine and wear gloves when handling chain. Keep chain sharp to reduce kickback risk.

Ignition System

Testing and replacing ignition components.

Spark Test: Remove spark plug, connect to wire, ground against cylinder. Pull starter; look for bright blue spark.

Module Test: Use multimeter to check primary resistance (approx. 0.5-2.0 Ω). Replace if out of spec.

Flywheel: Check for cracks and clean magnets. Torque nut to 40 Nm (30 ft-lb).

Reassembly Procedures

Reassembly in reverse order of disassembly.

1. Install new crankcase seals and gaskets.
2. Lubricate bearings and piston pin before assembly.
3. Torque crankcase screws in crisscross pattern to 10 Nm (7.5 ft-lb).
4. Set piston ring gap to 0.3-0.5 mm (0.012-0.020").
5. Prime fuel system before starting.

CAUTION! Use STIHL OEM parts for best fit. Test run without bar/chain first.

Parts List & Diagrams

Reference exploded views for part numbers and assembly order.

• Engine Group: Cylinder 1128 020 1205, Piston 1128 030 2001.
• Fuel System: Carburetor 1128 120 0601, Fuel Filter 0000 350 3500.
• Ignition: Spark Plug 0000 400 1300, Ignition Module 1128 400 1305.
• Chain Brake: Band 1128 650 0801, Handle 1128 790 4801.
• AV System: Buffer 1128 790 4701, Spring 1128 790 4702.

Ordering: Contact STIHL dealer with part numbers. Diagrams available at www.stihlusa.com.