STIHL MS 261 C-M Service Manual

The STIHL MS 261 C-M is a professional-grade chainsaw featuring the innovative M-Tronic engine management system for automatic carburetor adjustment, reduced emissions, and consistent performance under varying conditions. This manual provides comprehensive service, maintenance, and repair information for qualified technicians. Below are key sections covering safety, specifications, maintenance procedures, troubleshooting, and parts information.

1. Safety Information & Warnings 2. Technical Specifications 3. Maintenance Schedule 4. Engine & M-Tronic System 5. Fuel System & Carburetor 6. Ignition System 7. Chain & Bar Maintenance 8. Clutch & Sprocket 9. Troubleshooting Guide 10. Torque Specifications 11. Parts Diagrams & Ordering

Safety Information & Warnings

Service and repair of chainsaws involves significant hazards. Only trained personnel with proper tools and protective equipment (PPE) should perform these tasks.

Hazard	Precaution
Fuel & Fire	Work in well-ventilated area; no smoking or open flames.
Sharp Components	Always wear cut-resistant gloves when handling chain, bar, or cutter parts.
Hot Surfaces	Allow engine and muffler to cool completely before disassembly.
Spring-Loaded Parts	Use appropriate tools; clutch springs are under high tension.
Electrical Shock	Disconnect spark plug cap before any service to prevent accidental start.
Chemical Exposure	Use gloves when handling fuel, oil, or cleaning solvents.
Eye Protection	Always wear safety glasses or face shield.
Hearing Protection	Use ear defenders during testing or operation.

Technical Specifications

Model: STIHL MS 261 C-M
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 System: M-Tronic electronic engine management
Ignition System: Electronic, digital CDI
Fuel/Oil Mix: 50:1 with STIHL HP Ultra or equivalent ISO-L-EGD certified oil
Chain Oil: STIHL Chain Oil or equivalent
Guide Bar Lengths: 16" (40 cm) to 20" (50 cm)
Chain Pitch/Gauge: .325" / .063" (1.6 mm) or 3/8" PMM
Weight (powerhead): 5.2 kg (11.5 lbs)
Sound Pressure Level: 106 dB(A)
Vibration Levels (front/rear): 3.2/2.5 m/s²

Maintenance Schedule

Adherence to maintenance intervals is critical for performance, longevity, and safety.

Before Each Use: Check chain tension, sharpness, and lubrication. Inspect for loose fasteners and damage.
Daily (or after refueling): Clean air filter. Check fuel filter in tank.
Every 10 Operating Hours: Clean spark arrestor screen in muffler. Inspect clutch drum and sprocket.
Every 20 Operating Hours: Replace spark plug (STIHL ZAMA or equivalent). Clean cooling fins and cylinder fins.
Every 50 Operating Hours: Replace fuel filter. Clean carburetor air passage (if equipped with standard filter). Check all fuel lines for cracks.
Every 100 Operating Hours: Replace air filter (more often in dusty conditions). Check clutch springs and shoes for wear.
Annually or as needed: Replace chain, bar, and sprocket if worn. Check anti-vibration elements. Full professional inspection recommended.

WARNING! Use only genuine STIHL replacement parts to maintain performance, safety, and warranty.

Engine & M-Tronic System

The M-Tronic system automatically adjusts the fuel mixture based on engine load, altitude, temperature, and fuel quality.

System Components: Control unit (ECU), throttle position sensor, engine temperature sensor, intake air pressure sensor.

Service Notes: - Reset/Adaptation: After replacing ECU or major components, perform adaptation: Start saw and run at full throttle (no load) for 1 minute. Allow to idle for 1 minute. Repeat 2-3 times. - Diagnostics: No flashing LED codes on basic MS 261 C-M. Diagnosis requires STIHL diagnostic tool (HDS) for advanced troubleshooting. - Never adjust the fixed-jet carburetor. All mixture control is electronic.

CAUTION! Do not attempt to bypass or modify the M-Tronic system. Incorrect procedures can cause engine damage.

Fuel System & Carburetor

Fuel system includes tank, lines, filter, primer bulb (if equipped), and M-Tronic carburetor.

Fuel Line Replacement: Drain tank. Remove carburetor cover and air filter. Disconnect lines from carburetor and tank grommets. Install new lines, ensuring correct routing to prevent kinks.
Fuel Filter: Replace by pulling from tank with filter hook or wire. Soak new filter in fuel before installation.
Carburetor Service: Primarily limited to cleaning external vents and diaphragm inspection. Internal servicing requires specific training and parts kits.
Primer Bulb: Check for cracks. Replace by disconnecting fuel lines and pressing out of housing.

Tip: Always use fresh, properly mixed fuel. Stale fuel is a primary cause of starting and running issues.

Ignition System

Electronic CDI ignition with advanced timing control.

Spark Plug: STIHL ZAMA or equivalent (usually BPMR 7A). Gap: 0.5 mm (0.020").
Ignition Module Testing: Check for spark with tester gap set to 8 mm. No spark indicates faulty module, stop switch, or wiring.
Stop Switch: Test continuity. Wire should be open circuit when switch is OFF, closed when ON.
Flywheel & Coil Air Gap: Should be 0.3 mm (0.012"). Adjust by loosening coil screws and using a feeler gauge.

WARNING! Never crank engine with spark plug removed and connected if carburetor is attached. Risk of fire from fuel spray.

Chain & Bar Maintenance

Proper maintenance ensures cutting performance and safety.

Chain Sharpening: Use correct round file size (typically 4.0 mm for .325" chain). Maintain correct filing angle (usually 30°). File all cutters to equal length.
Depth Gauge Setting: Use a flat file and depth gauge tool to maintain proper height (approx. 0.65 mm / .025" below cutter tip).
Bar Maintenance: Clean bar groove regularly. Dress bar rails with a flat file if burred. Flip bar daily to ensure even wear.
Chain Tension: Adjust so chain snaps back when pulled from bar. Re-tension when chain is warm after initial use.
Oil Flow Check: Run saw at full throttle over light-colored surface (e.g., cardboard). A fine line of oil should be visible.

Clutch & Sprocket

Centrifugal clutch engages chain at approximately 2800 rpm.

Clutch Inspection: Remove cover and bar/chain. Check shoes for wear, cracking, or contamination with oil. Check springs for fatigue or breakage.
Clutch Drum & Sprocket: Inspect for wear, especially where chain drive links contact the sprocket. Replace if worn (visible hooking on sprocket teeth). Use rim sprocket system for easier replacement.
Bearing: Check crankshaft pilot bearing for smooth rotation. Replace if rough or noisy.

CAUTION! Clutch springs are under high tension. Use proper clutch removal tool to prevent injury.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine will not start	No spark, stale fuel, flooded, faulty M-Tronic	Check spark, use fresh fuel, clear flood, attempt M-Tronic adaptation.
Engine starts then dies	Fuel line leak, clogged filter, tank vent blocked	Inspect fuel system, replace filter, check tank vent.
Poor acceleration/loss of power	Clogged air filter, spark arrestor, exhaust port carbon, faulty M-Tronic sensor	Clean/replace filter, clean spark arrestor & exhaust, run adaptation.
Chain does not stop when idling	Clutch worn or oily, idle speed too high	Clean/replace clutch, check idle speed (should be just below engagement).
Excessive vibration	Loose fasteners, worn AV buffers, unbalanced chain, bent crankshaft	Tighten all fasteners, replace AV elements, balance chain, inspect crankshaft.
Chain oil not flowing	Empty oil tank, clogged oil passages, faulty oil pump	Fill tank, clean bar groove and oil ports, inspect/replace oil pump.

Torque Specifications

Use a calibrated torque wrench for critical fasteners.

Component	Thread Size	Torque (Nm)	Torque (lb-in / lb-ft)
Cylinder bolts	M6	10 Nm	88 lb-in
Muffler bolts	M5	8 Nm	71 lb-in
Clutch (left-hand thread)	-	40 Nm	30 lb-ft
Flywheel nut	M10x1.25	40 Nm	30 lb-ft
Spark plug	M14x1.25	20 Nm	15 lb-ft
Guide bar nuts	M8	12 Nm	106 lb-in
Handlebar through-bolts	M8	20 Nm	15 lb-ft

Parts Diagrams & Ordering

Always reference the specific serial number of your unit when ordering parts. Use the following group numbers for common assemblies:

Group 1: Cylinder, Piston, Crankshaft
Group 2: Carburetor & Fuel System
Group 3: Ignition System
Group 4: Clutch, Sprocket, Chain Brake
Group 5: Chain, Bar, Oiling System
Group 6: Housing, Handle, AV Elements
Group 7: M-Tronic Control Unit & Sensors

For Support: Contact an authorized STIHL servicing dealer. Do not attempt repairs beyond your skill level. Original parts and proper tools are essential.

Service Hotline (US): 1-800-467-8445

Website: www.stihlusa.com