Stihl MS 250 Repair Manual

The Stihl MS 250 is a professional-grade chainsaw featuring a powerful 45.4cc engine, STIHL Quickstop® inertia chain brake, Ematic™ lubrication system, and anti-vibration system. This repair manual provides comprehensive instructions for troubleshooting, maintenance, disassembly, and reassembly of key components including the engine, fuel system, ignition, chain brake, and cutting equipment. Below are essential sections for safety, specifications, troubleshooting, maintenance procedures, and parts diagrams.

1. Safety Precautions 2. Specifications & Technical Data 3. Troubleshooting Guide 4. Maintenance Schedule 5. Engine Repair 6. Fuel System 7. Ignition System 8. Chain Brake System 9. Cutting Equipment 10. Parts Diagrams 11. Torque Specifications

Safety Precautions

Always prioritize safety when performing repairs on the Stihl MS 250 chainsaw.

Precaution	Description
Personal Protective Equipment (PPE)	Wear safety glasses, gloves, hearing protection, and chainsaw chaps.
Work Area	Work in a well-ventilated area free from sparks or open flames.
Fuel Safety	Drain fuel tank before repair. Use approved fuel containers.
Chain & Bar	Remove guide bar and chain before internal repairs.
Disconnect Spark Plug	Always disconnect spark plug wire to prevent accidental starting.
Sharp Tools	Use proper tools; keep cutting tools sharp to avoid slippage.
Lifting	Lift with legs, not back, when handling heavy components.
Fire Extinguisher	Keep a Class B fire extinguisher nearby when working with fuel.

Specifications & Technical Data

Engine Displacement: 45.4 cc
Power Output: 2.6 kW (3.5 bhp)
Fuel Tank Capacity: 0.47 L (16 oz)
Oil Tank Capacity: 0.26 L (8.8 oz)
Guide Bar Lengths: 16" (40 cm) or 18" (45 cm)
Chain Pitch: 3/8" PMM3
Chain Gauge: 0.050" (1.3 mm)
Ignition System: Electronic, capacitor discharge
Idle Speed: 2,800 RPM
Max Power Speed: 13,000 RPM
Weight (powerhead): 5.2 kg (11.5 lbs)

Troubleshooting Guide

Common problems and solutions for the Stihl MS 250.

Symptom	Possible Cause	Corrective Action
Engine will not start	Empty fuel tank, clogged fuel filter, faulty spark plug, flooded engine	Refill fuel, replace filter, check spark plug gap (0.5mm), dry plug.
Engine starts then stalls	Dirty air filter, incorrect carburetor adjustment, faulty fuel line	Clean/replace air filter, adjust L and H screws, inspect fuel line.
Loss of power	Clogged spark arrestor, dirty carburetor, worn piston/cylinder	Clean spark arrestor screen, rebuild carburetor, inspect cylinder.
Excessive vibration	Loose components, unbalanced chain, worn anti-vibration mounts	Tighten bolts, balance chain, replace AV mounts.
Chain does not oil	Empty oil tank, clogged oil passages, faulty oil pump	Refill oil, clean oil ports, replace oil pump diaphragm.
Chain brake does not engage	Worn brake band, dirty mechanism, broken spring	Replace brake band, clean linkage, replace return spring.

Maintenance Schedule

Daily: Check chain tension, sharpness, and lubrication. Clean air filter. Inspect for loose nuts/bolts.
Weekly: Clean spark arrestor screen. Check fuel filter and fuel lines. Inspect chain brake function.
Monthly: Clean cooling fins. Check anti-vibration elements. Inspect clutch drum and sprocket.
Every 6 Months: Replace spark plug. Clean carburetor. Check cylinder compression (≥ 120 psi).
Annually: Replace fuel filter and lines. Inspect piston and cylinder for scoring. Replace worn guide bar.

WARNING! Use only genuine STIHL parts for replacement to ensure performance and safety.

Engine Repair

Procedures for disassembling and servicing the MS 250 engine.

Cylinder & Piston: Remove muffler, cylinder cover, and cylinder bolts. Inspect piston rings and cylinder walls for scoring.
Crankshaft & Bearings: Remove clutch, flywheel, and seals. Check crankshaft runout (max 0.1mm). Replace bearings if noisy.
Carburetor Adjustment: Initial settings: L screw 1 turn open, H screw 1 turn open. Fine-tune at operating temperature.
Muffler: Remove and clean carbon deposits from spark arrestor screen. Ensure gasket is intact.

CAUTION! Mark all parts during disassembly for correct reassembly. Use a torque wrench.

Fuel System

Servicing the fuel tank, lines, filter, and carburetor.

Drain fuel from tank.
Remove fuel filter through filler neck using a wire hook.
Inspect fuel lines for cracks; replace if hardened or leaking.
Carburetor rebuild kit includes diaphragms, gaskets, needle valve.
Clean all passages with compressed air; do not use wire.

Tip: Use STIHL MotoMix or 50:1 mix of high-quality 2-stroke oil with unleaded gasoline (min 89 octane).

Ignition System

Testing and replacing ignition components.

Spark Plug: NGK BPMR7A, gap 0.5 mm (0.020"). Replace if electrode is worn or insulator cracked.
Ignition Module: Check for spark with tester. Resistance: primary 0.2-0.3 Ω, secondary 4-6 kΩ.
Stop Switch: Test continuity; replace if circuit does not open when switch is pressed.
Flywheel & Key: Inspect for sheared key; torque flywheel nut to 40 Nm (30 ft-lb).

Chain Brake System

Repairing the Quickstop® inertia chain brake.

Remove side cover, clutch drum, and chain.
Inspect brake band for wear (replace if thickness < 1.5mm).
Check tension of return spring; replace if stretched or broken.
Clean all linkage points; lubricate with high-temperature grease.
Test function: With engine off, engage brake by hand; it should lock chain.

WARNING! Never operate chainsaw with a malfunctioning chain brake.

Cutting Equipment

Maintenance of guide bar, chain, and sprocket.

Guide Bar: Clean bar groove; flip weekly for even wear. Dress rails with flat file if burred.
Chain Sharpening: Use round file (4.0mm for 3/8" PMM3). Maintain 30° cutting angle, 10° top plate angle.
Drive Sprocket: Replace if worn (spur) or teeth are hooked (rim). Lubricate with grease.
Chain Tension: Adjust so chain snaps back when pulled from bar; should not sag.

Parts Diagrams

Reference diagrams for major assemblies (available in full manual).

Cylinder & Piston Assembly (Part # 1128 020 1201)
Carburetor (Zama C1Q-S267)
Ignition Module (Part # 4140 400 1301)
Chain Brake Assembly (Part # 1128 140 0801)
Clutch & Sprocket (Part # 1128 160 2003)
Anti-Vibration System (Part # 1128 790 0800)

Note: Always use the serial number of your unit when ordering parts.

Torque Specifications

Component	Torque	Notes
Cylinder bolts	10 Nm (88 in-lb)	Crisscross pattern
Muffler bolts	8 Nm (71 in-lb)	Use new gasket
Clutch nut	40 Nm (30 ft-lb)	Left-hand thread
Flywheel nut	40 Nm (30 ft-lb)	Use holder tool
Guide bar nuts	9 Nm (80 in-lb)	After tensioning chain
Spark plug	20 Nm (15 ft-lb)	Do not overtighten

Reset: After major repairs, reset carburetor to factory settings and retune.

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