STIHL FS 120 Repair Manual

The STIHL FS 120 is a professional-grade, straight-shaft trimmer/brushcutter designed for demanding clearing and maintenance work. This repair manual provides comprehensive technical information, troubleshooting procedures, and step-by-step instructions for maintenance, disassembly, reassembly, and repair of the FS 120 engine, drive train, cutting attachments, and controls. Essential for professional mechanics and experienced users.

1. Safety Precautions & Symbols 2. Specifications & Identification 3. Tools & Workshop Requirements 4. Engine System (FG-10) 5. Fuel System & Carburetor 6. Ignition System & Electronics 7. Drive Shaft & Gearhead 8. Cutting Attachments 9. Controls & Handle Assembly 10. Troubleshooting Flowcharts 11. Torque Specifications & Diagrams

Safety Precautions & Symbols

Before any repair, disconnect the spark plug boot. Use STIHL-approved tools. Work in a clean, well-ventilated area.

Symbol	Meaning	Action Required
⚠️	Warning/Caution	Risk of injury or damage. Follow instructions precisely.
🔧	Special Tool Needed	Use specified STIHL tool (e.g., piston stop, clutch holder).
📏	Measurement/Check	Use gauges (feeler, compression) to verify specification.
⚡	Electrical Hazard	Disconnect ignition before working on ignition system.
🔥	Fire Hazard	Drain fuel before repair. No sparks or open flames.

Specifications & Identification

Locate model and serial number on crankcase. Engine type: FG-10.

Engine: STIHL FG-10 single-cylinder, 2-stroke, air-cooled.
Displacement: 27.2 cm³ (1.66 cu in).
Power Output: 0.9 kW (1.2 hp) at 9000 rpm.
Fuel/Oil Mix: 50:1 (STIHL HP Ultra oil).
Idle Speed: 2800 rpm.
Max. RPM (WOT): 11000 rpm.
Drive Shaft: Solid, straight shaft.
Gearhead Ratio: 1.27:1 (standard).
Weight (powerhead): 5.9 kg (13.0 lbs).

Tools & Workshop Requirements

Essential tools for FS 120 repair.

STIHL Special Tools: Piston Stop 1129, Clutch Holder 0000 893 4300, Flywheel Puller 1128.
Standard Tools: Torx screwdrivers (T27), socket set (8-13mm), feeler gauges, torque wrench.
Measuring Tools: Compression tester, tachometer (digital), spark tester.
Consumables: STIHL grease, threadlocker (Loctite® 243), gasket kit, fuel line, diaphragm kits.

WARNING! Use genuine STIHL parts for safety and performance. Improper tools can damage components.

Engine System (FG-10)

Procedures for cylinder, piston, crankshaft, and bearings.

Cylinder Removal: Remove muffler, intake, cylinder base nuts. Lift cylinder off carefully.
Piston Inspection: Check for scoring, ring groove wear, pin bearing play. Replace if out of spec.
Crankshaft Seals: Replace if leaking (causes air leak, lean run). Use seal puller.
Decompression Valve: Test operation; clean or replace if stuck open/closed.
Muffler & Spark Arrestor: Clean carbon deposits regularly. Ensure screen is intact.

Torque Specs: Cylinder base nuts: 10 Nm (88 in-lb). Connecting rod bolts: 8 Nm (71 in-lb).

Fuel System & Carburetor

FS 120 typically uses Zama C1Q-S carburetor. Includes fuel lines, filter, tank vent.

Component	Function	Service Procedure
Fuel Filter	Strain debris	Replace annually or if clogged. Use STIHL 0000 350 3501.
Fuel Lines	Deliver fuel	Inspect for cracks/hardness. Replace as set (supply & return).
Carburetor Diaphragms	Pump & regulate fuel	Replace kit (gaskets, diaphragms) if engine runs lean/rich or won't idle.
Tank Vent	Equalize pressure	Clean or replace if fuel flow is restricted (causes vapor lock).
Adjustment Screws (L, H, LA)	Mixture control	Initial settings: L=1 turn open, H=1 turn open. Fine-tune with tachometer.

Ignition System & Electronics

Electronic ignition module (CDI), spark plug, stop switch, wiring.

Spark Plug: NGK BPMR7A. Gap: 0.5 mm (0.020 in). Replace if electrode eroded.
Ignition Module Test: Check for spark with tester. No spark? Check air gap (0.3 mm) to flywheel.
Flywheel & Key:Inspect key for shear. Tighten nut to 35 Nm (309 in-lb).

Stop Switch Circuit: Test continuity. Wire break or short to ground will kill engine.

Rewind Starter: Replace spring or rope if broken. Lubricate pulley bushing.

CAUTION! Never crank engine with spark plug removed and ignition connected—risk of CDI damage.

Drive Shaft & Gearhead

Transmits power from engine to cutting attachment.

Shaft Removal: Detach from clutch and gearhead. Inspect for bends or damage.

Clutch Assembly: Springs wear out—replace if engagement RPM is too high (>2800 rpm). Use clutch holder tool.

Gearhead Disassembly: Drain grease. Check gears (bevel) for pitting/wear. Replace seals.

Greasing: Pack with STIHL grease (0000 350 3500) annually or every 50 hours.

Bearing Inspection: Check shaft support bearings for smooth rotation. Replace if rough/noisy.

Torque Spec: Gearhead mounting bolts: 15 Nm (133 in-lb).

Cutting Attachments

Trimmer head, blades, circular saw blade compatibility.

AutoCut C 5-2 Head: For trimmer line. Replace bump feed mechanism if jammed.

Metal Blades: 3-tooth grass blade, 80-tooth circular saw blade. Ensure correct guard is installed.

Attachment Mounting: Use retaining ring (snap ring) and washer. Secure with gearhead spindle lock.

Balancing: Always balance metal blades before use to prevent vibration.

WARNING! Always wear cut-resistant chaps, eye/face protection, and gloves when servicing or using cutting attachments.

Controls & Handle Assembly

Throttle trigger, lockout, handlebar, harness connections.

Throttle Cable Adjustment: Cable should have slight free play at idle. Adjust at carburetor lever.

Throttle Lockout: Must engage before trigger can be pressed. Replace if worn or broken.

Handlebar Mounts: Check for cracks. Tighten bolts to 8 Nm (71 in-lb).

Anti-Vibration Elements: Inspect rubber dampers. Replace if hardened or cracked.

Harness Rings: Ensure secure attachment points for comfort and control.

Troubleshooting Flowcharts

Problem Check Solution

Engine won't start 1. Spark
2. Fuel delivery
3. Compression Test spark plug. Check fuel lines/filter. Compression >100 psi required.

Engine starts then dies 1. Tank vent
2. Carburetor diaphragms
3. Ignition coil (heat failure) Clean/replace vent. Rebuild carb. Test coil hot & cold.

Loss of power at WOT 1. Exhaust blocked
2. Fuel filter
3. H screw adjustment Clean spark arrestor. Replace filter. Adjust H screw richer.

Excessive vibration 1. Bent drive shaft
2. Unbalanced blade
3. Loose clutch/fan Straighten/replace shaft. Balance blade. Tighten clutch (35 Nm).

Clutch slipping/not engaging 1. Clutch springs
2. Drum inner surface Replace clutch assembly. Clean/replace drum if glazed.

Torque Specifications & Diagrams

Critical fastener torque values for reassembly.

Cylinder Base Nuts: 10 Nm (88 in-lb)

Spark Plug: 20 Nm (177 in-lb)

Flywheel Nut: 35 Nm (309 in-lb)

Clutch Nut: 35 Nm (309 in-lb) - Right-hand thread

Muffler Bolts: 8 Nm (71 in-lb)

Gearhead Bolts: 15 Nm (133 in-lb)

Handlebar Bolts: 8 Nm (71 in-lb)

Exploded View Diagrams: Refer to STIHL Parts Catalog 9910 850 0650 for component identification and ordering.

Service Support: For technical assistance or genuine parts, contact STIHL USA at 1-800-467-8445 or visit www.stihlusa.com.

STIHL FS 120 Repair Manual © - All Rights Reserved 2025. For professional technician use only.

Problem	Check	Solution
Engine won't start	1. Spark 2. Fuel delivery 3. Compression	Test spark plug. Check fuel lines/filter. Compression >100 psi required.
Engine starts then dies	1. Tank vent 2. Carburetor diaphragms 3. Ignition coil (heat failure)	Clean/replace vent. Rebuild carb. Test coil hot & cold.
Loss of power at WOT	1. Exhaust blocked 2. Fuel filter 3. H screw adjustment	Clean spark arrestor. Replace filter. Adjust H screw richer.
Excessive vibration	1. Bent drive shaft 2. Unbalanced blade 3. Loose clutch/fan	Straighten/replace shaft. Balance blade. Tighten clutch (35 Nm).
Clutch slipping/not engaging	1. Clutch springs 2. Drum inner surface	Replace clutch assembly. Clean/replace drum if glazed.