Stihl FS 120 Service Manual

The Stihl FS 120 is a professional-grade, straight-shaft grass trimmer/brushcutter designed for demanding clearing and trimming tasks. This manual provides essential information for the safe maintenance, troubleshooting, and repair of the FS 120, including its 2-stroke engine, fuel system, cutting attachments, drive shaft, and gearbox. Below are key sections covering safety, specifications, maintenance procedures, troubleshooting, and parts information.

Features at a Glance

Key components: High-performance 2-stroke engine, Anti-Vibration System (AVS), straight drive shaft, professional-grade gearbox, and compatibility with multiple cutting attachments.

Feature	Description
Engine Type	Single cylinder, 2-stroke, air-cooled
Displacement	Approx. 30.1 cm³ (1.84 cu. in.)
Power Output	High power-to-weight ratio for demanding tasks
Fuel System	Diaphragm carburetor with manual choke
Ignition System	Electronic, capacitor discharge (CDI)
Starting System	Rewind starter with decompression valve
Drive Shaft	Rigid straight shaft with flexible coupling
Gearbox	Professional-grade, grease-lubricated
Anti-Vibration	Stihl AVS system at handle and shaft
Cutting Attachments	Accepts trimmer heads, brush knives, circular blades

Safety Information

Always prioritize safety when servicing power tools. Disconnect the spark plug boot before performing any maintenance.

1. Personal Protective Equipment (PPE): Always wear safety glasses, gloves, hearing protection, and sturdy boots.
2. Work Area: Service in a clean, well-ventilated area away from sparks or open flames.
3. Fuel Safety: Drain fuel before service. Use only clean, fresh fuel-oil mix (50:1 with STIHL HP Ultra oil).
4. Sharp Tools: Handle cutting attachments with extreme care; wear cut-resistant gloves.
5. Hot Surfaces: Allow engine and muffler to cool completely before touching.

WARNING! Failure to follow safety instructions can result in serious injury or death. This manual is for qualified technicians.

Specifications

• Model: STIHL FS 120
• Engine Power: Approx. 1.3 kW (1.7 hp)
• Idle Speed: 2,800 rpm
• Max. Engine Speed: 12,500 rpm (with cutting tool)
• Fuel Tank Capacity: Approx. 0.55 liters (18.6 fl. oz.)
• Oil Tank Capacity: (For 2-MIX Oil Injection models, if equipped) - Refer to specific variant.
• Weight (powerhead): Approx. 6.2 kg (13.7 lbs) without cutting attachment
• Shaft Length: Standard straight shaft
• Sound Pressure Level: Typically 96 dB(A) - Wear hearing protection.
• Vibration Level: Reduced by AVS system.

Maintenance Schedule

Regular maintenance is crucial for performance, longevity, and safety.

Interval / Condition	Task
Before each use	Check for loose fasteners, damage, fuel leaks. Inspect cutting attachment.
Daily / After use	Clean exterior, check air filter, clean spark arrestor screen (if equipped).
Every 25 operating hours	Clean air filter more thoroughly. Check spark plug condition and gap.
Every 50 operating hours	Replace air filter (more often in dusty conditions). Clean fuel filter in tank.
Every 100 operating hours	Replace spark plug. Clean combustion chamber deposits. Check carburetor settings.
Annually or as needed	Replace fuel line and filter. Grease gearbox. Inspect drive shaft and couplings.
When performance drops	Check compression, fuel system, and muffler for carbon blockage.

Fuel & Lubrication

Fuel Mix: Use unleaded gasoline (min. 89 octane) mixed with a high-quality 2-stroke engine oil at a 50:1 ratio (STIHL HP Ultra recommended). Do not use automotive oil or old fuel.

Gearbox Lubrication: Use STIHL Gear Grease. Remove the gearbox cover, clean out old grease, and pack with new grease periodically (see Maintenance Schedule).

Drive Shaft: The shaft is lubricated from the factory. If disassembled, apply special shaft grease during reassembly.

CAUTION! Gasoline is extremely flammable. No smoking. Work in a well-ventilated area.

Engine Service

Common engine service procedures include cylinder/piston inspection, muffler cleaning, and crankcase seals.

1. Compression Check: Use a compression gauge. Reading below 110 psi indicates worn piston/rings or cylinder.
2. Cylinder & Piston: Remove muffler to inspect piston through exhaust port. Look for scoring or excessive carbon.
3. Muffler: Remove and clean carbon deposits from spark arrestor screen and internal baffles.
4. Crankcase Seals: Inspect for leaks. Failed seals cause lean running and engine damage. Replace using proper tools.
5. Decompression Valve: Check for proper operation and sealing. Clean or replace if leaking.

Drive Train & Gearbox

1. Drive Shaft Removal: Disconnect from engine and gearbox. Inspect for bends or damage. Check flexible coupling for wear.
2. Gearbox Disassembly: Remove cutting attachment and cover. Check gears for pitting or wear. Inspect bearings.
3. Greasing: Pack gearbox fully with STIHL Gear Grease during reassembly.
4. Clutch: Inspect clutch shoes for wear and springs for integrity. The clutch engages at approx. 2,800 rpm.

Tip: Always secure the drive shaft or gearbox when loosening the cutting attachment nut (reverse thread on some models).

Cutting Attachments

Supports trimmer heads (e.g., AutoCut), metal brush knives, and circular saw blades.

1. Installation: Ensure machine is off and spark plug disconnected. Use correct mounting hardware and guard.
2. Trimmer Line: For trimmer heads, use correct diameter STIHL line. Do not exceed recommended length.
3. Metal Blades: Inspect for cracks, missing teeth, or excessive wear. Sharpen or replace as needed. Balance is critical.
4. Guard: Always use the appropriate, undamaged guard for the attachment installed.

WARNING! Damaged or unbalanced blades can shatter causing severe injury. Replace damaged attachments immediately.

Carburetor Adjustment

The FS 120 typically uses a Zama or Walbro diaphragm carburetor with H (High), L (Low), and LA (Idle) adjustment screws.

1. Initial Settings: Gently turn H and L screws clockwise until seated, then back out: L - 1 turn, H - 1 turn.
2. Idle Adjustment: Start engine and warm up. Adjust LA screw for a steady idle (2,800 rpm) without the clutch engaging.
3. Low-Speed (L): From idle, quickly open throttle. If it hesitates, turn L counter-clockwise slightly. Adjust for smooth acceleration.
4. High-Speed (H): Under load (with cutting attachment), adjust H for maximum power without "leaning out" (screaming sound). Use a tachometer: Max 12,500 rpm.
5. Final Idle Check: Re-check idle after H/L adjustments. Engine should return to idle smoothly and not stall.

CAUTION! An improperly lean H setting will destroy the engine. If unsure, seek professional service.

Ignition System

The electronic ignition module (CDI) is non-adjustable. Primary checks:

• Spark Plug: Check for fouling, correct gap (0.020" / 0.5mm). Replace if electrodes are worn.
• Spark Test: Remove plug, connect to boot, hold against cylinder head. Pull starter; look for strong blue spark.
• Stop Switch: Check for continuity. Wire should be open circuit when switch is off, closed when on.
• Ignition Coil Air Gap: Check gap between flywheel magnets and coil legs (approx. 0.3 mm / 0.012"). Adjust if necessary.
• CDI Module: If no spark after checking above, module may be faulty. Test per STIHL specifications or replace.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No spark, flooded, fuel issue	Check spark, plug condition. Dry plug if flooded. Check fuel line, filter, and carburetor.
Engine starts but dies	Clogged fuel filter, dirty air filter, carb adjustment	Replace fuel filter. Clean air filter. Check carburetor L setting and idle speed.
Lacks power under load	Clogged muffler, worn piston/rings, lean H setting, dirty air filter	Clean muffler. Check compression. Adjust H screw richer. Clean air filter.
Excessive vibration	Unbalanced blade, bent shaft, loose components	Balance or replace blade. Inspect drive shaft. Tighten all fasteners.
Clutch slips or doesn't engage	Worn clutch shoes, weak springs, engine speed too low	Replace clutch assembly. Check and adjust idle speed.
Gearbox overheats or is noisy	Lack of grease, damaged gears/bearings	Disassemble, inspect, replace damaged parts, pack with new grease.

Note: Always diagnose from simple to complex causes (fuel, spark, compression).

Parts Diagrams

Refer to official STIHL parts diagrams (available through STIHL dealers or the STIHL service portal) for exploded views and part numbers for:

• Complete Powerhead Assembly
• Cylinder & Piston Group
• Crankcase & Crankshaft
• Carburetor & Air Filter System
• Ignition System & Rewind Starter
• Drive Shaft & Gearbox
• Handle & Control Assembly
• Cutting Attachment Mounting

Service: For genuine STIHL parts and technical support, contact an authorized STIHL servicing dealer. Manual © STIHL - All information subject to change.