Stihl FS 120 Manual

The Stihl FS 120 is a professional-grade straight-shaft brushcutter/trimmer designed for demanding clearing, cutting, and trimming tasks. It features a powerful 2-stroke engine with electronic ignition, a robust gearbox, a straight shaft for extended reach, a loop handle for control, a harness attachment point for operator comfort, and a comprehensive safety system including a throttle trigger lockout and a debris shield. Below are key sections for safety, features, operation, maintenance, and troubleshooting.

Features at a Glance

Key components: 2-stroke engine, Straight drive shaft, Loop handle, Throttle trigger with lockout, Stop switch, Fuel tank, Primer bulb, Decompression valve (if equipped), Gearbox, and attachment interface for cutting heads or blades.

Feature	Description
Engine Type	2-stroke, single-cylinder, air-cooled
Ignition System	Electronic ignition (CDI)
Drive Shaft	Solid straight shaft
Handle System	Loop handle with vibration damping
Throttle Control	Trigger with lockout for safety
Fuel System	Diaphragm carburetor with primer bulb
Stop Switch	Momentary contact stop switch
Gearbox	Heavy-duty gearbox for power transmission
Attachment	Accepts trimmer heads and metal blades
Harness Point	D-ring for attaching harness

Safety Instructions

Read and understand all instructions before operation. Always wear appropriate PPE.

1. Personal Protective Equipment (PPE): Wear safety glasses, hearing protection, heavy-duty gloves, long pants, and sturdy boots.
2. Clear Work Area: Remove stones, nails, wire, and other debris that could be thrown.
3. Fuel Safety: Refuel outdoors, with engine off and cool. Use clean, fresh fuel mix.
4. Operation Zone: Keep bystanders and pets at least 50 feet away.
5. Transport: Carry unit with cutting attachment stopped and pointed away.

WARNING! Risk of severe injury from rotating cutting attachment. Never operate without proper guard installed. Do not operate in poor lighting or on unstable ground.

Assembly & Preparation

Unpack and inspect all parts. Assemble according to instructions.

1. Attach Handle: Secure loop handle to shaft at desired position using provided bolts.
2. Install Cutting Attachment: Fit trimmer head or blade onto gearbox shaft, secure with retaining nut using appropriate wrench.
3. Install Guard: Mount the correct debris shield/guard for the cutting attachment being used.
4. Fuel Mix: Prepare fuel mixture of high-quality 2-stroke engine oil and unleaded gasoline (typically 50:1 ratio - check manual). Fill fuel tank.
5. Harness: Attivate the compatible harness to the D-ring on the unit.

CAUTION! Ensure all fasteners are tight before starting. Use only Stihl-approved attachments and accessories.

Specifications

• Engine Displacement: Approximately 30.1 cc (check specific model).
• Power Output: Professional-grade power for heavy vegetation.
• Fuel/Oil Ratio: 50:1 (Stihl HP Ultra oil recommended).
• Fuel Tank Capacity: Approximately 0.95 pints (450 ml).
• Drive System: Solid steel shaft.
• Cutting Attachment: Compatible with AutoCut, PolyCut heads, and circular metal blades.
• Weight (without attachment): Approximately 12.8 lbs (5.8 kg).
• Sound Power Level: High - hearing protection required.
• Vibration Level: Reduced by anti-vibration system.

Controls and Components

Main Controls: Throttle trigger, Throttle trigger lockout, Stop switch, Choke lever, Primer bulb, Decompression valve button (if equipped).

Handle Area: Throttle trigger, Lockout lever, Stop switch, Throttle cable. Engine Area: Air filter cover, Spark plug, Muffler, Fuel tank cap, Primer bulb, Choke lever. Gearbox Area: Attachment shaft, Retaining nut, Guard mounting points.

Operation: Squeeze lockout lever into handle, then press throttle trigger to increase engine speed. Release trigger for idle. Press stop switch to stop engine.

Starting & Stopping the Engine

Cold Start Procedure (with choke):

1. Place unit on clear, flat ground. Ensure cutting attachment is clear.
2. Press fuel primer bulb 5-10 times until fuel is visible.
3. Set choke lever to full choke position (usually symbol for closed).
4. If equipped, press decompression valve.
5. Hold unit securely. With left hand on shaft, place foot through loop handle to stabilize.
6. Pull starter rope briskly until engine fires (usually 1-3 pulls).
7. Move choke lever to half-choke or run position.
8. Pull starter rope again to start engine. Allow to warm up at idle.

Warm Start: Do not use choke. Follow steps from primer bulb or start directly.

Stopping: Release throttle, allow engine to idle briefly, press and hold stop switch until engine stops.

Operation & Cutting Techniques

Always operate with harness properly adjusted to reduce fatigue.

1. Basic Trimming: Use a smooth, sweeping motion from side to side, keeping cutting head parallel to ground.
2. Brush Cutting (with blade): Use a slow, deliberate swinging motion from right to left (blade rotation dependent). Let the blade do the work.
3. Slope Operation: Always stand across the slope, never up or down. Maintain secure footing.
4. Line Advance: For bump-feed heads, tap head on ground at idle speed to advance cutting line.
5. Avoid Kickback: Do not engage blade or line with tip. Cut with central portion of cutting attachment.

WARNING! Never operate above shoulder height. Be aware of hidden objects. Shut off engine before adjusting or clearing debris.

Maintenance Schedule

Regular maintenance ensures performance and longevity. Perform with engine off and cool.

Item	Frequency	Action
Air Filter	Daily or as needed	Clean with soapy water, dry, re-oil (if foam). Replace if damaged.
Spark Plug	Every 100 hours	Check gap, clean, replace if worn.
Fuel Filter	Every year or as needed	Replace if clogged.
Gearbox Grease	Every 25 hours	Add high-quality gear grease through fitting.
Cutting Attachment	Before each use	Inspect for damage, wear, secure mounting.
External Cleaning	After each use	Remove debris from cooling fins, cylinder area, and guard.
Carburetor Adjustment	By qualified technician	If engine runs poorly after basic checks.

Storage

For storage longer than 30 days, prepare unit to prevent fuel system damage.

1. Drain fuel tank completely and run engine until it stops to empty carburetor.
2. Remove spark plug, add a teaspoon of engine oil into cylinder, pull starter slowly 2-3 times to coat, reinstall plug.
3. Clean entire unit thoroughly.
4. Store in a dry, secure place, protected from dust and moisture.
5. Remove cutting attachment or coat metal parts with oil to prevent rust.

CAUTION! Never store unit with fuel in the tank for extended periods. Old fuel can damage engine components.

Warranty & Compliance

Stihl Limited Warranty: Refer to the separate warranty card for specific terms and duration. This product complies with relevant ANSI/OPEI safety standards and EPA emissions regulations. Engine emissions information on engine. For genuine parts, accessories, and service, see your authorized Stihl servicing dealer. For support, visit www.stihlusa.com or contact your local Stihl dealer.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No fuel, flooded, spark plug faulty, switch off	Check fuel, use correct starting procedure, check/clean spark plug, ensure stop switch is in run position.
Engine starts then dies	Clogged fuel filter, dirty air filter, carburetor issue	Replace fuel filter, clean air filter, see dealer for carb adjustment.
Loss of power	Dirty air filter, clogged spark arrestor, old fuel	Clean air filter, clean spark arrestor screen in muffler, use fresh fuel mix.
Excessive vibration	Damaged cutting attachment, loose parts, bent shaft	Inspect/replace cutting attachment, tighten all bolts, inspect shaft for damage.
Cutting line won't advance	Empty spool, tangled line, worn bump knob	Reload line spool, untangle line, replace bump feed head if worn.
Gearbox overheating	Low grease, damaged gears, overloading	Add grease, do not overload unit, have gearbox inspected by dealer.

Dealer Service: For repairs beyond basic maintenance, contact an authorized Stihl servicing dealer.

Support: www.stihlusa.com - Find a Dealer locator available on website.