STIHL FS 400 Manual

The STIHL FS 400 is a professional-grade, high-power brushcutter/string trimmer designed for demanding clearing and cutting tasks. It features a powerful 2-stroke engine with anti-vibration system, a straight drive shaft, a loop handle, and a harness for operator comfort and control. Below are key sections for safety, features, operation, maintenance, and troubleshooting.

Features at a Glance

Key components: Powerful 2-stroke engine, anti-vibration system, straight drive shaft, loop handle, quick-release harness, and compatible with a wide range of STIHL cutting attachments.

Feature	Description
Engine Type	2-stroke, single-cylinder, air-cooled
Anti-Vibration System	Reduces operator fatigue during extended use
Drive Shaft	Straight, solid shaft for high torque transmission
Handle System	Loop handle for two-handed control
Harness	Quick-release harness for balanced weight distribution
Fuel System	Diaphragm carburetor with purge bulb
Starter	STIHL Easy2Start™ system (if equipped)
Ignition	Electronic, capacitor discharge (CDI)
Cutting Tools	Accepts trimmer heads, metal blades, and brush knives
Throttle Control	Throttle trigger with lockout for safety

Safety Instructions

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

1. Personal Protective Equipment (PPE): Always wear safety glasses, hearing protection, sturdy gloves, long pants, and boots.
2. Work Area Safety: Keep bystanders and pets at least 50 feet away. Clear area of debris like stones and wire.
3. Fuel Safety: Refuel in a well-ventilated area with engine stopped and cool. Use only STIHL MotoMix® or gasoline mixed with STIHL HP Ultra oil at 50:1 ratio.
4. Operation: Hold unit firmly with both hands. Do not operate in poor lighting, bad weather, or when fatigued.
5. Kickback Zone: Be aware of the danger zone (approximately 2 o'clock to 5 o'clock position of the cutting attachment).

WARNING! Failure to follow safety instructions can result in serious injury or death. The cutting tool continues to rotate after the throttle is released.

Assembly & Preparation

Unpack and inspect contents. Assemble according to these steps.

1. Attach the Shaft: Align the shaft with the engine unit and tighten the coupling securely.
2. Install the Handle: Slide the loop handle onto the shaft at a comfortable position and tighten the clamping screws.
3. Fit the Harness: Connect the harness hooks to the attachment points on the unit. Adjust straps for a snug, balanced fit.
4. Install Cutting Attachment: Ensure the engine is off. Install the chosen trimmer head or blade, securing it with the appropriate nut and guard. Use the included combination wrench.
5. Add Fuel & Oil: Fill the fuel tank with the correct 50:1 fuel/oil mixture. Fill the gearbox with STIHL gear oil (if applicable).

CAUTION! Ensure all fasteners are tight before starting. Do not use without the correct cutting attachment guard installed.

Specifications

• Engine Displacement: Approximately 40.2 cc (2.45 cu. in.)
• Power Output: High power output for professional use.
• Fuel/Oil Ratio: 50:1 (STIHL HP Ultra oil)
• Fuel Tank Capacity: Approx. 0.95 liters (32.1 fl. oz.)
• Drive System: Straight solid drive shaft.
• Sound Power Level: Typically exceeds 100 dB(A). Hearing protection required.
• Vibration Levels: Reduced by anti-vibration system.
• Weight (without cutting attachment): Approximately 7.5 kg (16.5 lbs).
• Recommended Cutting Attachments: STIHL PolyCut heads, circular saw blades, brush knives.

Controls and Components

Main controls located on the handle and engine unit.

Handle Area: Throttle trigger, throttle trigger lockout, stop switch (I/O), handle mounting clamp.

Engine Unit: Starter grip, fuel tank cap, purge bulb (primer), choke lever, master control lever (combines choke/run/stop functions on some models), air filter cover, spark plug.

Shaft/Gearhead: Drive shaft coupling, gearbox, cutting attachment mounting flange, debris shield.

Harness: D-rings, quick-release buckle, adjustable shoulder and waist straps.

Starting & Stopping the Engine

Follow this procedure for a cold start. For a warm start, omit the choke step.

1. Pre-Start Check: Place unit on clear, level ground. Ensure cutting attachment is clear of obstacles.
2. Fuel & Ignition: Press purge bulb 5-10 times until fuel is visible. Set master control lever to choke position (closed).
3. Starting: Hold the unit securely with your right hand on the rear handle and your foot in the carrying ring. Pull the starter rope briskly until the engine fires (usually 1-3 pulls).
4. Warm-Up: Immediately move the master control lever to the half-throttle or run position. Allow engine to run at idle for 30-60 seconds to warm up.
5. Stopping: Release the throttle trigger and allow the engine to return to idle. Press and hold the stop switch (I/O) to "O" until the engine stops completely.

Tip: Do not pull the starter rope to its full extent or let the starter grip snap back.

Operation & Cutting Techniques

Proper technique ensures efficiency and safety.

1. Basic Stance: Stand firmly with feet shoulder-width apart. Let the harness support the unit's weight.
2. Swinging Technique: Use a smooth, sweeping motion from right to left (for right-handed operators), keeping the cutting attachment parallel to the ground. Cut with the tip of the line or blade.
3. Tall Grass/Weeds: Use a multi-pass technique, cutting the top first and then lower layers.
4. Brushes/Saplings: Use a brush knife or circular saw blade. Use a scything motion, contacting with the forward portion of the blade.
5. Avoiding Kickback: Never swing the unit so the cutting attachment contacts an object in the 2-5 o'clock zone. Always cut at full throttle.

WARNING! Never use a metal blade for trimming lawn or near fences, buildings, or rocks. Always inspect the work area for hidden objects.

Maintenance Schedule

Regular maintenance ensures long life and safe operation. Perform these tasks at the intervals specified in your manual.

Task	Before Each Use	Every 25 Hours	Every 100 Hours/Annually
Check fasteners for tightness	✓
Inspect cutting attachment & guard	✓
Clean air filter		✓
Check fuel filter & line		✓
Clean spark plug, check gap		✓
Clean cooling fins			✓
Check/replace spark arresting screen (muffler)			✓
Grease gearbox		✓
Inspect drive shaft			✓

CAUTION! Always disconnect the spark plug boot before performing any maintenance.

Storage

Prepare the unit for storage if it will not be used for more than 30 days.

1. Fuel System: Run the engine until it stops to empty the carburetor. Drain all fuel from the tank into an approved container.
2. Cleaning: Thoroughly clean the exterior of the unit, removing all grass, dirt, and debris.
3. Maintenance: Perform any overdue maintenance tasks (e.g., clean air filter).
4. Storage Location: Store in a dry, secure place, out of reach of children. Cover the unit to keep dust off.
5. Battery (if any): Remove and store separately in a cool, dry place.

CAUTION! Never store the unit with fuel in the tank in an enclosed area. Fuel vapors are explosive.

Warranty & Support

STIHL Limited Warranty: Refer to the warranty card included with your product for specific terms and duration. This warranty covers defects in material and workmanship. Regular maintenance as outlined in this manual is required to maintain warranty coverage. For genuine STIHL parts, replacement accessories, and service, contact an authorized STIHL servicing dealer. For support, visit www.stihlusa.com or contact STIHL Incorporated at 1-800-467-8445.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No fuel, flooded engine, dirty air filter, faulty spark plug, choke not set correctly.	Check fuel level. Set choke to open/run and pull starter repeatedly. Clean/replace air filter and spark plug.
Engine starts but dies	Clogged fuel filter, dirty carburetor, faulty fuel line.	Replace fuel filter. Clean carburetor. Inspect fuel line for cracks.
Loss of power	Dirty air filter, clogged spark arresting screen, old fuel.	Clean air filter and muffler screen. Use fresh fuel mix.
Excessive vibration	Damaged cutting tool, loose fasteners, bent drive shaft.	Stop engine! Inspect and replace damaged blade/head. Tighten all fasteners.
Cutting attachment does not spin	Broken drive cable (flexible shaft models), damaged gearbox, clutch slipping.	Stop engine! Inspect drive shaft and gearbox. Have unit serviced by a dealer.
Line not feeding	Empty bump head, line tangled, worn bump head.	Reload with correct line length. Clean head or replace if worn.

Dealer Service: For complex repairs, always take your STIHL FS 400 to an authorized STIHL servicing dealer.

Support: 1-800-467-8445; www.stihlusa.com