Stihl FS 400 Workshop Manual

The Stihl FS 400 is a professional-grade brushcutter/string trimmer designed for demanding clearing and maintenance work. This workshop manual provides comprehensive technical information for service, repair, and maintenance, covering the engine, fuel system, ignition, cutting attachments, gearbox, and safety systems. Below are key sections for specifications, safety, disassembly, troubleshooting, and assembly procedures.

Specifications & Technical Data

Key technical parameters for the Stihl FS 400 engine and drive system.

Safety Instructions & Symbols

Critical safety warnings and explanation of symbols found on the unit and in this manual.

1. Personal Protective Equipment (PPE): Always wear safety glasses, hearing protection, gloves, sturdy boots, and long pants.
2. Workshop Safety: Ensure adequate ventilation when working with fuel. Keep work area clean and free of ignition sources.
3. Before Disassembly: Allow engine to cool completely. Drain fuel tank. Disconnect spark plug wire and secure away from plug.
4. Symbols: Understand warning (triangle), prohibition (crossed circle), and mandatory action (blue circle) symbols on the machine.

WARNING! Failure to follow safety instructions can result in serious injury or death. The cutting attachment can continue to rotate after the engine is shut off (inertia).

Tools & Workshop Equipment

Recommended Stihl special tools and standard workshop equipment for servicing the FS 400.

1. Stihl Piston Stop Tool (to lock crankshaft for flywheel/coupling nut removal).
2. Stihl Flywheel Puller (2- or 3-arm puller).
3. Stihl Carburetor Adjustment Tool (for H, L, LA screws).
4. Stihl Compression Tester (for 2-stroke engines).
5. Standard Tools: Torque wrench (5-50 Nm), socket set, screwdrivers, feeler gauges, circlip pliers.
6. Cleaning Tools: Parts cleaning tank, compressed air, brushes.

CAUTION! Use correct tools to avoid damage to components. Do not use impact drivers on plastic or magnesium parts.

Engine System

• Cylinder & Piston: Inspect for scoring, carbon buildup, and ring wear. Measure piston clearance.
• Crankshaft & Bearings: Check for play and smooth rotation. Replace if axial/radial play exceeds limits.
• Muffler & Spark Arrestor: Clean carbon deposits from exhaust port and spark arrestor screen regularly.
• Decompression Valve: Test for proper sealing and spring function.
• Engine Mounts: Inspect rubber anti-vibration elements for cracks or deterioration.

Fuel System & Carburetor

Service procedures for the carburetor (typically Walbro or Zama), fuel lines, filter, and tank.

Carburetor (HD-type): Adjust idle speed (L screw) and high-speed (H screw) using tachometer. Standard baseline: H=1 turn open, L=1 turn open from lightly seated. Final adjustment must be made under load.

Fuel System: Replace cracked or stiff fuel lines. Clean fuel filter in tank. Check tank vent for blockage. Ensure fuel cap seal is intact.

Ignition System

Testing and replacement of ignition module (coil), spark plug, and stop switch.

1. Remove spark plug, check electrode gap (0.5mm / 0.020"). Replace if worn or fouled.
2. Check spark with tester: Connect tester between plug wire and ground, pull starter. Look for strong blue spark.
3. Ignition Module: Measure primary and secondary resistance with multimeter (refer to values in technical supplement).
4. Stop Switch: Test for continuity between black wire and ground when switch is pressed.

Tip: A weak or no spark is often caused by a faulty ignition module, broken wire, or damaged flywheel magnets.

Cutting Attachments & Gearbox

Servicing the drive shaft, gearbox, and cutting head or blade attachment.

1. Drive Shaft: Inspect for straightness. Check flexible coupling for wear.
2. Gearbox: Drain old grease. Inspect bevel gears for pitting or wear. Repack with Stihl gear grease.
3. Output Shaft: Check bearing for smooth rotation. Replace seal if leaking.
4. Cutting Attachment: For blade, check balance and sharpness. For trimmer head, replace worn line or bump feed mechanism.
5. Clutch: Inspect clutch shoes for wear and springs for integrity. Clean drum interior.

WARNING! Always ensure cutting attachment is securely fastened and guard is properly installed before testing.

Disassembly Sequence

Recommended order for major disassembly.

1. Remove cutting attachment and guard.
2. Remove fuel tank and air filter cover/carburetor cover.
3. Disconnect fuel lines and throttle linkage from carburetor.
4. Remove starter assembly and fan housing.
5. Lock piston and remove flywheel nut, then pull flywheel.
6. Remove ignition module.
7. Separate crankcase halves (requires heating for case bond).

CAUTION! Mark mating parts and take photos during disassembly for correct reassembly. Do not force components.

Troubleshooting & Diagnostics

Assembly & Torque Specifications

Critical torque values and assembly notes.

• Cylinder Head Screws: 20 Nm (15 ft-lb) - tighten in crisscross pattern.
• Flywheel Nut: 40 Nm (30 ft-lb) - use piston stop.
• Clutch Nut: 40 Nm (30 ft-lb) - left-hand thread.
• Gearbox Housing Screws: 10 Nm (7.5 ft-lb).
• Muffler Screws: 10 Nm (7.5 ft-lb).
• Spark Plug: 25 Nm (18 ft-lb).

Assembly Tip: Use new gaskets and seals. Apply a thin coat of 2-stroke oil to piston and cylinder during assembly. Ensure crankshaft seals are properly seated.

Maintenance Schedule

Recommended service intervals based on hours of operation or annually, whichever comes first.

Reset: After major service, reset maintenance counter if equipped.

Service & Parts: For genuine Stihl parts and technical bulletins, contact an authorized Stihl servicing dealer or visit www.stihl.com.