Stihl FS 40 Service Manual

The Stihl FS 40 is a lightweight, professional-grade trimmer designed for demanding use. This service manual provides detailed instructions for maintenance, troubleshooting, and repair of the FS 40 trimmer, including its engine, cutting head, drive shaft, and controls. Below are key sections covering safety, specifications, maintenance procedures, troubleshooting, and parts information.

Features at a Glance

Key components: 2-stroke engine, loop handle, straight shaft, PolyCut head, throttle trigger lockout, and decompression valve.

Feature	Description
Engine Type	2-stroke, single-cylinder, air-cooled
Displacement	27.2 cc
Power Output	0.9 kW (1.2 hp)
Fuel Tank Capacity	0.42 liters
Cutting Width	Up to 42 cm with optional blade
Drive System	Flexible drive shaft
Cutting Head	Stihl PolyCut head (standard)
Starting System	Rewind starter with decompression valve
Weight (without cutting attachment)	Approx. 4.9 kg
Vibration Damping	Integrated anti-vibration system

Safety Precautions

Always follow safety guidelines to prevent injury and equipment damage.

1. Wear protective gear: safety glasses, gloves, ear protection, long pants, and sturdy boots.
2. Inspect the trimmer before each use for loose parts, damage, or fuel leaks.
3. Use only Stihl-approved fuel mix (50:1 gasoline to 2-stroke oil).
4. Operate in a well-ventilated area; avoid inhaling exhaust fumes.
5. Keep bystanders and pets at a safe distance (at least 15 meters).

WARNING! Never operate a damaged trimmer. Disconnect the spark plug before performing any maintenance.

Specifications

• Model: Stihl FS 40
• Engine: 2-stroke, 27.2 cc
• Power: 0.9 kW at 9000 rpm
• Idle Speed: 2800 rpm
• Maximum Engine Speed: 11500 rpm
• Fuel/Oil Mix: 50:1 (Stihl HP Ultra or equivalent)
• Spark Plug: Bosch WSR 6 F or equivalent, gap 0.5 mm
• Cutting Tools: PolyCut head, circular grass blade, metal brush knife
• Sound Pressure Level: 96 dB(A)
• Vibration Level: Front handle: 3.5 m/s²; Rear handle: 4.5 m/s²

Maintenance Schedule

Regular maintenance ensures optimal performance and longevity.

Interval	Task
Before each use	Check for loose fasteners, fuel leaks, and cutting attachment condition.
Every 25 hours	Clean air filter; check spark plug; inspect fuel filter.
Every 50 hours	Replace air filter; clean spark arrestor screen; check carburetor settings.
Every 100 hours	Replace fuel filter; clean cooling fins; inspect drive shaft and gearbox.
Annually	Replace spark plug; check anti-vibration elements; full inspection.

Note: Service intervals may vary based on operating conditions.

Engine Service

Procedures for engine maintenance and adjustment.

1. Cylinder & Piston: Inspect for scoring or carbon buildup. Clean with solvent and soft brush.
2. Carburetor Adjustment: Use tachometer to set idle (L) screw to 2800 rpm and high-speed (H) screw to 11500 rpm.
3. Ignition Timing: Fixed timing; check flywheel key for shearing.
4. Compression Test: Should read at least 90 psi. Low compression indicates worn piston rings or cylinder.
5. Muffler: Remove and clean spark arrestor screen regularly to prevent overheating.

CAUTION! Engine components are precision parts. Use proper tools and torque specifications.

Fuel System

Maintaining a clean fuel system is critical for reliable operation.

1. Fuel Tank: Drain and clean every season. Inspect for cracks.
2. Fuel Filter: Replace if clogged. Located inside fuel tank.
3. Fuel Lines: Check for cracks or hardening. Replace if necessary.
4. Carburetor Diaphragm: Inspect for tears or stiffness. Replace diaphragm kit if fuel delivery is poor.
5. Fuel Mix: Always use fresh, unleaded gasoline (min. 89 octane) mixed with Stihl 2-stroke oil at 50:1 ratio.

Tip: Use a fuel stabilizer if storing the trimmer for more than 30 days.

Ignition System

The ignition system provides the spark for combustion.

1. Spark Plug: Remove and inspect electrode. Clean or replace if worn. Set gap to 0.5 mm.
2. Ignition Module: Test with multimeter. Primary resistance: 0.5-1.5 Ω; secondary: 5-15 kΩ.
3. Stop Switch: Ensure it grounds the ignition module when engaged.
4. Flywheel: Check for cracks and clean magnets. Torque nut to 35 Nm.
5. Air Gap: Set between flywheel and ignition module to 0.3 mm using a feeler gauge.

WARNING! Always disconnect the spark plug wire before servicing the ignition system.

Cutting Head & Drive Shaft

Proper maintenance of the cutting attachment and drive system ensures efficient cutting.

1. PolyCut Head: Replace cutting line when worn. Ensure spool is wound correctly to prevent tangling.
2. Drive Shaft: Lubricate with Stihl drive shaft grease every 25 hours of operation.
3. Gearbox: Check for grease level and wear. Replace seals if leaking.
4. Shaft Coupling: Inspect for wear or damage. Ensure it engages properly with engine output.
5. Cutting Blades: Sharpen metal blades regularly. Balance after sharpening to reduce vibration.

CAUTION! Always ensure the engine is off and cool before servicing cutting attachments.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine won't start	No spark, clogged fuel system, flooded engine	Check spark plug; clean fuel filter; hold throttle open while pulling starter.
Engine runs poorly	Dirty air filter, incorrect carburetor adjustment, fouled spark plug	Clean/replace air filter; adjust L and H screws; replace spark plug.
Excessive vibration	Unbalanced blade, bent drive shaft, loose mounting bolts	Balance or replace blade; inspect drive shaft; tighten all fasteners.
Loss of power	Clogged muffler, worn piston rings, dirty cooling fins	Clean spark arrestor; check compression; clean cylinder fins.
Cutting head doesn't spin	Broken drive shaft, stripped gearbox, engaged clutch	Inspect drive shaft; check gear teeth; ensure clutch is disengaged at idle.

Reset: For electronic ignition issues, disconnect spark plug wire for 30 seconds to reset module.

Parts Diagram

Refer to the official Stihl FS 40 parts diagram (Part No. 0000 350 4100) for exploded views and part numbers. Key assemblies include:

• Cylinder & Piston Assembly (1128 030 2001)
• Carburetor Kit (1130 007 1000)
• Ignition Module (1130 400 1301)
• PolyCut Head (4130 713 3600)
• Drive Shaft (4135 713 0600)
• Fuel Tank (4130 350 0601)

Note: Always use genuine Stihl parts to maintain warranty and performance.

Warranty & Support

Stihl FS 40 comes with a limited warranty covering defects in materials and workmanship. Warranty period varies by region (typically 1-2 years for private use). Proof of purchase required.

For technical support, parts, or service:

• Contact your authorized Stihl dealer.
• Visit the Stihl website: www.stihl.com
• Customer service hotline: 1-800-467-8445 (USA)

Disclaimer: This manual is for reference only. Stihl reserves the right to make changes without notice. Always follow the official Stihl service manual for repair procedures.