STIHL FS55 Repair Manual

The STIHL FS55 is a lightweight, professional-grade string trimmer/brushcutter designed for demanding clearing and trimming tasks. This manual provides essential information for the safe maintenance, troubleshooting, and repair of the FS55 powerhead. It covers engine specifications, fuel system, ignition, cooling, starting, drive shaft, gearhead, cutting attachments, and routine service procedures. Always refer to the official STIHL manual for complete safety and technical details.

Safety Precautions

Before performing any repair, ensure the engine is off, cool, and the spark plug boot is disconnected. Wear appropriate personal protective equipment (PPE). Use only genuine STIHL replacement parts.

Hazard	Precaution
Fuel Fire/Explosion	Work in well-ventilated area away from ignition sources. Drain fuel before certain repairs.
Hot Surfaces	Allow engine and muffler to cool completely before handling.
Sharp Tools/Cutters	Wear heavy-duty gloves. Ensure cutting attachment is secured and not under tension.
Unexpected Start-up	Always disconnect spark plug boot before disassembly.
Toxic Fumes	Do not run engine indoors. Ensure proper carburetor adjustment to minimize exhaust.

Specifications & Identification

Key identifiers and technical data for the STIHL FS55.

• Engine Type: STIHL single-cylinder, 2-stroke, air-cooled.
• Displacement: 27.2 cm³ (1.66 cu. in.).
• Power Output: Approximately 0.9 kW (1.2 bhp).
• Fuel/Oil Mix: 50:1 with STIHL HP Ultra or equivalent ISO-L-EGD (JASO FD, ISO EGD) certified 2-stroke oil.
• Ignition System: Electronic, capacitor discharge ignition (CDI).
• Starting System: STIHL ErgoStart (Easy2Start) or standard recoil.
• Carburetor: Zama C1Q-S series or equivalent.
• Idle Speed: 2,800 - 3,200 rpm.
• Maximum Power Speed: 12,500 rpm (WOT).
• Serial Number Location: Stamped on the crankcase or on a label on the rear handle.

Maintenance Schedule

Regular maintenance is critical for performance and longevity.

1. Before Each Use: Check for loose fasteners, damage, and fuel leaks. Inspect cutting attachment.
2. Daily/After Use: Clean exterior, check air filter, clean cooling fins.
3. Every 3 Months or 50 Hours: Clean/replace air filter, check spark plug, clean spark arrestor screen (if equipped).
4. Annually or 100 Hours: Replace fuel filter, check fuel lines, clean carburetor (if needed), check gearhead grease.
5. As Needed: Replace drive shaft, gearhead bearings, piston/cylinder, clutch.

WARNING! Use only STIHL-approved lubricants and parts. Improper maintenance voids warranty.

Fuel System

Components: Fuel tank, cap, filter, lines, carburetor.

1. Fuel Filter Replacement: Drain tank. Use hooked wire to extract filter and line. Attach new filter to new line, feed back into tank.
2. Fuel Line Replacement: Remove carburetor cover and air filter. Disconnect lines from carburetor and tank grommet. Install new lines, ensuring secure connections.
3. Carburetor Adjustment (Basic): With engine warm, adjust idle speed screw (LA) to achieve stable idle (2,800-3,200 rpm). High-speed screw (H) is factory-set; fine-tune only if necessary for smooth high-speed operation.
4. Carburetor Cleaning/Repair Kit: Requires disassembly, cleaning jets and diaphragms with carb cleaner, and reassembly with kit.

CAUTION! Fuel is flammable. Work in a well-ventilated area.

Ignition System

Components: Flywheel, ignition module (coil), spark plug, stop switch, wiring.

1. Spark Plug Check/Replacement: Use STIHL recommended plug (e.g., BOSCH WSR6F or equivalent). Gap: 0.5 mm (0.020"). Check for fouling, cracks, or incorrect gap.
2. Ignition Module Testing: Check for spark with spark tester. No spark? Check stop switch circuit (disconnect wire from module). If still no spark, measure coil resistance (primary: ~0.2-2 Ω, secondary: ~5-20 kΩ) or replace module.
3. Air Gap Setting: Gap between ignition module poles and flywheel magnets should be 0.2-0.4 mm (0.008-0.016"). Use feeler gauge.

Tip: A weak or no spark is a common cause of no-start conditions.

Engine & Cylinder

Major components: Crankcase, crankshaft, connecting rod, piston, cylinder, bearings, seals.

1. Compression Test: Use compression gauge. Should be over 100 psi (healthy). Low compression indicates worn piston/rings, cylinder scoring, or leaking crank seals.
2. Piston/Cylinder Inspection: Remove muffler to view piston through exhaust port. Look for scoring, aluminum transfer (seizure), or broken ring. Severe damage requires cylinder kit replacement.
3. Crankcase Seal Replacement: Leaking seals cause air leaks, leading to lean run condition and engine damage. Requires splitting crankcase.
4. Decompression Valve: If equipped, ensure it seals properly when closed. Can cause hard starting if leaking.

WARNING! Engine disassembly requires special tools and expertise. Consider professional service for major repairs.

Cooling System

Components: Cylinder fins, fan wheel, fan housing, deflectors.

Ensure cooling fins on cylinder head are not clogged with grass, dirt, or debris. Clean with a brush or compressed air. Check that the fan wheel on the flywheel is intact and the fan housing is not cracked. Overheating, indicated by loss of power, metallic pinging, or seizure, is often caused by a dirty cooling system or improper carburetion (too lean).

Starting System

Components: Starter pulley, rope, spring, pawls, ErgoStart mechanism (if equipped).

1. Recoil Rope Replacement: Wind spring carefully. Route rope correctly through pulley and handle.
2. ErgoStart (Easy2Start): This mechanism reduces pulling force. If it fails, check the starter pawls and spring inside the flywheel cup for wear or breakage.
3. Starter Engagement: Pawls should engage flywheel dogs cleanly. Clean and lubricate with light grease if sticky.

CAUTION! The recoil spring is under tension. Wear eye protection and gloves when servicing.

Drive Shaft & Gearhead

Transfers power from engine to cutting attachment.

1. Drive Shaft Removal: Remove gearhead. Withdraw shaft from tube. Inspect for kinks, corrosion, or damaged flex cable.
2. Gearhead Greasing: Remove plug, fill with STIHL gear grease until it emerges from the bearing. Reinstall plug.
3. Bearing Replacement: Worn bearings cause vibration and noise. Press out old bearings and press in new ones.
4. Clutch Inspection: The centrifugal clutch should engage smoothly around 2,800-3,200 rpm. Worn shoes or broken springs require clutch replacement.

Cutting Attachments

Supports trimmer heads and brush knives.

• Trimmer Line Heads: Ensure correct line diameter is used. Replace worn bump feed mechanisms or spools.
• Metal Blades: Inspect for cracks, nicks, and balance. Sharpen with a file, maintaining original angle. Always use correct blade washer and nut tightening procedure.
• Attachment Installation: Use the correct combination of washers and the shoulder nut. Secure with the combination wrench, holding the shaft to prevent rotation.

WARNING! Always ensure the cutting attachment is properly installed, balanced, and secured before operation.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No spark, no fuel, flooded	Check spark, fuel flow, carburetor. Dry plug if flooded.
Engine starts then dies	Clogged fuel filter, dirty air filter, carb adjustment	Replace fuel filter, clean air filter, adjust carburetor.
Loss of power at high speed	Clogged spark arrestor, dirty air filter, exhaust port carbon	Clean spark arrestor screen, air filter, decarbonize exhaust port.
Excessive vibration	Bent drive shaft, unbalanced blade, loose fasteners	Inspect/replace drive shaft, balance blade, tighten all fasteners.
Clutch slipping/not engaging	Worn clutch shoes, oily clutch drum, weak springs	Replace clutch assembly, clean drum.
Gearhead overheating/noisy	Low grease, worn bearings	Repack with grease, replace bearings.

Service Note: For complex repairs, consult an authorized STIHL servicing dealer.

STIHL Support: Contact an authorized STIHL dealer or visit www.stihlusa.com for service manuals and parts diagrams.