Stihl FS55R Repair Manual

The Stihl FS55R is a professional-grade, lightweight string trimmer/brushcutter designed for demanding residential and commercial use. This manual provides comprehensive repair, maintenance, and troubleshooting information for the FS55R model, including its 2-stroke engine (FS 55 R), fuel system, ignition system, cutting head, drive shaft, and safety features. Below are key sections covering safety, specifications, disassembly, component repair, assembly, and troubleshooting procedures.

1. Safety Precautions & Warnings 2. Technical Specifications 3. Tools & Equipment Required 4. Engine System Repair 5. Fuel System Repair 6. Ignition System Repair 7. Drive Shaft & Gearhead Repair 8. Cutting Head & Trimmer Line Repair 9. Carburetor Adjustment & Tuning 10. Reassembly & Final Checks 11. Troubleshooting Guide

Safety Precautions & Warnings

Before attempting any repair, ensure the unit is completely cool, the spark plug boot is disconnected, and all fuel has been drained. Always work in a well-ventilated area.

Hazard	Precaution
Fuel Fire/Explosion	Drain fuel tank before disassembly. Keep away from sparks, flames, or heat sources.
Sharp Components	Wear cut-resistant gloves when handling cutting blades, trimmer line, or drive shaft.
Spring Tension	The starter recoil spring is under high tension. Use proper tools and follow specific procedures for removal.
Chemical Exposure	Use safety glasses and gloves when handling fuel, oil, or cleaning solvents.
Electrical Shock	Disconnect spark plug boot and secure it away from the plug before any work.

Technical Specifications

Model: STIHL FS 55 R
Engine Type: Single-cylinder, 2-stroke, air-cooled
Displacement: 27.2 cc (1.66 cu. in.)
Power Output: 0.8 kW (1.1 bhp)
Fuel/Oil Mix: 50:1 (STIHL HP Ultra or equivalent ISO-L-EGD certified oil)
Fuel Tank Capacity: 0.53 liters (18 fl. oz.)
Ignition System: Electronic, capacitor discharge (CDI)
Starting System: Manual recoil starter
Drive System: Flexible drive shaft with bevel gearhead
Cutting Attachment: STIHL AutoCut C5-2 cutting head (standard), accepts trimmer line and optional blades
Weight (without cutting attachment): Approx. 5.2 kg (11.5 lbs)

Tools & Equipment Required

Basic mechanic's toolset plus STIHL specialty tools recommended for efficient repair.

STIHL Spark Plug Wrench
STIHL Flywheel Holder (or equivalent)
STIHL Piston Stop Tool
Torx screwdriver set (T10, T15, T20, T25, T27)
Socket set (8mm, 10mm, 13mm)
Flathead and Phillips screwdrivers
Needle-nose pliers & Circlip pliers
Carburetor adjustment tool (splined)
Compression tester & Vacuum/pressure tester for fuel system
Feeler gauges (for spark plug gap)

WARNING! Use only STIHL-approved replacement parts to maintain performance and safety certifications.

Engine System Repair

Covers cylinder, piston, crankshaft, and connecting rod inspection and replacement.

Cylinder & Piston Inspection: Remove muffler to inspect cylinder walls for scoring. Check piston rings for free movement and wear.
Crankshaft Seal Replacement: Critical for maintaining engine compression. Remove flywheel and clutch to access seals.
Decompression Valve: If equipped, test for leaks. Replace if valve does not seal properly.
Muffler & Spark Arrestor: Clean carbon deposits from spark arrestor screen regularly. Ensure muffler gasket is intact.

CAUTION! Label all parts and fasteners during disassembly. Take photos for reference during reassembly.

Fuel System Repair

Includes tank, filter, lines, and primer bulb service.

Component	Procedure
Fuel Tank	Inspect for cracks. Clean interior with compressed air if debris present.
Fuel Filter	Replace annually or if clogged. Use STIHL part # 0000 350 3501.
Fuel Lines	Check for brittleness or cracks. Replace both supply and return lines as a set.
Primer Bulb	Test for cracks and proper seal. Replace if it does not draw fuel.
Fuel Cap & Vent	Ensure vent is clear. Cap should seal without leaks.

Ignition System Repair

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

Spark Plug: Gap should be 0.5 mm (0.020 in). Replace if electrodes are worn or fouled. Use STIHL recommended plug.
Ignition Coil Air Gap: Set to 0.3 mm (0.012 in) using a feeler gauge between coil leg and flywheel magnet.
Stop Switch Test: Use multimeter to test for continuity. Wire should break circuit when switch is pressed.
Flywheel Key: Inspect for shearing. A sheared key will cause timing issues and prevent starting.

Drive Shaft & Gearhead Repair

Servicing the flexible drive shaft and bevel gearhead assembly.

Remove cutting attachment and drive tube covers.
Inspect drive shaft for kinks or damage. Lubricate with STIHL drive shaft grease during reassembly.
Gearhead: Check for worn gears, damaged seals, and proper grease level. Replace output shaft seal if leaking.
Bearing Inspection: Check input and output bearings for smooth rotation without play or noise.

Tip: Apply thread locker to gearhead mounting bolts as specified by STIHL.

Cutting Head & Trimmer Line Repair

Focus on the STIHL AutoCut C5-2 bump-feed head.

Head Disassembly: Remove spool and spring. Clean inside housing of debris.
Line Feed Mechanism: Ensure bump knob moves freely and engages the spool correctly.
Spool & Line Replacement: Wind .095" (2.4mm) diameter trimmer line onto spool as per directional arrows. Do not overfill.
Blade Compatibility: For brush cutting, use only STIHL-approved metal blades (e.g., 4-tooth grass blade). Ensure blade bolt is torqued to specification.

Carburetor Adjustment & Tuning

Procedures for the Zama C1Q-S carburetor common on the FS55R.

Initial Settings (if carburetor was replaced or rebuilt):
• Low-Speed Mixture Screw (L): Turn gently clockwise until seated, then open 1 turn.
• High-Speed Mixture Screw (H): Turn gently clockwise until seated, then open 1 turn.
• Idle Speed Screw (LA): Set so engine idles without clutch engagement.

Final Tuning (with warm engine):
1. Adjust idle speed (LA) so cutting attachment does not rotate.
2. Adjust Low-Speed (L) for smoothest acceleration from idle.
3. Adjust High-Speed (H) for maximum power without "four-cycling" (sputtering at full throttle under load).

WARNING! Incorrect carburetor adjustment can cause engine damage or unsafe operation.

Reassembly & Final Checks

Systematic reassembly and operational verification.

Follow disassembly steps in reverse order, referring to photos and notes.
Torque all fasteners to STIHL specifications (e.g., cylinder bolts, clutch nut).
Refill with fresh, properly mixed fuel (50:1).
Start engine and check for leaks (fuel, exhaust).
Verify throttle trigger operation, idle stability, and stop switch function.
Test under load with cutting attachment installed.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine will not start	No spark, flooded, fuel issue	Check spark plug, boot, and ignition coil. Ensure fuel is fresh and mix is correct. Check carburetor diaphragm.
Engine starts but dies	Clogged fuel filter, carburetor adjustment, air leak	Replace fuel filter. Adjust carburetor L and LA screws. Test for crankcase leaks.
Loss of power under load	Clogged air filter, exhaust port carbon, faulty spark arrestor	Clean/replace air filter. Decarbonize muffler and exhaust port. Clean spark arrestor screen.
Excessive vibration	Bent drive shaft, unbalanced cutting attachment, loose components	Inspect drive shaft. Check cutting head/blade for damage. Tighten all mounting bolts.
Cutting head does not spin	Sheared drive shaft pin, clutch slipping, gearhead failure	Inspect shear pin/clutch drum. Check clutch shoes for wear. Inspect gearhead gears.
Trimmer line does not feed	Worn bump knob, tangled line, incorrect line size	Disassemble and clean head. Replace worn parts. Use correct diameter line wound properly.

Technical Support: For genuine STIHL parts and detailed technical bulletins, contact an authorized STIHL servicing dealer or visit www.stihlusa.com.