The Stihl FS 55 is a lightweight, professional-grade string trimmer/brushcutter designed for demanding use. This manual provides comprehensive repair, maintenance, and troubleshooting information for the FS 55 model, including its 2-stroke engine (FS 55, FS 55 R), fuel system, ignition, cutting head, drive shaft, and safety systems. Below are key sections covering safety, specifications, disassembly, component repair, assembly, and troubleshooting procedures.
Critical safety procedures must be followed before any repair work. The FS 55 contains sharp components, flammable fuel, and a high-speed engine.
| Hazard | Precaution |
|---|---|
| Fuel Fire/Explosion | Work in well-ventilated area; no sparks or flames; drain fuel before disassembly. |
| Sharp Cutting Tools | Wear cut-resistant gloves; secure cutting head; disconnect spark plug. |
| Hot Surfaces | Allow engine to cool completely before handling. |
| Spring-Loaded Components | Use appropriate tools; wear eye protection during disassembly. |
| Chemical Exposure | Use gloves when handling fuel, oil, or cleaning solvents. |
WARNING! Always disconnect the spark plug cap to prevent accidental starting during repair.
Specialized Stihl tools and standard mechanics tools are required for proper repair.
CAUTION! Use only Stihl-approved replacement parts to maintain performance and safety.
Follow this sequence to avoid damage and ensure proper reassembly.
Tip: Take photos during disassembly and organize screws by location.
Common issues include clogged filters, leaking lines, and faulty carburetor diaphragms.
| Component | Inspection | Replacement Procedure |
|---|---|---|
| Fuel Tank | Check for cracks, vent operation | Replace tank if cracked; clean vent with compressed air. |
| Fuel Lines | Check for brittleness, leaks | Replace both supply and return lines; ensure proper routing. |
| Fuel Filter | Inspect for debris | Replace if dirty; always use genuine Stihl filter. |
| Carburetor | Check diaphragms, gaskets, needle valve | Rebuild with OEM kit; clean all jets with solvent. |
WARNING! Never use compressed air on carburetor diaphragms as they can tear.
The electronic ignition module (coil) and spark plug are key components.
Note: A weak spark is often caused by incorrect air gap or faulty ignition module.
Maintenance of the cutting head and drive shaft ensures optimal cutting performance.
CAUTION! Never operate trimmer without cutting attachment or debris shield installed.
Proper adjustment is critical for performance and engine longevity.
Initial Settings (screws turned gently to stop):
Fine Tuning Procedure:
Critical torque values must be observed to prevent damage.
| Component | Torque | Notes |
|---|---|---|
| Crankcase Screws | 10 Nm (88 in-lb) | Tighten in criss-cross pattern |
| Cylinder Base Nuts | 20 Nm (177 in-lb) | Apply copper anti-seize |
| Muffler Bolts | 15 Nm (133 in-lb) | Replace gasket |
| Clutch Nut | 40 Nm (30 ft-lb) | Right-hand thread; use holder tool |
| Flywheel Nut | 35 Nm (26 ft-lb) | Apply thread locker |
Tip: Apply Stihl assembly lubricant to crankshaft bearings and seals during reassembly.
Regular maintenance prevents major repairs and extends equipment life.
| Symptom | Possible Cause | Corrective Action |
|---|---|---|
| Engine won't start | No spark, flooded, fuel issue | Check spark plug; dry cylinder; verify fuel flow to carb. |
| Engine starts then dies | Clogged fuel filter, vent issue | Replace fuel filter; check tank vent. |
| Loss of power | Clogged air filter, exhaust screen | Clean/replace air filter; clean spark arrestor. |
| Excessive vibration | Bent drive shaft, unbalanced head | Inspect shaft; replace if bent. Balance cutting head. |
| Cutting head won't feed line | Worn bump head, tangled line | Replace bump head; rewind spool correctly. |
| Clutch slipping | Worn shoes, grease contamination | Replace clutch assembly; clean drum. |
Reset: For electronic issues, disconnect spark plug for 1 minute to reset ignition module.
Service Support: Authorized Stihl dealers only. Use OEM parts (1-800-467-8445).