The Stihl FS66 is a professional-grade, lightweight trimmer/brushcutter designed for demanding clearing and trimming tasks. This manual provides comprehensive repair, maintenance, and troubleshooting information for the FS66 engine, fuel system, ignition, cutting head, drive shaft, and gearbox. Below are key sections covering safety, specifications, disassembly, component repair, assembly, and troubleshooting procedures.
Critical safety information for handling fuel, working on ignition systems, and using cutting tools.
| Symbol/Area | Precaution |
|---|---|
| Fuel Handling | Work in well-ventilated area; no open flames or sparks. |
| Ignition System | Disconnect spark plug boot before any repair to prevent accidental start. |
| Cutting Tools | Wear protective gloves; ensure cutting attachment is stopped and disconnected. |
| Hot Surfaces | Allow engine and muffler to cool completely before disassembly. |
| Sharp Edges | Handle cutting blades and metal parts with care to avoid injury. |
| Pressurized Systems | Depressurize fuel tank by opening cap slowly before servicing. |
Required tools for FS66 repair: Standard metric wrench/socket set, Torx bits, feeler gauges, compression tester, spark tester, carburetor adjustment tool, gearbox grease, locking compound, torque wrench.
Special Tools: Stihl piston stop tool (for flywheel removal), flywheel puller, clutch removal tool, gearbox bearing press tool.
WARNING! Label all parts during disassembly. Replace gaskets and seals during reassembly.
Components: Fuel tank, filter, lines, carburetor (Zama C1Q-S series).
CAUTION! Use only ethanol-resistant fuel lines. Clean fuel tank if contaminated.
Components: Spark plug (Bosch WSR6F or equivalent), ignition module (coil), stop switch, wiring.
Tip: Weak or no spark often indicates failed ignition module or improper gap.
Drive shaft connects engine to gearbox. Cutting attachments: AutoCut C5-2 trimmer head, circular saw blades, brush knives.
WARNING! Always use Stihl-approved cutting attachments and genuine replacement parts.
Gearbox reduces speed and increases torque. Centrifugal clutch engages at ~2800 rpm.
CAUTION! Do not run gearbox without grease. Ensure seals are properly seated to prevent leaks.
Critical steps for reassembly and final adjustment.
| Interval | Task |
|---|---|
| Daily/Before Use | Check for loose fasteners; inspect cutting attachment; clean air filter. |
| 25 Hours | Clean spark arrestor screen; check fuel filter; lubricate drive shaft. |
| 50 Hours | Replace spark plug; clean carburetor air passage; check clutch shoes. |
| 100 Hours | Replace fuel lines; clean/replace carburetor kit; inspect gearbox grease. |
| Annual | Full inspection: crankshaft seals, piston rings, gearbox bearings. |
| Symptom | Possible Cause | Corrective Action |
|---|---|---|
| Engine won't start | No spark, fuel issue, compression | Check spark, fuel flow, compression (should be >100 psi). |
| Engine starts then dies | Clogged fuel filter, carburetor diaphragm | Replace fuel filter; rebuild carburetor. |
| Loss of power | Clogged air filter, exhaust port carbon, ignition timing | Clean/replace air filter; decarbon exhaust; check flywheel key. |
| Excessive vibration | Bent drive shaft, unbalanced blade, loose mounting | Inspect shaft; balance or replace blade; tighten all mounts. |
| Gearbox overheating | Low grease, worn bearings, misalignment | Refill with grease; replace bearings; check shaft alignment. |
| Clutch slipping | Worn shoes, weak springs, grease contamination | Replace clutch assembly; clean clutch drum. |
Reset: After carburetor rebuild, reset screws to initial settings (1 turn out each).
Service Support: For genuine Stihl parts and technical assistance, contact authorized Stihl dealers or visit www.stihlusa.com.