Stihl Trimmer FS 45 Repair Manual
The Stihl FS 45 is a lightweight, straight-shaft grass trimmer designed for residential use. This manual provides detailed repair and maintenance procedures for the FS 45, including troubleshooting, disassembly, component replacement, and reassembly. Key sections cover safety, specifications, engine repair, fuel system, ignition system, cutting head, gearbox, and routine maintenance.
Safety Precautions
Before performing any repair, ensure the trimmer is off and cool. Disconnect the spark plug boot to prevent accidental starting. Work in a well-ventilated area. Use proper tools and personal protective equipment (gloves, safety glasses).
| Hazard | Precaution |
|---|
| Fuel Fire/Explosion | Drain fuel tank before repair. No smoking or open flames. |
| Sharp Components | Wear cut-resistant gloves when handling cutting head or line. |
| Hot Surfaces | Allow engine to cool completely before disassembly. |
| Spring Tension | Note orientation of springs (starter, throttle) during disassembly. |
| Chemical Exposure | Use gloves when handling fuel, oil, or cleaning solvents. |
Specifications & Identification
Identify your unit via serial number plate on the shaft housing.
- Model: FS 45
- Engine Type: 2-stroke, air-cooled
- Displacement: 27.2 cc
- Fuel/Oil Mix: 50:1 (Stihl HP Ultra or equivalent)
- Powerhead Weight: Approx. 4.6 kg (10.1 lbs)
- Shaft Type: Straight, solid drive shaft
- Cutting Attachment: Standard bump-feed trimmer head
- Ignition: Electronic, capacitor discharge (CDI)
- Carburetor: Diaphragm type (Zama or Walbro)
WARNING! Use only genuine Stihl parts or approved equivalents for repair.
Troubleshooting Guide
Systematic approach to diagnose common FS 45 problems.
| Symptom | Possible Cause | Check/Repair |
|---|
| Engine won't start | No spark, fuel issue, flooded | Check spark plug, fuel flow, air filter. Pull cord with throttle open to clear flood. |
| Engine starts then dies | Clogged fuel filter, carb adjustment | Inspect fuel lines and filter. Adjust idle and high-speed screws per manual. |
| Loss of power | Clogged air filter, exhaust screen, bad piston rings | Clean/replace filter and spark arrestor. Check compression. |
| Excessive vibration | Bent shaft, unbalanced cutting head, loose parts | Inspect shaft for straightness. Balance or replace head. Tighten all fasteners. |
| Line not feeding | Worn head, tangled line, dirty bump mechanism | Disassemble head, clean, replace spool if worn. |
| Gearbox overheating | Low grease, damaged gears, misalignment | Remove gearbox, inspect gears, repack with Stihl grease. |
Routine Maintenance
Regular maintenance prevents major repairs.
- Daily/Pre-use: Check for loose fasteners, inspect cutting attachment, verify throttle operation.
- After each use: Clean exterior, remove debris from cooling fins and air intake.
- Every 25 hours: Clean/replace air filter. Check spark plug condition and gap (0.020 in / 0.5 mm).
- Every 50 hours: Clean fuel filter. Inspect fuel lines for cracks. Check gearbox grease.
- Every 100 hours: Decarbonize exhaust port and spark arrestor screen. Check piston through exhaust port for scoring.
CAUTION! Always use a fuel stabilizer if storing for more than 30 days. Run engine dry before long-term storage.
Fuel System Repair
Components: Fuel tank, cap, lines, filter, carburetor.
- Drain Tank: Remove fuel cap, tip unit to drain into approved container.
- Replace Fuel Filter: Pull old filter from line inside tank. Attach new filter, ensuring line is not kinked.
- Inspect Fuel Lines: Replace if cracked, hardened, or leaking. Note routing.
- Carburetor Service: Remove carb (disconnect linkages, fuel line). Clean with carb cleaner, replace diaphragms and gaskets if kit available. Set mixture screws to factory baseline: L (low speed) - 1 turn out; H (high speed) - 1 turn out. Fine-tune when running.
Tip: Take photos before disconnecting linkages for easier reassembly.
Ignition System Repair
Components: Spark plug, boot, wire, ignition module (coil).
- Test for Spark: Remove spark plug, reconnect boot, ground plug base to cylinder. Pull starter cord – look for bright blue spark.
- No Spark: Check plug condition and gap. Replace if fouled or electrode worn.
- Still No Spark: Check wire and boot for cracks. Test ignition module resistance with multimeter (refer to specs). Replace module if out of spec.
- Module Replacement: Remove starter cover, flywheel (use puller), and old module. Set air gap between module and flywheel magnets (0.3 mm / 0.012 in) using a feeler gauge.
WARNING! Never pull starter with plug disconnected and boot not grounded – can damage ignition module.
Engine Repair (Cylinder & Piston)
For low compression, scoring, or piston ring failure.
- Remove Engine Covers: Take off starter assembly, air filter housing, and carburetor.
- Remove Cylinder: Unbolt cylinder base nuts. Gently tap cylinder to break gasket seal. Lift off.
- Inspect: Check cylinder walls for scoring. Light scoring can be honed. Deep scratches require cylinder replacement. Inspect piston for cracks, ring groove wear. Check ring end gap in cylinder.
- Replace Piston & Rings: Remove circlips from piston pin. Drive out pin (support piston). Install new rings with gap aligned to pin. Lubricate with 2-stroke oil. Reinstall cylinder with new base gasket.
- Torque: Cylinder base nuts: 10 Nm (88 in-lb).
CAUTION! Label all parts. Keep work area clean to avoid debris entering engine.
Gearbox & Drive Shaft
For vibration, noise, or loss of drive.
- Remove Cutting Head: Hold head, turn locknut clockwise (reverse thread) to remove.
- Separate Shaft: Remove screws at gearbox housing. Pull powerhead away from shaft.
- Inspect Drive Shaft: Check for bends or damage. Replace if bent.
- Gearbox Disassembly: Unbolt gearbox halves. Inspect gears for wear or missing teeth. Check bearings for smooth rotation.
- Repack Grease: Clean old grease. Pack with Stihl gear grease (approx. 15g). Reassemble, ensuring seals are seated.
Tip: Apply thread locker to gearbox screws on reassembly.
Cutting Head & Trimmer Line
Bump-feed head maintenance and line replacement.
- Disassemble Head: Depress tabs, remove cover. Remove spool and spring.
- Clean: Remove debris, check for worn parts.
- Reload Line: Use correct diameter (typically 2.0 mm). Wind line evenly onto spool in direction of arrow. Leave 15-20 cm tails.
- Reassemble: Replace spring, spool, and cover. Ensure tabs click into place.
- Adjustment: If line doesn't feed, check bump mechanism for smooth operation.
WARNING! Always use genuine Stihl trimmer line. Incorrect line can cause overheating and head failure.
Reassembly & Final Checks
After repair, reassemble in reverse order of disassembly.
- Ensure all gaskets and seals are new and properly seated.
- Tighten fasteners to specified torque values.
- Reconnect all linkages, cables, and fuel lines.
- Refill with fresh fuel mix (50:1).
- Start & Test: Start engine, allow to warm. Check idle stability. Test throttle response. Engage cutting head to verify smooth operation and no vibration.
- Check for leaks (fuel, exhaust).
Final Safety Check: Verify stop switch functions, guard is secure, and all covers are fastened.
Parts List & Diagrams
Refer to official Stihl FS 45 IPL (Illustrated Parts List) for part numbers and exploded views. Common replacement parts:
- Spark Plug: BPMR7A (NGK) or equivalent.
- Air Filter: Stihl Part # XXXX
- Fuel Filter: Stihl Part # XXXX
- Carburetor Repair Kit: Zama C1U or Walbro K10-WAT kit (confirm model).
- Piston Ring Set: Stihl Part # XXXX
- Trimmer Head Spool: Stihl Part # XXXX
Support: For technical assistance or parts, contact an authorized Stihl dealer or visit www.stihlusa.com.
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