STIHL FS 90 Repair Manual

The STIHL FS 90 is a professional-grade, lightweight trimmer/brushcutter designed for demanding clearing and trimming work. This manual provides detailed repair, maintenance, and troubleshooting information for the FS 90, including its 2-stroke engine, fuel system, ignition, cutting attachments, and safety systems. Below are key sections covering safety, specifications, disassembly, component repair, assembly, and troubleshooting procedures.

1. Safety Information & Precautions 2. Technical Specifications 3. Tools & Workshop Requirements 4. Engine Disassembly Procedure 5. Fuel System Repair 6. Ignition System Service 7. Cutting Head & Drive Shaft 8. Carburetor Adjustment & Repair 9. Reassembly & Torque Specifications 10. Maintenance Schedule 11. Troubleshooting Guide

Safety Information & Precautions

Before performing any repair, read all safety warnings. Disconnect the spark plug boot. Allow engine to cool. Work in a clean, well-ventilated area. Use STIHL-approved tools and replacement parts.

Hazard	Precaution
Fuel Fire/Explosion	Drain fuel tank before repair. No sparks or flames nearby.
Sharp Cutting Tools	Wear heavy-duty gloves. Secure cutting attachment.
Hot Surfaces	Allow muffler and cylinder to cool completely.
Spring-Loaded Components	Use appropriate tools; parts may eject under tension.
Chemical Exposure	Use gloves when handling fuel, oil, or solvents.

Technical Specifications

Model: STIHL FS 90
Engine Type: Single cylinder, 2-stroke, air-cooled
Displacement: 27.2 cm³
Power Output: 0.9 kW / 1.2 hp
Fuel/Oil Mix: 50:1 (STIHL HP Ultra or equivalent)
Ignition System: Electronic, breakerless
Carburetor: STIHL Zama diaphragm carburetor
Starting System: Rewind starter
Drive Shaft: Solid, with anti-vibration system
Standard Cutting Attachment: STIHL AutoCut C 5-2 head
Weight (powerhead): 5.4 kg

Tools & Workshop Requirements

Required tools for most repairs: Torx screwdrivers (T27), socket set (8mm-13mm), feeler gauges, spark plug wrench, circlip pliers, piston stop tool, flywheel puller (STIHL 1122 893 4500), crankshaft seal installer.

Clean workspace free of debris.
Organize parts using labeled containers.
Use genuine STILL service tools for critical operations (e.g., flywheel removal).
Have service data (torque specs, clearances) readily available.

WARNING! Using incorrect tools can damage components and void warranty.

Engine Disassembly Procedure

Step-by-step teardown of the FS 90 powerhead.

Remove cutting attachment and drive shaft cover.
Drain fuel tank and remove fuel line from carburetor.
Remove starter assembly and fan housing.
Remove muffler and cylinder cover.
Secure piston at Top Dead Center (TDC) using piston stop tool.
Remove clutch assembly and flywheel (using puller).
Remove cylinder head and cylinder.
Remove connecting rod and crankshaft from crankcase (requires case separation).

CAUTION! Mark all parts for orientation during reassembly. Do not force components.

Fuel System Repair

Diagnosis and repair of fuel lines, tank, filter, and pump.

Common Issues: Fuel leaks, hard starting, poor performance.

Inspect fuel lines for cracks, hardening, or leaks. Replace if damaged.
Replace in-tank fuel filter annually or if clogged.
Check fuel tank vent for blockage; clean or replace.
Test fuel pump (integrated with carburetor): Apply light vacuum to impulse line; fuel should flow.
Replace complete fuel line set (STIHL part #xxxx) as an assembly if multiple lines are aged.

Ignition System Service

Testing and replacement of spark plug, ignition module, and stop switch.

Component	Test Procedure	Specification
Spark Plug	Check electrode gap, condition	Gap: 0.5 mm (0.020 in). Replace if fouled/worn.
Ignition Module	Check for spark with tester	Strong blue spark required. No spark indicates faulty module.
Stop Switch Wire	Continuity test to ground	Wire should show continuity when switch is pressed.
Flywheel Magnets	Visual inspection	No cracks or debris. Clean with soft cloth.

Note: Ignition module air gap to flywheel: 0.3 mm (0.012 in).

Cutting Head & Drive Shaft

Service for the AutoCut head, trimmer line, drive shaft, and gearhead.

Line Head: Tap to advance line. Replace spool when empty or damaged.
Drive Shaft: Inspect for bends or damage. Lubricate with STIHL shaft grease during reassembly.
Gearhead: Check for worn gears, leaking seals. Refill with high-temperature grease.
Anti-Vibration Mounts: Inspect rubber dampers for tears; replace if hardened or cracked.

Tip: Use only genuine STIHL cutting attachments and replacement line for optimal performance and safety.

Carburetor Adjustment & Repair

Service the Zama carburetor: cleaning, diaphragm replacement, and adjustment.

Standard Screw Settings (initial): LA: 1 turn open. H: 1 turn open. L: 1 turn open.

Remove carburetor from engine.
Disassemble: Remove cover, diaphragms (pump and metering), gaskets, needle valve.
Clean all parts in ultrasonic cleaner or carburetor cleaner. Blow out passages with compressed air.
Replace diaphragm and gasket kit. Ensure lever height is set correctly.
Reinstall and adjust at operating temperature: Set idle speed (~2800 RPM), adjust L for smooth acceleration, adjust H for max RPM (13,000 RPM).

Reassembly & Torque Specifications

Critical torque values for engine and drive components.

Component	Torque	Notes
Cylinder Head Nuts	20 Nm (15 ft-lb)	Tighten in criss-cross pattern
Clutch	40 Nm (30 ft-lb)	Use piston stop, left-hand thread
Flywheel Nut	40 Nm (30 ft-lb)	Use flywheel holder
Muffler Screws	10 Nm (7.5 ft-lb)	Replace gasket
Spark Plug	25 Nm (18 ft-lb)	Do not over-tighten

Assembly Tip: Lubricate piston rings, cylinder wall, and bearings with 2-stroke oil during assembly.

Maintenance Schedule

Daily: Check for loose fasteners, clean air filter, inspect cutting attachment.
Every 50 Hours: Clean spark arrestor screen, check fuel filter, lubricate drive shaft.
Every 100 Hours: Replace spark plug, clean carburetor, check ignition system.
Annually or 300 Hours: Decarbonize exhaust port and muffler, replace fuel lines, inspect crankshaft seals.
As Needed: Replace worn cutting line, damaged anti-vibration mounts, worn gearhead grease.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine will not start	No spark, flooded, fuel issue	Check spark, dry plug, verify fuel flow and mix.
Engine starts then dies	Clogged fuel filter, tank vent, carb issue	Replace filter, clean vent, service carburetor.
Loss of power	Clogged air filter, exhaust screen, lean/rich mix	Clean/replace filter and screen, adjust carburetor.
Excessive vibration	Bent drive shaft, unbalanced cutting head, worn mounts	Inspect shaft, balance head, replace mounts.
Cutting head does not turn	Sheared drive pin, clutch slipping, shaft disconnect	Replace shear pin, inspect clutch, reconnect shaft.
Oil leaking from gearhead	Worn seals, overfilled grease	Replace seals, clean and refill with correct amount.

Final Check: After repair, run engine at idle and mid-range to verify operation before full load.

STIHL Service Support: For technical assistance or genuine parts, contact an authorized STIHL servicing dealer or visit www.stihlusa.com.