Stihl FS 80 Repair Manual

The Stihl FS 80 is a professional-grade trimmer/brushcutter designed for demanding applications. This repair manual provides comprehensive instructions for troubleshooting, maintenance, and repair of the FS 80 powerhead, including its engine, fuel system, ignition system, cutting attachments, and drive shaft. Below are key sections covering safety, specifications, disassembly, component repair, assembly, and troubleshooting procedures.

1. Safety Precautions 2. Specifications & Identification 3. Tools & Special Equipment 4. Engine Disassembly 5. Fuel System Repair 6. Ignition System Service 7. Drive Shaft & Gearhead 8. Cutting Attachments 9. Reassembly & Adjustment 10. Troubleshooting Guide 11. Torque Specifications

Safety Precautions

Before attempting any repair, ensure the unit is completely cool and the spark plug boot is disconnected. Work in a clean, well-ventilated area. Use proper personal protective equipment (PPE) including safety glasses and gloves.

Hazard	Precaution
Fuel Fire/Explosion	Drain fuel tank before service. No smoking or open flames.
Sharp Components	Handle cutting blades and trimmer heads with extreme care.
Spring Force	Use caution when handling recoil starter assembly.
Chemical Exposure	Use gloves when handling fuel, oil, or cleaning solvents.
Unexpected Start-up	Always disconnect spark plug wire and secure it away from plug.

Specifications & Identification

Identify your unit using the serial number plate on the crankcase.

Engine Type: STIHL single-cylinder, 2-stroke, air-cooled.
Displacement: 30.1 cc (1.84 cu in).
Power Output: 1.3 kW (1.8 hp).
Fuel/Oil Mix: 50:1 (STIHL HP Ultra or equivalent ISO-L-EGD).
Idle Speed: 2,800 RPM.
Max Power Speed: 9,500 RPM.
Ignition System: Electronic, capacitor discharge (CDI).
Carburetor: Zama C1Q-S series (diaphragm type).
Drive Shaft: Solid steel, with flexible coupling.
Standard Cutting Attachment: PolyCut head 2-2 or solid drive shaft for blade.

Tools & Special Equipment

Required tools: Standard metric socket set, Torx drivers (T27), feeler gauges, spark plug wrench, piston stop tool (Stihl 5910 890 4500), flywheel puller (Stihl 5910 890 4003), crankshaft seal installer, carburetor adjustment tool (Stihl 5910 890 2200), compression tester, and a clean parts tray.

NOTE: Use genuine Stihl tools or equivalents to prevent damage.

Engine Disassembly

Follow this sequence for complete engine teardown.

Remove cutting attachment and drive shaft housing.
Remove fuel tank and air filter cover/carburetor assembly.
Remove muffler and cylinder cover.
Secure crankshaft using piston stop tool; remove flywheel nut and pull flywheel.
Remove ignition module (coil).
Remove cylinder base nuts; carefully separate cylinder from crankcase.
Remove piston pin circlips, heat piston, and drive out piston pin to remove piston.
Remove crankcase halves by separating after removing all fasteners.

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

Fuel System Repair

Common issues: hard starting, poor performance, flooding.

Carburetor Service: Disassemble carburetor. Clean all jets, channels, and diaphragms in ultrasonic cleaner. Inspect diaphragms for stiffness or tears. Replace complete kit (diaphragms, gaskets, needle valve).
Fuel Lines & Filter: Replace cracked or stiff fuel lines. Replace in-tank fuel filter.
Fuel Tank Vent: Check tank vent for blockage; replace if necessary.
Adjustment: After reassembly, set initial carburetor screws: LA (idle) = 1 turn open, H (high speed) = 1 turn open. Fine-tune with engine at operating temperature.

Ignition System Service

Check for spark first. Remove spark plug, reconnect boot, ground plug base to cylinder, pull starter rope. Look for strong blue spark.

No Spark: Check/replace spark plug (BPMR7A). If still no spark, test ignition coil resistance (primary: ~0.5-2 Ω, secondary: ~5-15 kΩ). Replace coil if out of spec. Check stop switch wire for short to ground.
Weak/Intermittent Spark: Check flywheel magnets for strength and cleanliness. Check air gap between coil legs and flywheel (0.3-0.4 mm).
Flywheel Key: Inspect Woodruff key for shearing; replace if damaged to maintain correct timing.

Drive Shaft & Gearhead

To service the drive shaft or gearhead, remove the shaft from the powerhead.

Remove the clutch drum and clutch assembly.
Withdraw the drive shaft from the shaft tube.
Inspect shaft for straightness and spline wear.
For gearhead service (if equipped), disassemble to inspect gears, bearings, and seals. Replace worn bearings and seals. Pack gearbox with Stihl grease.
Reinstall shaft, ensuring proper engagement with engine crankshaft and gearhead/clutch drum.

Tip: Lubricate drive shaft splines with anti-seize compound during reassembly.

Cutting Attachments

FS 80 supports trimmer heads and cutting blades.

PolyCut Head: To replace line, remove spool, wind new line following directional arrows, feed ends through eyelets.
Metal Blades: Use only Stihl-approved blades. Ensure blade bolt is torqued to specification (35-40 Nm) and blade carrier is secure. Always use appropriate blade guard.
Drive Shaft Coupling: Inspect flexible coupling for cracks or wear; replace if damaged to prevent vibration.

Reassembly & Adjustment

Reverse disassembly sequence. Critical steps:

Crankcase Sealing: Use new crankcase gasket and sealant (Stihl Dirko) on mating surfaces. Torque crankcase screws in a crisscross pattern.
Crankshaft Seals: Always install new seals. Lubricate lip with grease before pressing into place.
Cylinder/Piston: Lubricate piston rings and cylinder wall with 2-stroke oil. Ensure ring end gaps are spaced around piston. Torque cylinder nuts evenly.
Final Adjustments: Set carburetor. Adjust idle speed to 2,800 RPM. Check clutch engagement (engages around 3,800-4,200 RPM).

CAUTION! Do not over-torque fasteners, especially in magnesium crankcase.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine won't start	No spark, no fuel, flooded	Check spark, fuel delivery, dry plug. Check carburetor diaphragms.
Engine starts then dies	Fuel starvation, air leak	Check fuel filter, lines, tank vent. Pressure test crankcase for leaks.
Loss of power	Clogged exhaust, bad piston/cylinder, air leak	Decarbonize muffler. Check compression (should be >120 psi). Inspect for scored cylinder.
Excessive vibration	Bent drive shaft, unbalanced blade, loose components	Inspect shaft and blade. Tighten all mounting fasteners.
Clutch slipping/not engaging	Worn clutch shoes, weak springs, grease contamination	Replace clutch assembly. Clean clutch drum.
Overheating	Lean fuel mixture, clogged cooling fins, insufficient lubrication	Check carburetor adjustment. Clean cylinder fins. Ensure correct fuel/oil mix.

Torque Specifications

Critical fastener torques (Nm):

Cylinder nuts: 10 Nm
Crankcase screws: 8 Nm
Flywheel nut: 35 Nm
Clutch: 35-40 Nm (left-hand thread)
Spark plug: 20-25 Nm
Muffler bolts: 8 Nm
Gearhead bolts (if applicable): 12 Nm

Reset: After major repair, allow engine to break in at varied throttle for first tank of fuel.

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