STIHL FS 85 Repair Manual

The STIHL FS 85 is a professional-grade, lightweight trimmer/brushcutter designed for demanding clearing and trimming work. This manual provides comprehensive repair, maintenance, and troubleshooting information for the FS 85 engine and cutting head systems, including the fuel system (carburetor, fuel lines, tank), ignition system (spark plug, ignition module), air filtration, cooling system, drive shaft, gearbox, and cutting attachments. Below are key sections for safety, specifications, disassembly, component repair, assembly, and troubleshooting.

Safety & General Information

Critical safety warnings for repair work. Always disconnect the spark plug boot before beginning any repair. Work in a clean, well-ventilated area. Use genuine STIHL replacement parts.

Warning Symbol	Meaning
⚠️	General hazard alert. Read manual before proceeding.
🔥	Fire hazard. Fuel is highly flammable. No smoking.
⚡	Ignition hazard. Disconnect spark plug before service.
✋	Cut hazard. Handle cutting attachments with care.

WARNING! Failure to follow safety instructions can result in serious injury or death. The cutting attachment can start unexpectedly.

Specifications & Identification

Engine and unit specifications for the STIHL FS 85.

• Engine Type: STIHL 2-MIX engine, single cylinder, air-cooled, 2-stroke.
• Displacement: 27.2 cm³ (1.66 cu. in.)
• Power Output: 0.9 kW (1.2 bhp)
• Idle Speed: 2,800 rpm
• Max. Engine Speed: 12,500 rpm
• Fuel/Oil Mix: 50:1 with STIHL HP Ultra or equivalent ISO-L-EGD certified oil.
• Fuel Tank Capacity: 0.53 liters (18 fl. oz.)
• Gearbox Ratio: Standard gearhead.
• Drive System: Flexible drive shaft.
• Weight (without cutting attachment): Approx. 5.2 kg (11.5 lbs).
• Serial Number Location: Stamped on the crankcase.

Tools & Special Equipment

Required tools for repair. Standard metric tools plus STIHL special tools.

1. STIHL Piston Stop Tool (for flywheel/crankshaft holding).
2. Flywheel Puller (STIHL tool or equivalent).
3. Clutch Removal Tool.
4. Torque Wrench (calibrated in Nm).
5. Feeler Gauges (for spark plug gap, points if applicable).
6. Carburetor Adjustment Tool (splined).
7. Compression Gauge.
8. Vacuum/Pressure Tester (for crankcase seals).

Note: Using incorrect tools can damage components.

Fuel System Repair

Servicing the carburetor, fuel filter, lines, and tank.

1. Carburetor Removal: Remove air filter cover and filter. Disconnect fuel line and throttle linkage. Remove carburetor mounting nuts.
2. Carburetor Overhaul: Disassemble, clean all jets (H, L, idle) with compressed air. Inspect diaphragm and gaskets for cracks. Replace kit if necessary.
3. Fuel Line/Filter: Replace cracked or stiff fuel lines. Replace in-tank fuel filter annually or if clogged.
4. Fuel Tank: Inspect for cracks. Clean interior if contaminated.
5. Adjustment: Initial settings: LA screw = 1 turn open, H screw = 1 turn open. Fine-tune for smooth acceleration and peak RPM.

CAUTION! Use only ethanol-resistant fuel lines and diaphragms. Ethanol can degrade old components.

Ignition System Repair

Testing and replacing the ignition module, spark plug, and stop switch.

Spark Plug: NGK BPMR7A or equivalent. Gap: 0.5 mm (0.020 in.). Replace if electrodes are worn or fouled.

Ignition Module Test: Use an ignition tester. Correct spark should jump a 6mm gap. No spark indicates a faulty module, broken wire, or faulty stop switch.

Stop Switch Test: Disconnect wire from module. If spark returns, stop switch is faulty.

Air Gap: Between ignition module and flywheel magnets. Set to 0.3 mm (0.012 in.) with a feeler gauge.

Flywheel: Check for sheared key if timing is off. Use puller for removal; do not hammer.

Engine Top End (Cylinder & Piston)

Inspecting, measuring, and replacing the cylinder, piston, rings, and wrist pin.

1. Remove muffler and cylinder cover.
2. Remove cylinder base nuts. Carefully lift cylinder off piston.
3. Inspection: Check cylinder for scoring, transfer, or excessive wear. Measure bore diameter. Inspect piston for scuffs, ring groove wear, and pin bore.
4. Piston Ring: Check ring end gap in cylinder. Replace if over limit or broken.
5. Reassembly: Lubricate piston and rings with 2-stroke oil. Compress rings with ring compressor or fingers. Ensure arrow on piston crown points toward exhaust port.
6. Torque: Cylinder base nuts: 10 Nm (88 in. lb).

WARNING! Do not scratch piston or cylinder walls. Debris causes rapid failure.

Engine Crankcase & Crankshaft

Seal replacement, crankshaft inspection, and crankcase splitting.

Crankcase Seals: Leaking seals cause air leaks, leading to lean run and engine damage. Replace if leak is detected (pressure/vacuum test). Pry out old seals; press in new ones flush.

Crankshaft Bearings: Check for play or roughness. Replace bearings and seals as a set if worn.

Crankcase Splitting: Requires special tooling. Mark halves for alignment. Heat case gently if necessary. Clean sealing surfaces thoroughly before reassembly with sealant.

Connecting Rod: Check for up/down play at the big end bearing. Excessive play requires crankshaft assembly replacement.

Drive Shaft & Gearbox

Servicing the flexible drive shaft and gearhead.

1. Drive Shaft Removal: Remove cutting attachment and rear handle. Disconnect from engine and gearbox.
2. Inspection: Check shaft for kinks or broken wires. Lubricate with STIHL drive shaft grease during reassembly.
3. Gearbox Disassembly: Remove retaining ring and gear. Inspect gears for wear or chipping. Replace worn bearings.
4. Gearbox Grease: Pack with high-quality, high-temperature grease (STIHL Grease or equivalent).
5. Reassembly: Ensure seal is in place to prevent grease leakage and dirt ingress.

Tip: A stiff or vibrating drive shaft is often damaged and should be replaced.

Cutting Attachment Repair

Maintaining the AutoCut C 5-2 head, solid knife, or circular saw blade.

AutoCut Head: To replace line, press in tabs, remove spool, wind new line (2.4mm/0.095" diameter), feed ends through eyelets.

Solid Blade/Knife: Check for cracks, nicks, or imbalance. Sharpen with a file, maintaining original angle. Replace if damaged.

Mounting: Ensure blade or head is securely fastened with the correct washer and nut. Use a wrench to hold the shaft.

CAUTION! Always wear heavy gloves and eye protection when handling cutting attachments. Ensure unit is off and spark plug disconnected.

Reassembly & Adjustment

Final steps after repair.

1. Reinstall all components in reverse order of disassembly.
2. Torque all fasteners to specification.
3. Refill with fresh, properly mixed fuel (50:1).
4. Start-up: Prime carburetor (if equipped). Set choke. Start unit. Allow brief warm-up.
5. Carburetor Final Tune: Adjust LA screw for smooth idle. Adjust H screw to achieve max RPM (12,500 rpm) without leaning out (four-stroking slightly at peak is acceptable). Readjust idle speed if necessary.
6. Check for leaks (fuel, exhaust). Listen for abnormal noises.

Troubleshooting & Testing

Symptom	Possible Cause	Corrective Action
Engine will not start	No spark, no fuel, flooded	Check spark, fuel flow, compression. Dry plug if flooded.
Engine starts but dies	Clogged fuel filter, carburetor issue, air leak	Replace filter, clean carb, pressure test crankcase.
Loss of power	Clogged air filter, exhaust screen, bad piston/rings	Clean filter/screen, check compression.
Excessive vibration	Bent drive shaft, unbalanced blade, loose parts	Inspect shaft and attachment. Tighten all fasteners.
Gearbox overheating	Low grease, damaged gears, misalignment	Disassemble, inspect, repack with grease.
Cutting head won't feed line	Tangled line, worn head, improper line size	Disassemble head, clean, install correct line.

Compression Test: Should be above 120 psi for a healthy engine.

Support: For technical assistance or parts, contact an authorized STIHL servicing dealer.