Stihl FS 85 Service Manual

The Stihl FS 85 is a professional-grade brushcutter/string trimmer designed for demanding clearing and trimming tasks. This manual provides comprehensive service, maintenance, and repair information for the FS 85 model, including engine specifications, fuel system, ignition system, cutting attachments, troubleshooting procedures, and safety precautions. Below are key sections covering safety, specifications, maintenance intervals, disassembly/assembly, and troubleshooting.

1. Safety Instructions 2. Technical Specifications 3. Maintenance Schedule 4. Fuel System 5. Ignition System 6. Cutting Attachments 7. Carburetor Adjustment 8. Disassembly & Assembly 9. Torque Specifications 10. Troubleshooting 11. Parts List & Diagrams

Safety Instructions

Before performing any service, read all safety warnings. Use proper personal protective equipment (PPE).

Hazard	Precaution
Fuel Fire/Explosion	Work in well-ventilated area; no smoking or open flames.
Hot Surfaces	Allow engine to cool before servicing.
Sharp Cutting Tools	Wear cut-resistant gloves; ensure tool is stopped.
Rotating Drive Shaft	Disconnect spark plug boot before working on drive system.
Chemical Exposure	Use gloves when handling fuel, oil, or cleaning solvents.

Technical Specifications

Model: Stihl FS 85
Engine Type: 2-stroke, single-cylinder, air-cooled
Displacement: 27.2 cm³ (1.66 cu. in.)
Power Output: 0.9 kW (1.2 hp) at 9000 rpm
Fuel/Oil Mix: 50:1 (Stihl HP Ultra or equivalent ISO-L-EGD)
Fuel Tank Capacity: 0.53 L (18 fl. oz.)
Ignition System: Electronic, capacitor discharge (CDI)
Spark Plug: Bosch WSR 6 F or equivalent, gap 0.5 mm (0.020 in.)
Idle Speed: 2800 rpm
Maximum Powerhead Speed: 11500 rpm
Drive System: Flexible shaft, curved shaft
Cutting Attachment Options: PolyCut head, circular grass blade, metal brush knife

Maintenance Schedule

Regular maintenance ensures optimal performance and longevity.

Before Each Use: Check for loose fasteners, fuel leaks, and cutting attachment security.
Every 10 Hours: Clean air filter; check spark plug condition and gap.
Every 25 Hours: Clean spark arrestor screen (if equipped); inspect fuel filter.
Every 50 Hours: Clean cooling fins; check carburetor adjustment; inspect drive shaft and gearhead.
Every 100 Hours: Replace fuel filter; decarbonize exhaust port and muffler.
Seasonal/Annual: Replace spark plug; inspect and grease gearhead bearings.

WARNING! Always use genuine Stihl parts for replacement to maintain performance and safety.

Fuel System

Components: Fuel tank, cap, filter, lines, carburetor (Zama C1Q-S series).

Fuel Mix: Use 50:1 mix of unleaded gasoline (min. 89 octane) with Stihl HP Ultra 2-stroke oil.
Fuel Filter Replacement: Disconnect fuel line, remove old filter from tank, attach new filter, reconnect line.
Fuel Line Inspection: Check for cracks, hardening, or leaks. Replace if damaged.
Carburetor Diaphragms: Inspect for tears or stiffness; replace kit if necessary.

CAUTION! Do not use automotive oil or mixing ratios other than 50:1. This can cause engine damage.

Ignition System

CDI module, spark plug, stop switch, wiring.

Spark Plug Testing: Remove plug, reconnect boot, ground electrode against cylinder, pull starter. Look for strong blue spark.

Ignition Coil Air Gap: Set between 0.2-0.4 mm (0.008-0.016 in.) using feeler gauge.

Stop Switch Test: With spark plug grounded, stop switch should kill spark when engaged.

Cutting Attachments

Installation and maintenance of PolyCut head, blades, and drive shaft.

PolyCut Head: Load with correct diameter trimmer line. Ensure head is securely locked.
Metal Blade Installation: Use correct blade washer and nut. Tighten counterclockwise (left-hand thread).
Drive Shaft Lubrication: Apply Stihl drive shaft grease through grease fitting at gearhead.
Gearhead Inspection: Check for worn gears, bearing play, and seal integrity.

WARNING! Always use the appropriate guard for the cutting attachment. Never operate without a properly installed guard.

Carburetor Adjustment

Adjustment screws: L (low speed), H (high speed), LA (idle speed).

Start engine and warm up for 5 minutes.
Initial Setting: Turn L and H screws clockwise until lightly seated, then open 1 turn out.
Idle Speed (LA): Adjust so chain/attachment does not rotate at idle.
Low Speed (L): Adjust for smooth acceleration from idle without hesitation.
High Speed (H): Adjust for maximum RPM without exceeding 11500 rpm (use tachometer).
Re-check idle speed after L and H adjustments.

Note: Final adjustment may require emission analyzer for compliance. Limit caps may be present.

Disassembly & Assembly

Major component removal sequence.

Remove Cutting Attachment and Debris Shield.
Remove Fuel Tank: Disconnect fuel line and impulse line (if present).
Remove Carburetor and Air Filter Assembly.
Remove Cylinder Cover and Muffler.
Remove Cylinder and Piston: Remove cylinder base nuts, tap cylinder gently to break gasket seal.
Remove Crankshaft from Crankcase: Requires press or special puller tools.
Reassembly: Reverse order using new gaskets and seals. Apply sealant as specified.

CAUTION! Mark all components for reassembly orientation. Use a service manual with detailed diagrams.

Torque Specifications

Component	Torque	Notes
Cylinder Base Nuts	20 Nm (15 ft-lb)	M8 thread
Spark Plug	25 Nm (18 ft-lb)	M14 x 1.25
Muffler Bolts	10 Nm (88 in-lb)	Apply anti-seize compound
Clutch Assembly	40 Nm (30 ft-lb)	Right-hand thread
Cutting Attachment Nut	Hand-tight + 1/4 turn	Left-hand thread

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No spark, fuel issue, flooded	Check spark, fuel mix, choke operation. Dry plug if flooded.
Engine starts but dies	Clogged fuel filter, dirty air filter, carb adjustment	Replace fuel filter, clean air filter, adjust L screw.
Loss of power	Clogged spark arrestor, air leak, exhaust port carbon	Clean spark arrestor, pressure test crankcase, decarbonize.
Excessive vibration	Bent drive shaft, unbalanced blade, loose components	Inspect shaft, balance or replace blade, tighten fasteners.
Cutting attachment does not spin	Sheared clutch spring, broken drive cable, engaged brake	Inspect clutch, replace drive cable, disengage brake.

Parts List & Diagrams

Refer to official Stihl FS 85 IPL (Illustrated Parts List) for part numbers and exploded views. Common replacement parts:

Air Filter: Part # 0000 120 1620
Fuel Filter: Part # 0000 350 3501
Spark Plug: Part # 0000 400 1300 (Bosch WSR 6 F)
Carburetor Repair Kit: Part # 0000 120 0650 (for Zama C1Q-S)
Drive Shaft Grease: Part # 0000 890 1001

Service Support: For technical assistance or to order parts, contact an authorized Stihl servicing dealer or visit www.stihlusa.com.