STIHL FS 80 Service Manual

The STIHL FS 80 is a professional-grade, lightweight brushcutter/grass trimmer designed for demanding clearing and trimming work. This manual provides essential information for the safe operation, maintenance, troubleshooting, and repair of the FS 80. It covers specifications, safety precautions, fuel mixture, starting procedures, routine maintenance, carburetor adjustment, and common fault diagnosis. Always refer to this manual and wear appropriate personal protective equipment (PPE) when servicing the unit.

1. Safety Information & Symbols 2. Specifications & Technical Data 3. Component Identification 4. Fuel & Lubrication 5. Starting & Stopping Procedures 6. Routine Maintenance Schedule 7. Carburetor Adjustment (H/L/LA Screws) 8. Cutting Attachment Maintenance 9. Ignition System (Spark Plug & Coil) 10. Fuel System Service 11. Troubleshooting Guide

Safety Information & Symbols

Understanding and adhering to all safety symbols and warnings is paramount to prevent serious injury or death.

Symbol/Area	Description & Precaution
General Warning	Indicates a potentially hazardous situation. Read and understand all instructions before use.
Hot Surfaces	Muffler and cylinder get extremely hot during/after operation. Do not touch.
Rotating Cutting Tool	Keep hands and feet away. Ensure cutting attachment has stopped before handling. Use shoulder strap.
Hearing Protection	Prolonged exposure to noise can cause hearing loss. Always wear approved hearing protection.
Eye Protection	Flying debris can cause permanent eye injury. Always wear safety glasses or a face shield.
Fuel Handling	Fuel is highly flammable. Refuel outdoors, with engine stopped and cool. No smoking.

WARNING! Never operate the unit without proper PPE: eye protection, hearing protection, gloves, long pants, and sturdy boots.

Specifications & Technical Data

Model: STIHL FS 80
Engine Type: Single-cylinder, two-stroke, air-cooled
Displacement: 27.2 cm³ (1.66 cu. in.)
Power Output: Approx. 0.8 kW (1.1 hp)
Fuel/Oil Mix Ratio: 50:1 (Using STIHL HP Ultra or equivalent ISO-L-EGD certified oil)
Fuel Tank Capacity: 0.47 liters (0.12 US gal)
Ignition System: Electronic, capacitor discharge (CDI)
Spark Plug: Bosch WSR 6 F or equivalent, gap 0.5 mm (0.020 in)
Carburetor: Zama C1Q-S series (typical)
Recommended Cutting Attachments: PolyCut head 2-0, circular grass blade, metal brush knife
Drive System: Flexible drive shaft

Component Identification

Key components for operation and service.

Throttle Trigger & Lockout: Controls engine speed. Lockout must be depressed to engage throttle.
Stop Switch: Moves to "I" (On) or "O" (Off) position.
Choke Lever (Master Control Lever): Positions: • = Run, ø = Choke (cold start), ø+ = Warm start/Decompression.
Purge Bulb (Primer): Press several times to fill carburetor with fuel for cold starts.
Air Filter Cover: Houses the air filter. Secured with a latch or screw.
Spark Plug Boot: Connects the spark plug to the ignition coil.
Fuel Tank Cap: Ventilated cap. Ensure it is securely tightened after refueling.
Muffler: Equipped with a spark arrestor screen (mandatory in some areas).

Fuel & Lubrication

Using the correct fuel mixture is critical for engine performance and longevity.

Fuel Mixture: Use 89+ octane unleaded gasoline mixed with a high-quality two-stroke engine oil at a 50:1 ratio.
- Example: For 1 gallon of gas, add 2.6 oz of STIHL HP Ultra oil.
Mixing Procedure: Always mix in a clean, approved fuel can. Add oil to the can first, then add gasoline. Close can and shake thoroughly.
Gearbox Grease: The drive shaft gearbox at the cutting attachment requires periodic lubrication with STIHL gear grease. Remove plug and grease until it emerges from the vent.
Storage: For storage over 30 days, either drain the fuel system completely or add a fuel stabilizer to a full tank and run the engine for 2 minutes.

CAUTION! Never use automotive motor oil, old or pre-mixed fuel older than 30 days, or ethanol-blended fuel above E10. This can cause engine damage.

Starting & Stopping Procedures

Cold Start:

Place unit on clear, flat ground. Engage the harness.
Press the purge bulb (primer) 5-10 times until fuel is visible.
Set the Master Control Lever to the CHOKE (ø) position.
Hold the unit firmly. Press the throttle trigger lockout and fully depress the throttle trigger once, then release both. This sets the starting throttle.
Pull the starter rope briskly until the engine fires (usually 2-5 pulls).
Immediately move the Master Control Lever to the WARM START/HALF-CHOKE (ø+) position.
Pull the starter rope again. The engine should start and run.
Briefly blip the throttle to disengage the starting throttle. Allow to warm up at idle for 30 seconds.

Warm Start: Set Master Control Lever to RUN (•). Pull starter rope.

Stopping: Release throttle and allow engine to idle. Move the stop switch to the "O" position.

Routine Maintenance Schedule

Interval (Operating Hours)	Task	Procedure / Check
Before each use	General Inspection	Check for loose fasteners, damaged parts, fuel leaks. Ensure cutting attachment is secure and sharp.
Every 10 hours	Air Filter	Clean foam filter with warm soapy water, dry, and re-oil lightly with STIHL filter oil.
Every 20 hours	Spark Plug	Check electrode gap (0.5mm). Clean with wire brush. Replace if electrodes are worn or ceramic is cracked.
Every 50 hours	Fuel Filter	Replace in-tank fuel filter. Clean exterior of carburetor.
Every 100 hours	Gearbox Grease	Repack gearbox with high-temperature grease.
Annually	Muffler / Spark Arrestor	Remove and clean carbon deposits from spark arrestor screen (if equipped).
As Needed	Cutting Tool	Sharpen metal blades. Replace worn nylon cutting line or blades.

Carburetor Adjustment (H/L/LA Screws)

Note: Factory settings are usually best. Adjust only if necessary (poor idle, hesitation, lack of power). Engine must be at full operating temperature.

L Screw (Low Speed): Controls fuel mixture at idle and low RPM.
- Turn clockwise (lean) until engine speed increases, then counter-clockwise (rich) 1/4 turn to find smoothest idle.
H Screw (High Speed): Controls fuel mixture at full throttle. DO NOT LEAN BEYOND FACTORY SETTING.
- With engine at full throttle (no load), turn clockwise until RPM just peaks, then turn counter-clockwise 1/8 turn.
LA Screw (Idle Speed): Controls idle RPM.
- Adjust so the cutting tool does not rotate at idle, but the engine runs smoothly.

WARNING! An excessively lean H screw setting can cause engine overheating and seizure. If unsure, have adjustment performed by an authorized STIHL dealer.

Cutting Attachment Maintenance

Proper maintenance ensures efficient cutting and safety.

PolyCut Head (Nylon Line):
- Replace line when worn short. Use only STIHL approved nylon line diameter.
- To reload: Remove spool, wind new line in direction of arrow, feed ends through eyelets.
Metal Blades:
- Inspect for cracks, nicks, or excessive wear. Replace damaged blades immediately.
- Sharpen with a flat file, maintaining the original cutting angle. Balance the blade after sharpening.
Drive Shaft / Gearhead:
- Check for smooth rotation. Lubricate gearhead as per schedule.
- Ensure the attachment is locked securely to the drive shaft with the appropriate retaining nut or clamp.

Ignition System (Spark Plug & Coil)

Common service points for no-start or poor running conditions.

Spark Plug Test: Remove spark plug, reconnect boot. Ground plug electrode against cylinder head. Pull starter rope. Look for strong blue spark. No spark indicates faulty plug, boot, or ignition module.
Spark Plug Gap: Adjust to 0.5 mm (0.020 in).
Ignition Coil Air Gap: The gap between the flywheel magnets and the ignition coil legs should be 0.3 mm (0.012 in). Check with a feeler gauge. Adjust by loosening coil mounting screws.
Stop Switch Wire: A faulty or grounded stop switch wire can prevent spark. Disconnect wire from coil to test if spark returns.

Fuel System Service

Diagnosing and servicing fuel delivery issues.

Fuel Filter: Located inside the fuel tank. Replace if clogged or annually. Use STIHL part.
Fuel Lines: Inspect for cracks, brittleness, or leaks. Replace both supply and return lines if damaged.
Carburetor Diaphragms & Gaskets: Over time, the pump and metering diaphragms can harden. A carburetor repair kit (diaphragms, gaskets, needle valve) is required for service.
Fuel Tank Vent: A clogged tank vent (in cap) can create a vacuum and starve the engine. Clean vent hole.

Tip: Always use a clean workspace and compressed air to blow out passages when servicing the carburetor.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine will not start	1. No spark 2. No fuel 3. Flooded	1. Check spark plug, boot, coil gap, stop switch. 2. Check fuel in tank, purge bulb, fuel filter/lines. 3. Set control to WARM START (ø+), hold throttle wide open, pull starter 10+ times.
Engine starts but dies	1. Clogged fuel filter/vent 2. Dirty air filter 3. Faulty carburetor diaphragm	1. Replace fuel filter, clean tank vent. 2. Clean/replace air filter. 3. Rebuild carburetor with kit.
Lacks power at high speed	1. Clogged muffler screen 2. Incorrect H screw setting 3. Worn piston/cylinder	1. Clean spark arrestor screen. 2. Adjust H screw (see Section 8). 3. Perform compression test; rebuild if low.
Excessive vibration	1. Bent drive shaft 2. Unbalanced blade 3. Loose mounting bolts	1. Inspect and replace shaft. 2. Sharpen blade evenly or replace. 3. Tighten all engine and handle fasteners.
Cutting attachment does not spin	1. Worn clutch shoes/springs 2. Broken drive shaft 3. Gearbox failure	1. Replace clutch assembly. 2. Replace drive shaft. 3. Inspect gear teeth; replace gearbox.

Dealer Service: For major repairs (crankshaft seals, crankshaft bearings, cylinder/piston), contact an authorized STIHL servicing dealer.

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