Stihl FS 100 Service Manual

The Stihl FS 100 is a professional-grade trimmer/brushcutter designed for demanding clearing and maintenance work. This manual provides essential information for the safe operation, maintenance, troubleshooting, and repair of the FS 100 power tool. It covers specifications, safety precautions, fuel/oil mixture, starting procedures, routine maintenance, carburetor adjustment, and common fault diagnosis.

1. Safety Instructions 2. Specifications & Features 3. Assembly & First Use 4. Fuel & Lubrication 5. Starting & Operation 6. Routine Maintenance 7. Carburetor Adjustment 8. Cutting Attachments 9. Storage 10. Troubleshooting 11. Exploded Views & Parts List

Safety Instructions

Always read and understand the manual before use. Wear appropriate PPE: safety glasses, hearing protection, gloves, long pants, and sturdy boots.

Hazard	Precaution
Kickback	Keep cutting attachment below waist level; maintain firm grip.
Thrown Objects	Clear work area of bystanders, especially children and pets.
Fire & Explosion	Refuel in well-ventilated area, engine off and cool. No smoking.
Carbon Monoxide	Operate only in well-ventilated outdoor areas.
Vibration	Take regular breaks to reduce risk of Hand-Arm Vibration Syndrome (HAVS).
Hot Surfaces	Avoid contact with muffler during and immediately after operation.

Specifications & Features

Model: Stihl FS 100
Engine Type: 2-stroke, single-cylinder, air-cooled
Displacement: 31.8 cc (1.94 cu. in.)
Power Output: 1.3 kW (1.7 hp)
Fuel/Oil Ratio: 50:1 (Stihl HP Ultra or equivalent ISO-L-EGD certified oil)
Fuel Tank Capacity: 0.55 liters (18.6 fl. oz.)
Idle Speed: 2800 rpm
Max. Powerhead Speed: 9500 rpm
Drive System: Solid drive shaft with gearbox
Standard Cutting Attachment: PolyCut head 25-2 or circular grass blade
Weight (powerhead only): Approx. 5.9 kg (13.0 lbs)
Anti-Vibration System: Standard
Ignition System: Electronic, CDI
Starter: Rewind starter with decompression valve

Assembly & First Use

Handle Assembly: Attach loop handle or bike handle to shaft as per kit instructions. Secure all bolts.
Cutting Attachment: Install trimmer head or blade onto gearbox output shaft. Secure with appropriate retaining nut (left-hand thread for trimmer heads).
Debris Shield: Always install the correct guard for the cutting attachment being used.
Check Controls: Ensure throttle trigger, throttle trigger lockout, and stop switch function correctly.
Fuel Mix: Prepare fresh 50:1 fuel/oil mixture. Fill fuel tank.

WARNING! Never operate the tool without the proper cutting attachment guard installed.

Fuel & Lubrication

Fuel: Use unleaded gasoline with a minimum octane rating of 89 (RON). Do not use fuel containing more than 10% ethanol (E10).

2-Stroke Oil: Use a high-quality, air-cooled engine oil meeting ISO-L-EGD (JASO FD, API TC) standards. Stihl HP Ultra is recommended.

Mixing: Always mix fuel and oil in a clean, approved container before filling the tank. Ratio: 50 parts gasoline to 1 part oil (2%).

Gearbox Lubrication: Lubricate the gearbox with Stihl Gear Grease after every 25 hours of operation or if contamination is suspected.

CAUTION! Never use straight gasoline or motor oil. Improper mixture will cause severe engine damage.

Starting & Operation

Cold Start:

Place unit on clear, flat ground.
Press primer bulb 5-10 times until fuel is visible.
Set choke lever to closed position (full choke).
Pull starter rope slowly until resistance is felt, then pull briskly. Do not pull rope to full extension.
After engine fires, move choke lever to half-choke/open position.
Pull starter rope again to start. Allow engine to warm up at idle for 30 seconds.

Warm Start: Do not use choke. Follow steps 1 and 6.

Operation: Use a smooth, sweeping motion from right to left. Maintain a steady engine speed; do not "saw" with the tool. Allow the tool to do the work.

Routine Maintenance

Interval	Task	Procedure
Daily / Before Use	General Inspection	Check for loose fasteners, damaged parts, fuel leaks. Clean air intake and cooling fins.
Every 25 hrs	Air Filter Clean/Replace	Remove foam filter, wash in soapy water, dry, re-oil lightly with filter oil.
Every 25 hrs	Spark Plug Inspection	Clean electrode, check gap (0.5 mm / 0.020 in). Replace if worn.
Every 25 hrs	Gearbox Grease	Remove fill screw, apply Stihl Gear Grease until it emerges from vent hole.
Every 100 hrs	Fuel Filter Replace	Replace in-tank fuel filter. Clean fuel tank cap vent.
As Needed	Cutting Attachment	Sharpen blade, replace trimmer line, check for wear/cracks.

Carburetor Adjustment

Tools Required: T-handle adjustment tool or small flat-head screwdriver.

Standard Screw Settings (initial):
L (Low Speed Mixture): 1 turn open from lightly seated.
H (High Speed Mixture): 1 turn open from lightly seated.
LA (Idle Speed): Adjust to achieve stable idle (~2800 rpm) and proper clutch disengagement.

Start engine and warm up for 5 minutes.
Set Idle Speed (LA): Turn LA screw clockwise to increase idle until cutting attachment begins to rotate, then counter-clockwise just until rotation stops. Engine should run smoothly.
Set High Speed (H): With throttle fully open, adjust H screw for maximum RPM. Then turn counter-clockwise slightly (approx. 1/8 turn) to enrich mixture. Engine should "4-cycle" slightly at full throttle under no load.
Set Low Speed (L): From idle, quickly open throttle. If engine hesitates or dies, turn L screw counter-clockwise to enrich. If engine smokes or runs rough, turn clockwise to lean. Aim for smooth acceleration.
Re-check idle speed (LA) after L adjustment.

CAUTION! An overly lean mixture (H screw too far in) will cause engine overheating and permanent damage.

Cutting Attachments

PolyCut Head 25-2: For grass and light weeds. Use 2.0 mm or 2.4 mm round line. Tap head on ground to advance line (Bump Feed).
Circular Grass Blade (4-tooth): For thick grass, weeds, and light brush up to 1/2 inch diameter.
Circular Brush Knife (80-tooth): For heavy brush, saplings, and woody vegetation.
Metal Blade Guard: MUST be used with all metal cutting blades.

WARNING! Always ensure the cutting attachment is properly installed, balanced, and secured. Never use a damaged or cracked blade.

Storage

Long-term (>30 days): Drain fuel tank and carburetor completely. Run engine until it stops from fuel starvation.
Remove spark plug, add a teaspoon of 2-stroke oil into cylinder. Pull starter rope slowly 2-3 times to coat internals. Reinstall spark plug.
Clean entire unit thoroughly, removing grass, dirt, and debris.
Store in a dry, well-ventilated place, protected from dust and moisture.
Short-term: Store with fuel stabilizer added to a full tank of fresh fuel mix.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No spark, flooded, bad fuel, clogged filter	Check spark plug, air filter, fuel filter. Use fresh fuel mix. Clear flood (remove plug, pull starter).
Engine starts then dies	Clogged fuel filter/tank vent, dirty carburetor	Replace fuel filter, clean tank vent, clean/rebuild carburetor.
Loss of power	Clogged air filter, spark arrestor, carb adjustment	Clean/replace air filter. Clean spark arrestor screen in muffler. Adjust carburetor.
Excessive vibration	Bent drive shaft, unbalanced/damaged blade, loose parts	Inspect shaft and cutting attachment. Tighten all bolts and nuts.
Cutting attachment does not spin	Broken clutch springs, worn clutch shoes, gearbox failure	Inspect clutch assembly and gearbox. Replace worn parts.
Engine overheats	Lean carburetor setting, clogged cooling fins, low oil mix	Enrich H screw. Clean cylinder fins. Verify correct 50:1 fuel mix.

Exploded Views & Parts List

Refer to official Stihl FS 100 IPL (Illustrated Parts List) for detailed breakdown of assemblies:

Powerhead Assembly (Cylinder, Piston, Crankshaft)
Carburetor & Fuel System
Ignition System (Flywheel, Coil, Spark Plug)
Starter Assembly
Clutch & Gearbox
Shaft & Handle Assembly
Cutting Attachment Interfaces

Note: Use genuine Stihl replacement parts to ensure performance, safety, and warranty compliance. For parts ordering, contact an authorized Stihl dealer with your model and serial number.

Service Support: For technical assistance or to locate a dealer, visit www.stihlusa.com or call 1-800-467-8445.