STIHL FS 91 Manual

The STIHL FS 91 is a professional-grade gasoline-powered brushcutter/string trimmer designed for demanding clearing and trimming tasks. It features a powerful 2-stroke engine with STIHL anti-vibration system, a straight shaft, a loop handle, and a semi-automatic tap-and-go cutting head. Below are key sections for safety, features, operation, maintenance, and troubleshooting.

Features at a Glance

Key components: 2-stroke engine, straight drive shaft, loop handle, semi-automatic cutting head (Tap 'n' Go), anti-vibration system, and quick-release fuel and oil caps.

Feature	Description
Engine Type	STIHL 2-MIX engine, air-cooled, single cylinder
Displacement	27.2 cm³ (1.66 cu. in.)
Power Output	0.9 kW (1.2 bhp)
Fuel/Oil Mix	50:1 (STIHL HP Ultra 2-stroke oil)
Drive Shaft	Solid, straight shaft
Cutting Attachment	Tap 'n' Go trimmer head (standard), optional blade kits
Anti-Vibration System	STIHL standard AV system
Starter	STIHL ErgoStart or standard recoil starter
Handle	Loop handle with throttle trigger lockout
Weight (without cutting attachment)	Approx. 5.2 kg (11.5 lbs)

Safety Instructions

Read and understand all instructions before operation. Wear appropriate PPE.

1. Personal Protective Equipment (PPE): Always wear safety glasses, hearing protection, heavy-duty gloves, long pants, and sturdy boots.
2. Work Area Safety: Clear area of bystanders, especially children and pets. Keep a safe distance from hard objects (rocks, fences).
3. Fuel Safety: Refuel outdoors only, with engine stopped and cool. Use approved fuel container. Wipe up spills.
4. Operation Safety: Hold unit firmly with both hands. Do not operate above shoulder height. Beware of thrown objects.
5. Kickback Zone: Be aware of the cutting attachment's "kickback zone" (upper quadrant of the cutting circle).

WARNING! Failure to follow safety instructions can result in serious injury or death. The cutting tool continues to rotate after the throttle is released.

Assembly & Preparation

Assemble the unit before first use. Check all parts are included.

1. Attach the Handle: Slide the loop handle onto the shaft at the desired position and tighten the clamping screws securely.
2. Attach the Cutting Attachment: For the Tap 'n' Go head, align and push onto the drive shaft until it clicks. Pull to ensure it is locked.
3. Install the Debris Shield: Secure the shield to the shaft using the provided clamp.
4. Fuel Mixing: Mix gasoline (min. 89 octane) with STIHL HP Ultra oil at a 50:1 ratio in an approved container. Shake well.
5. Filling Fuel & Oil: Fill the fuel tank with the mixed fuel. The gearbox uses STIHL HP gear oil (filled at factory).
6. Check Fasteners: Ensure all nuts, bolts, and screws are tight.

CAUTION! Use only STIHL-approved attachments and accessories. Incorrect assembly can lead to failure and injury.

Specifications

• Model: STIHL FS 91
• Engine Power: 0.9 kW (1.2 bhp)
• Displacement: 27.2 cm³
• Idle Speed: 2,800 rpm
• Max. Powerhead Speed: 11,500 rpm
• Fuel Tank Capacity: 0.53 L (18 fl. oz.)
• Sound Pressure Level (LpA): 99 dB(A)
• Sound Power Level (LWA): 112 dB(A)
• Vibration Level (ah, V): Front Handle: 4.5 m/s², Rear Handle: 6.5 m/s²
• Warranty: Limited warranty, see your STIHL dealer for terms.

Controls and Components

Familiarize yourself with all controls before starting.

Main Components: Throttle trigger, throttle trigger lockout, stop switch, choke lever, primer bulb, fuel tank cap, oil tank cap (gearbox), spark plug, air filter cover, muffler, debris shield, shoulder strap eyelets.

Controls:
Throttle Trigger: Controls engine speed. Release to return to idle.
Throttle Trigger Lockout: Must be depressed before throttle trigger can be engaged.
Stop Switch: Move to STOP position to shut off engine.
Choke Lever: Used for cold starting (position I or II).
Primer Bulb: Press several times to draw fuel into carburetor for easier starting.

Starting & Stopping the Engine

Follow these steps precisely. Unit must be on firm, clear ground.

1. Cold Start:
   • Press fuel tank cap to vent.
   • Set stop switch to RUN position (I).
   • Set choke lever to cold start position (II).
   • Press primer bulb 5-10 times until fuel is visible.
   • Place unit on ground, hold securely with foot in handle.
   • Pull starter rope briskly until engine fires (usually 2-5 pulls).
   • Immediately move choke lever to warm start position (I).
   • Pull starter rope again to start engine.
2. Warm Start: Choke lever in position I or 0. Pull starter rope.
3. Stopping: Release throttle, allow engine to idle briefly. Move stop switch to STOP position (0).

WARNING! Never drop-start or "throw" start the unit. Keep hands and feet clear of cutting attachment.

Operation & Cutting Techniques

Use proper technique for efficiency and safety. Adjust shoulder strap for balance.

1. Basic Trimming (String Head): Hold unit parallel to ground. Use a smooth, sweeping motion from side to side, cutting with the tip of the line. Avoid pushing head into ground.
2. Clearing (Blade): For heavy brush, use a multi-tooth or circular saw blade. Use a swinging, scything motion from right to left. Let the blade do the work.
3. Tall Grass: Use a two-cut method: top half first, then bottom half.
4. Advancing Line (Tap 'n' Go): With engine at high speed, tap the head firmly on the ground. Centrifugal force will advance fresh line.
5. Slope Operation: Always stand across the slope, never up or down. Be extra cautious of footing.

Tip: Frequently check the cutting attachment for wear, damage, or loose parts.

Maintenance Schedule

Regular maintenance ensures long life and safe operation. Refer to manual for detailed procedures.

Interval	Task
Before each use	Check for loose fasteners, damage. Check fuel level.
Daily	Clean air filter (more often in dusty conditions). Check cutting attachment.
Every 25 hours	Clean spark plug, check electrode gap (0.5 mm / 0.020 in).
Every 50 hours	Clean cooling fins, fuel filter, and exhaust screen. Check gearbox grease.
Every 100 hours	Replace spark plug. Have carburetor adjusted by STIHL dealer if needed.
Annually or as needed	Replace fuel filter, air filter. Check all cables and linkages.

CAUTION! Always disconnect the spark plug boot before performing any maintenance.

Storage

Prepare unit for storage if it will not be used for more than 30 days.

1. Fuel System: Run engine until it stops to empty carburetor. Drain all fuel from tank into approved container.
2. Cleaning: Thoroughly clean the entire unit, especially the cooling fins and air filter area.
3. Lubrication: Apply light oil to the cutter drive shaft end. Grease the gearbox if specified.
4. Storage Location: Store in a dry, secure place, out of reach of children. Cover unit to keep dust free.
5. Battery (if equipped with ErgoStart): Remove battery and store separately in a cool, dry place.

WARNING! Never store unit with fuel in the tank indoors or in an enclosed space due to fire and fume hazards.

Warranty Information

STIHL FS 91 is covered by a limited warranty. Proof of purchase from an authorized STIHL dealer is required.

• Warranty Period: Refer to the specific warranty statement provided by your dealer or included with the unit.
• What is Covered: Defects in material and workmanship under normal use and maintenance.
• What is Not Covered: Damage from misuse, neglect, improper maintenance, commercial use (if sold as residential), or use of non-STIHL parts/fuel.
• Service: Warranty repairs must be performed by an authorized STIHL servicing dealer.

For full warranty terms, contact your STIHL dealer or visit www.stihlusa.com.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No fuel, flooded engine, dirty air filter, faulty spark plug, stop switch in STOP position	Check fuel, move choke to 0 and pull starter 10+ times (clear flood), clean/replace air filter, check/replace spark plug, ensure switch is in RUN.
Engine starts but dies	Clogged fuel filter, dirty carburetor, old fuel, vent in fuel cap blocked	Replace fuel filter, have carburetor serviced, use fresh fuel mix, clean fuel cap vent.
Loss of power	Dirty air filter, clogged spark arrestor screen, carbon buildup in exhaust	Clean air filter, clean spark arrestor screen, have dealer clean exhaust port.
Excessive vibration	Damaged cutting attachment, bent drive shaft, loose fasteners	Inspect and replace damaged cutter, check shaft, tighten all nuts and bolts.
Line not advancing	Line worn too short, line tangled in head, worn head	Tap head on ground at high speed, disassemble head to remove tangles, replace head if worn.
Cutting attachment does not spin	Drive shaft disconnected, gearbox failure, clutch slipping	Check shaft connection, inspect gearbox for damage/seizure, have dealer check clutch.

Dealer Service: For complex issues, take your STIHL FS 91 to an authorized STIHL servicing dealer.

Support: 1-800-467-8445 (US); www.stihlusa.com