Stihl FS 81 Manual

The Stihl FS 81 is a professional-grade brushcutter/trimmer designed for demanding clearing, trimming, and cutting tasks. It features a powerful 2-stroke engine, a robust drive shaft, a variety of cutting attachments (including nylon line heads and metal blades), anti-vibration system, easy-start technology, and ergonomic handles. Below are key sections for safety, features, assembly, operation, maintenance, and troubleshooting.

1. Features at a Glance 2. Safety Instructions 3. Assembly & Preparation 4. Specifications 5. Controls and Components 6. Starting & Stopping the Engine 7. Operating Instructions 8. Maintenance Schedule 9. Storage 10. Warranty Information 11. Troubleshooting

Features at a Glance

Key components: 2-stroke engine, drive shaft, cutting attachment interface, anti-vibration system, throttle trigger, stop switch, fuel tank, and ergonomic handle.

FeatureDescription
Engine Type2-stroke, single-cylinder, air-cooled
DisplacementApprox. 40.2 cc
Power OutputHigh power-to-weight ratio for professional use
Fuel SystemDiaphragm carburetor
Starting SystemStihl Easy2Start™ technology (where equipped)
Drive SystemSolid drive shaft with gearhead
Cutting AttachmentsAccepts nylon line heads, metal blades, brush knives
Anti-VibrationIntegrated damping elements in handle and shaft
Handle TypeLoop handle or bicycle handle (model dependent)
WeightApprox. 6.8 kg (without cutting attachment)

Safety Instructions

Read and understand all instructions before use. Failure to follow warnings can result in serious injury or death.

  1. Personal Protective Equipment (PPE): Always wear safety glasses, hearing protection, sturdy gloves, long pants, and boots.
  2. Clear Work Area: Remove stones, nails, wire, and other debris that could be thrown.
  3. Fuel Safety: Refuel in a well-ventilated area with engine stopped and cool. Use only Stihl MotoMix or gasoline/oil mix (50:1).
  4. Operation: Keep bystanders and pets at least 15 meters away. Hold unit firmly with both hands.
  5. Hot Surfaces: Avoid contact with muffler and cylinder during/after operation.

WARNING! This is a powerful cutting tool. Never operate without proper training, PPE, or in unsafe conditions.

Assembly & Preparation

Unpack all parts. Assemble on a clean, flat surface.

  1. Attach Shaft: Connect the two shaft sections (if applicable) and tighten locking collar.
  2. Install Handle: Slide handle onto shaft at desired position and tighten clamping screws.
  3. Install Cutting Attachment: Ensure engine is off. Fit chosen head or blade onto gearhead shaft, secure with retaining nut using appropriate wrench.
  4. Fuel Mix: Fill fuel tank with correct 2-stroke mixture (50:1 Stihl HP Ultra oil with unleaded gasoline).
  5. Check Controls: Ensure throttle trigger moves freely and stop switch functions.

CAUTION! Ensure all fasteners are tight before starting. Use only Stihl-approved attachments.

Specifications

Controls and Components

Main Controls: Throttle trigger, throttle trigger lockout, stop switch, choke lever, primer bulb, decompression valve (if equipped).

Components: Fuel tank cap, air filter cover, spark plug, muffler, carrying ring, shoulder harness attachment points, gearhead, shaft.

Starting Procedure: Set stop switch to "I" (Run), set choke, press primer bulb, engage decompression valve (if equipped), pull starter rope firmly.

Stopping: Release throttle, press and hold stop switch to "0" (Stop).

Starting & Stopping the Engine

Cold Start:

  1. Place unit on clear, flat ground.
  2. Press fuel tank cap vent (if equipped).
  3. Set stop switch to "I".
  4. Set choke lever to full choke position.
  5. Press primer bulb 5-10 times until fuel is visible.
  6. Pull starter rope sharply until engine 'pops'.
  7. Move choke lever to half-choke or run position.
  8. Pull starter rope again to start engine.
  9. Allow to warm up at idle for 30 seconds.

Warm Start: Omit choke step; start from half-choke or run position.

Stopping: Release throttle, press stop switch to "0".

Operating Instructions

Always use shoulder harness. Maintain secure footing.

  1. Swing Technique: Use a smooth, sweeping motion from right to left (for right-handed users).
  2. Cutting Height: Keep cutting attachment parallel to ground and slightly above grass/weed level.
  3. Blade Use: For heavy brush, use a saw blade or brush knife. Let blade do the work; avoid forcing.
  4. Line Head Use: Advance line by tapping head on ground at high RPM (bump feed).
  5. Avoid Kickback: Never use blade near wire, fences, or hard objects. Be aware of tip of blade.

Tip: Take frequent breaks to reduce fatigue. Clear debris from cutting attachment regularly.

Maintenance Schedule

IntervalTask
Before each useCheck for loose fasteners, damage. Clean air filter if dirty.
Every 25 hoursClean spark arrestor screen (if equipped). Check spark plug.
Every 50 hoursReplace air filter. Clean fuel filter. Check gearhead oil.
Every 100 hoursReplace spark plug. Clean carburetor (or as needed).
SeasonallyReplace gearhead oil. Inspect drive shaft and bearings.

CAUTION! Always disconnect spark plug wire before performing maintenance.

Storage

For storage longer than 30 days:

  1. Drain fuel tank and run carburetor dry.
  2. Remove spark plug, add a teaspoon of engine oil into cylinder, pull starter slowly 2-3 times, reinstall plug.
  3. Clean entire unit, especially cooling fins and air filter.
  4. Store in a dry, secure place, out of reach of children.
  5. Protect cutting attachment from corrosion with light oil.

WARNING! Never store unit with fuel in tank in an enclosed area due to fire and fume hazards.

Warranty Information

Stihl FS 81 includes a limited warranty for commercial use. Duration and terms vary by region. Proof of purchase from an authorized Stihl dealer is required. Warranty does not cover damage from misuse, improper fuel, lack of maintenance, or unauthorized repairs. For warranty service, contact the authorized Stihl dealer where the unit was purchased.

Troubleshooting

SymptomPossible CauseCorrective Action
Engine will not startNo fuel, flooded, spark plug faulty, choke setting wrongCheck fuel; dry plug; set choke correctly; check/replace spark plug.
Engine runs poorly or stallsDirty air filter, old fuel, clogged fuel filter, carburetor adjustmentClean/replace air filter; use fresh fuel; clean fuel filter; adjust carburetor (by dealer if under warranty).
Excessive vibrationDamaged blade, loose cutting attachment, bent shaftStop engine! Check blade for damage/balance; tighten attachment; inspect shaft.
Cutting attachment does not turnDrive shaft disconnected, gearhead damaged, clutch slippingCheck shaft couplings; inspect gearhead for damage; service clutch (by dealer).
Line will not advanceEmpty line spool, tangled line, worn bump headRefill spool with correct line; untangle or replace spool; replace head if worn.

Service: For repairs not covered here, contact an authorized Stihl servicing dealer. Use only genuine Stihl parts.

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