Stihl Trimmer FS 90 Manual

The Stihl FS 90 is a professional-grade gasoline-powered 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 (trimmer head, metal blades), an anti-vibration system, an easy-start system, and a harness for operator comfort. Below are key sections for safety, features, assembly, operation, maintenance, and troubleshooting.

Features at a Glance

Key components: 2-stroke engine, Drive shaft, Handlebar with controls, Cutting attachment interface, Harness attachment points, Fuel tank.

Feature	Description
Engine	Powerful 2-stroke gasoline engine with easy-start system
Drive System	Solid drive shaft for reliable power transmission
Anti-Vibration	Integrated system to reduce operator fatigue
Cutting Attachments	Compatible with trimmer heads, circular metal blades, and brush knives
Handlebar	Adjustable loop handle with throttle trigger and stop switch
Harness	Comfort harness for balanced weight distribution
Fuel Tank	Transparent tank for easy fuel level inspection
Spark Arrestor Muffler	Reduces fire hazard and noise
Throttle Interlock	Safety feature to prevent accidental throttle activation
Decompression Valve	Eases starting by reducing cylinder pressure

Safety Instructions

Read and understand all instructions before use. Always wear appropriate PPE.

1. Personal Protective Equipment (PPE): Always wear safety glasses, hearing protection, sturdy gloves, long pants, and boots.
2. Clear Area: Clear work area of bystanders, especially children and pets. Maintain a 50-foot safety zone.
3. Fuel Safety: Refuel outdoors, with engine off and cool. Use clean, unleaded gasoline mixed with high-quality 2-stroke oil (50:1 ratio). Wipe up spills.
4. Operation: Keep firm footing. Do not operate on steep slopes. Never cut near wire fences or hidden objects.
5. Hot Surfaces: Avoid contact with muffler and cylinder during and immediately after operation.

WARNING! Risk of severe injury or death from blade contact, thrown objects, fire, or carbon monoxide. Operate with extreme caution.

Assembly & Preparation

Assemble the unit before first use.

1. Attach Shaft to Engine: Align and secure the drive shaft to the engine housing using the provided bolts.
2. Install Handlebar: Slide the loop handle onto the shaft at a comfortable height and tighten the clamping screws.
3. Install Cutting Attachment: Fit the chosen trimmer head or blade onto the drive shaft end. Secure with the retaining nut using the appropriate spanner. Ensure guard is correctly installed.
4. Adjust Harness: Connect the harness to the attachment points on the unit. Adjust straps for a snug, balanced fit.
5. Fuel Mix: Prepare fuel: Mix unleaded gasoline (min. 89 octane) with STIHL 2-stroke engine oil at a 50:1 ratio in an approved container. Shake well. Fill the tank.

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

Specifications

• Engine Type: STIHL 2-stroke, single cylinder.
• Displacement: [e.g., 31.8 cc] (Refer to specific model plate).
• Power Output: [e.g., 1.5 kW / 2.0 hp].
• Fuel/Oil Ratio: 50:1 (High-quality 2-stroke oil).
• Fuel Tank Capacity: [e.g., 0.55 liters].
• Drive System: Solid steel drive shaft.
• Cutting Attachment: Standard trimmer head (line). Optional: Circular blades, brush knives.
• Sound Power Level: Complies with relevant directives (e.g., L_WA 112 dB(A)).
• Weight (without attachment): Approximately [e.g., 6.2 kg].
• Vibration Levels (ah, K): Left/Right Handle: [e.g., 4.5/5.5 m/s²].

Controls and Components

Main controls located on the handlebar and engine housing.

Handlebar: Throttle trigger with interlock, Stop switch (I/O), Throttle trigger lockout.

Engine Housing: Choke lever (for cold start), Decompression valve button, Primer bulb, Fuel tank cap, Muffler.

Other Components: Drive shaft, Attachment interface, Harness connectors, Spark plug, Air filter cover.

Operation States: STOP (0): Engine off. RUN (I): Normal operation. START (Choke symbol): For cold starting.

Starting & Stopping the Engine

Follow these steps precisely.

1. Cold Start:
   • Place unit on clear, flat ground. Attach harness.
   • Press the fuel primer bulb 5-10 times until fuel is visible.
   • Set stop switch to "I" (RUN).
   • Push the choke lever to the closed position (symbol).
   • Press the decompression valve (if equipped).
   • Hold the unit securely with one hand on the handle and your foot in the rear handle. Pull the starter cord briskly until the engine 'fires' (usually 2-5 pulls).
   • Immediately move the choke lever to the open/half-throttle position.
   • Pull the starter cord again. The engine should start. Allow to warm up at idle for ~30 seconds.
2. Warm Start: Choke lever must be open. Follow steps 2, 3, and 8 from above.
3. Stopping: Release throttle trigger. Let engine idle briefly. Move stop switch to "O" (STOP).

WARNING! Never start the engine without the cutting attachment properly installed and the guard in place.

Operation & Cutting Techniques

Use proper technique for efficiency and safety.

1. Harness: Always use the harness. Adjust so the unit hangs comfortably with the cutting attachment off the ground.
2. Basic Trimming (Line Head): Use a sweeping motion from side to side, keeping the head parallel to the ground. Advance slowly. Let the tip of the line do the work (approx. 2-3 inch cutting swath).
3. Brushing/Clearing (Blade): For heavy weeds and brush. Use a scything motion from right to left (for right-handed operators). Keep blade at an angle; do not force it. Beware of kickback.
4. Slopes: Always stand across the slope, never up or down. Be extra cautious of footing.
5. Line Advance: For bump-feed heads: Tap the head firmly on the ground (at full throttle) to advance fresh line.

Tip: Clear area of rocks, metal, and other debris before cutting to avoid damage and thrown objects.

Maintenance Schedule

Regular maintenance ensures long life and safe operation.

Component	Before Each Use	Every 25 Hours	Every 100 Hours/Annually
Air Filter	Check, clean if dirty	Clean thoroughly	Replace
Spark Plug	Check condition	Clean, check gap	Replace
Fuel Filter	-	-	Replace
Spark Arrestor Screen	-	Check, clean	Replace if damaged
Cutting Attachment	Inspect for damage/wear, tighten nut	Sharpen blade (if applicable)	Replace worn parts
Drive Shaft Grease	-	-	Lubricate (use STIHL grease)
External Screws/Nuts	Check tightness	Tighten as needed	-

CAUTION! Perform maintenance only when the engine is off and cool. Disconnect the spark plug boot for major work.

Storage

Prepare the unit for storage periods longer than 30 days.

1. Fuel System: Run the engine until it stops to empty the carburetor. Drain all fuel from the tank into an approved container.
2. Engine: Remove the spark plug. Pour about 1 teaspoon of 2-stroke oil into the cylinder. Pull the starter cord slowly 2-3 times to distribute oil. Reinstall spark plug.
3. Cleaning: Clean the entire unit of dirt, grass, and debris.
4. Storage Location: Store in a dry, well-ventilated place, out of reach of children. Cover the unit.
5. Battery (if electric start): Remove and store battery in a cool, dry place. Charge periodically.

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

Warranty Information

STIHL Limited Warranty: This STIHL product is backed by a [e.g., 1-year] limited warranty for personal use against defects in material and workmanship when purchased from an authorized STIHL dealer. Commercial use may have different terms. Proof of purchase is required. This warranty does not cover damage from misuse, neglect, improper maintenance, or use of non-STIHL parts/fuel. For full warranty details, contact your authorized 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, choke position incorrect	Check fuel level and mix. Wait if flooded. Clean/replace air filter. Check/replace spark plug. Follow correct starting procedure.
Engine starts but dies	Clogged fuel filter, dirty carburetor, faulty fuel line, vent in fuel cap blocked	Replace fuel filter. Clean carburetor (dealer service). Check fuel line. Clean fuel tank cap vent.
Loss of power	Dirty air filter, clogged spark arrestor screen, old fuel, carbon buildup	Clean air filter. Clean spark arrestor screen. Use fresh fuel mix. Decarbonize cylinder/exhaust (dealer).
Excessive vibration	Loose cutting attachment, bent drive shaft, unbalanced blade, damaged clutch	Tighten attachment nut. Inspect shaft for damage. Balance or replace blade. Have clutch inspected by dealer.
Cutting attachment does not turn	Drive shaft disconnected, clutch worn, gearhead damaged	Check shaft connection. Inspect clutch and gearhead (dealer service).
Line not advancing	Empty line spool, line tangled, worn bump head	Refill spool with correct line. Untangle or replace spool. Replace worn bump head components.

Dealer Service: For complex repairs, contact your authorized STIHL dealer.

Support: www.stihlusa.com | Find your dealer: 1-800-467-8445