STIHL FS 90 Trimmer 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 system, a variety of cutting attachments, and ergonomic controls for extended use. Below are key sections for safety, features, operation, maintenance, and troubleshooting.

1. Features at a Glance 2. Safety Instructions 3. Assembly & Preparation 4. Specifications 5. Controls and Components 6. Fuel & Lubrication 7. Starting & Operation 8. Cutting Attachments 9. Maintenance & Storage 10. Regulatory Information 11. Troubleshooting

Features at a Glance

Key components: 2-stroke engine, Anti-Vibration System (AV), throttle trigger with lockout, stop switch, fuel tank, gearbox, and attachment interface.

Feature	Description
Engine	Powerful 2-stroke, air-cooled engine
Anti-Vibration System (AV)	Reduces operator fatigue during extended use
Throttle Trigger Lockout	Prevents accidental throttle engagement
Stop Switch	Quickly stops the engine
Fuel Tank	Integrated, see-through for fuel level check
Gearbox	Robust design for transferring power to cutting head
Attachment Interface	Accepts various STIHL cutting heads and blades
Debris Shield	Protects operator from thrown debris
Handlebar	Adjustable loop handle for operator control
Carrying Harness (optional)	Reduces operator strain during use

Safety Instructions

Read and understand all instructions before operation. Always wear appropriate Personal Protective Equipment (PPE).

PPE: Always wear safety glasses, hearing protection, sturdy gloves, long pants, and boots.
Area Inspection: Clear work area of bystanders, pets, and obstacles like stones or wire.
Fuel Safety: Refuel in a well-ventilated area, with engine off and cool. Use approved fuel containers.
Operation: Keep firm footing. Do not operate above shoulder height. Be aware of kickback zone.
Maintenance: Always stop engine and disconnect spark plug boot before performing maintenance.

WARNING! Improper use can cause serious or fatal injury. Never operate without proper training and PPE.

Assembly & Preparation

Unpack and inspect contents. Assemble handlebar, cutting attachment, and debris shield.

Attach Handlebar: Secure to shaft using provided bolts. Adjust to comfortable position.
Install Cutting Attachment: Follow specific instructions for trimmer head or blade. Ensure gearbox is properly greased.
Attach Debris Shield: Secure shield to gearbox housing to protect operator.
Add Fuel & Oil: Fill fuel tank with correct gasoline/oil mixture. Fill gearbox with specified grease.
Attach Harness (if used): Connect carrying harness to unit for balanced operation.

CAUTION! Ensure all fasteners are tight before starting the engine.

Specifications

Engine Type: STIHL 2-stroke, single cylinder.
Displacement: Approx. 31.8 cc (FS 90 specific).
Power Output: High power-to-weight ratio for demanding tasks.
Fuel/Oil Mix: Use high-quality 2-stroke engine oil mixed with unleaded gasoline at 50:1 ratio (check manual).
Drive System: Solid drive shaft.
Cutting Attachments: Compatible with STIHL trimmer heads, circular blades, and brush knives.
Sound Power Level: Meets applicable standards (wear hearing protection).
Vibration Levels: Reduced by Anti-Vibration System.
Weight (without attachment): Approx. 5.7 kg (12.6 lbs).

Controls and Components

Main Controls: Throttle trigger, throttle trigger lockout, stop switch, choke lever, primer bulb.

Handle Area: Throttle trigger (with lockout), stop switch, handlebar. Engine Area: Choke lever, primer bulb, air filter cover, spark plug, muffler, fuel tank cap. Drive Area: Gearbox, attachment interface, debris shield mount.

Operation: Squeeze throttle trigger lockout, then throttle trigger to increase engine speed. Use stop switch to stop engine.

Fuel & Lubrication

Use fresh, unleaded gasoline with a minimum octane rating of 89. Mix with STIHL 2-stroke engine oil at the specified ratio (e.g., 50:1).

Fuel Mixing: Use a clean, approved container. Add correct amount of oil first, then gasoline. Shake well.
Refueling: Clean area around fuel cap. Fill tank slowly to avoid spills. Tighten cap securely.
Gearbox Lubrication: Fill gearbox with STIHL gear grease using a grease gun until grease appears at the output shaft.

Tip: Never use fuel older than 60 days. Always use the specified high-quality 2-stroke oil.

Starting & Operation

Starting Procedure (Cold Engine):

Place unit on clear, flat ground. Ensure cutting attachment is clear.
Press primer bulb 5-10 times until fuel is visible.
Set choke lever to full choke position.
Press throttle trigger lockout and throttle trigger simultaneously. Set stop switch to "I" (On).
Pull starter handle sharply until engine fires (usually 2-5 pulls).
Move choke lever to half-choke position. Pull starter again until engine starts.
Allow engine to warm up at idle for ~30 seconds before applying load.

Operating Technique: Use a smooth, sweeping motion from right to left. Let the tool do the work; do not force it.

WARNING! Never start the engine indoors or in poorly ventilated areas due to carbon monoxide risk.

Cutting Attachments

Supports various attachments: Trimmer heads for grass/weeds, circular blades for thick brush/saplings, brush knives for heavy vegetation.

Trimmer Head (e.g., AutoCut): For grass and light weeds. Tap head on ground to advance line.
Circular Saw Blade: For clearing brush and saplings up to a specified diameter. Ensure correct blade guard is installed.
Brush Knife: For heavy weeds and vines.
Installation/Removal: Follow specific instructions for each attachment. Always disconnect spark plug boot first.

WARNING! Always use the correct guard/shield for the attachment. Inspect blades for damage before use.

Maintenance & Storage

Regular maintenance ensures long life and safe operation.

Daily/Air Filter: Clean air filter regularly. Replace if damaged.
Spark Plug: Check and clean periodically. Replace as needed.
Fuel Filter: Replace annually or if clogged.
Gearbox: Check grease level regularly and replenish.
Cutting Attachment: Inspect for wear/damage. Sharpen blades as needed.
Storage: For long-term storage, drain fuel tank and run engine dry. Store in a dry, secure place.

CAUTION! All maintenance must be performed with the engine off and spark plug boot disconnected.

Regulatory Information

This product complies with relevant safety and emissions standards. It incorporates safety features to reduce risks but does not eliminate them. The muffler contains spark-arresting screens (in certain models) for use in fire-prone areas. STIHL is a registered trademark. For genuine parts, accessories, and service, contact an authorized STIHL servicing dealer.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No fuel, flooded, spark plug	Check fuel level. For flooded engine, move choke to off, open throttle fully, pull starter. Check/clean spark plug.
Engine runs poorly/dies	Clogged air filter, old fuel, carburetor	Clean/replace air filter. Use fresh fuel mix. May require carburetor adjustment by dealer.
Excessive vibration	Loose parts, bent shaft, damaged attachment	Tighten all fasteners. Inspect drive shaft and cutting attachment for damage.
Cutting attachment does not spin	Drive shaft, clutch, gearbox	Check for obstructions. Inspect clutch (dealer service). Check gearbox for grease and damage.
Loss of power	Clogged muffler screen, worn piston/cylinder	Clean spark arrestor screen (if equipped). May indicate need for engine service by dealer.

Service: For repairs beyond basic maintenance, contact an authorized STIHL servicing dealer. Use only STIHL-approved replacement parts.

Support: Visit www.stihlusa.com or contact your local STIHL dealer.