Stihl FS 90 Weedeater Manual

The Stihl FS 90 is a professional-grade, lightweight string trimmer designed for demanding clearing and trimming tasks. It features a powerful 2-stroke engine, a straight shaft with a gearbox, a loop handle for comfortable operation, a Tap Advance™ cutting head for easy line feed, and a comprehensive safety system. Below are key sections for safety, features, operation, maintenance, troubleshooting, and specifications.

1. Features at a Glance 2. Safety Information 3. Assembly & First Use 4. Specifications 5. Controls and Components 6. Fuel & Lubrication 7. Operation 8. Cutting Attachment Use 9. Maintenance & Storage 10. Troubleshooting

Features at a Glance

Key components: 2-stroke engine, Straight shaft with gearbox, Loop handle, Tap Advance™ cutting head, Throttle trigger lockout, Decompression valve, Anti-vibration system.

Feature	Description
Engine	Powerful 2-stroke, air-cooled engine with electronic ignition
Shaft	Straight solid drive shaft with heavy-duty gearbox
Handle	Loop handle with adjustable harness connection points
Cutting Head	Tap Advance™ head for bump-feed line advancement
Safety	Throttle trigger lockout, rear hand guard, debris shield
Starting	Decompression valve for easier pull starts
Vibration	Integrated anti-vibration system in handle and shaft
Fuel System	Transparent fuel tank with integrated fuel filter
Air Filtration	Pre-cleaner and heavy-duty air filter
Spark Arrestor	Muffler with USDA approved spark arrestor screen

Safety Information

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

Personal Protective Equipment (PPE): Always wear eye protection, hearing protection, sturdy gloves, long pants, and closed-toe shoes.
Clear Area: Inspect work area for stones, nails, wire, or other debris that could be thrown.
Fuel Safety: Refuel in a well-ventilated area with engine stopped and cool. Use only STIHL MotoMix® or gasoline/oil mixture per manual.
Operation Zone: Keep bystanders, children, and pets at least 50 feet away.
Hot Surfaces: Avoid contact with muffler and cylinder during/after operation.

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

Assembly & First Use

Unpack and inspect all parts. Assemble according to instructions.

Handle Attachment: Slide loop handle onto shaft at desired position and tighten clamping screws securely.
Cutting Attachment: Ensure engine is off. Install Tap Advance™ head or optional blade per instructions, using appropriate guard.
Harness: Attach STIHL harness to connection points on handle and adjust for comfort and balance.
Fuel Mix: Fill tank with fresh fuel mixture (50:1 gasoline to STIHL 2-stroke oil) or STIHL MotoMix®.
Starting: Place unit on clear, flat ground. Set master control lever to cold start position (choke on). Pull starter rope briskly until engine fires, then move lever to run position and pull to start.

CAUTION! Ensure cutting attachment is properly installed and guard is secure before starting engine.

Specifications

Engine Displacement: 31.4 cc (1.9 cu. in.)
Power Output: 1.5 kW (2.0 bhp)
Fuel/Oil Ratio: 50:1 (STIHL 2-stroke oil)
Fuel Tank Capacity: 0.53 US qt (0.5 liter)
Idle Speed: 2,800 rpm
Maximum Powerhead Speed: 9,500 rpm
Drive System: Solid steel drive shaft, spiral bevel gearbox
Shaft Length: Straight shaft
Cutting Swath (Line): Approx. 17 inches
Weight (Powerhead): 11.7 lbs (5.3 kg)
Sound Pressure Level: 96 dB(A)
Vibration Level (Handle): Left: 4.5 m/s², Right: 6.5 m/s²

Controls and Components

Master Control Lever: Combines stop switch, choke, and run positions. Throttle Trigger: Controls engine speed. Throttle Trigger Lockout: Must be depressed to engage throttle. Decompression Valve: Reduces compression for easier starting. Primer Bulb: Press several times to prime fuel system (if equipped).

Main Components: Fuel Tank Cap, Air Filter Cover, Spark Plug, Muffler, Rear Hand Guard, Debris Shield, Gearbox, Cutting Attachment Retaining Nut, Harness Connection Points.

Fuel & Lubrication

Use only high-quality, unleaded gasoline with a minimum octane rating of 89 (RON). Mix with STIHL 2-stroke engine oil at a 50:1 ratio. For convenience and guaranteed quality, use STIHL MotoMix® pre-mixed fuel.

Clean area around fuel cap before opening.
Use a clean, approved fuel container and funnel.
Do not overfill. Leave space for expansion.
Tighten fuel cap securely after refueling.
Gearbox Lubrication: Lubricate gearbox with STIHL special gear grease periodically as per maintenance schedule.

WARNING! Gasoline is extremely flammable. No smoking. Avoid sparks. Store fuel in a cool, well-ventilated place.

Operation

Starting, running, and stopping procedures.

Starting (Cold): Set master control to cold start (choke) position. Press decompression valve (if equipped). Press primer bulb 5 times. Hold unit securely on ground. Pull starter rope briskly until engine fires. Move master control to run position. Pull starter rope to start.
Starting (Warm): Set master control to run position. Pull starter rope.
Running: Allow engine to warm up at idle for 30 seconds. Depress throttle trigger lockout and squeeze throttle trigger to increase speed.
Stopping: Release throttle trigger. Allow engine to idle briefly. Move master control lever to STOP (0) position.
Trimming Technique: Use a smooth, sweeping motion from right to left. Cut from top of grass downward. Do not force tool; let the tip speed do the work.

Cutting Attachment Use

Tap Advance™ Cutting Head: To advance line, tap head firmly on ground while engine is at high speed. Use only STIHL .095" diameter trimmer line.

Optional Attachments: Can be fitted with various STIHL blades (e.g., circular grass blade, brush knife) using appropriate guard and mounting hardware. Ensure cutting attachment is correctly installed and balanced.

WARNING! Always use the correct guard for the cutting attachment. Inspect blades for damage or cracks before each use. Never use metal blades for edging or near hard surfaces.

Maintenance & Storage

Regular maintenance ensures performance and longevity.

Daily/Air Filter: Clean air filter regularly. Replace if damaged.
Spark Plug: Check and clean electrode gap (0.020"). Replace annually.
Fuel Filter: Replace fuel filter in tank every season or as needed.
Gearbox: Check for grease leakage. Lubricate with special gear grease per manual.
Exterior: Keep unit clean. Remove grass and debris from cooling fins, cylinder area, and guard.
Storage: For long-term storage, run engine dry of fuel or add fuel stabilizer. Store in a dry, secure place, out of reach of children.

CAUTION! Perform maintenance only when engine is cool and spark plug wire is disconnected.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No fuel, flooded, bad spark plug, master control position	Check fuel; set control to run; check spark plug; wait if flooded.
Engine starts then dies	Clogged fuel filter, dirty air filter, carburetor adjustment	Replace fuel filter; clean air filter; see authorized dealer.
Loss of power	Dirty air filter, clogged spark arrestor, old fuel	Clean air filter; clean spark arrestor screen; use fresh fuel.
Excessive vibration	Damaged cutting attachment, loose parts, bent shaft	Inspect and replace damaged parts; tighten all fasteners.
Line does not advance	Empty spool, tangled line, worn head	Reload spool with correct line; clean head; replace if worn.
Gearbox overheats	Low grease, damaged gears	Lubricate with special grease; inspect gears for damage.

Service: For repairs beyond basic maintenance, contact an authorized STIHL servicing dealer.

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