Stihl FS 90 Instruction Manual

The Stihl FS 90 is a professional-grade gasoline-powered string trimmer/brushcutter designed for demanding clearing, trimming, and cutting tasks. It features a powerful 2-stroke engine with anti-vibration system, a straight shaft with bicycle handles for optimal control, a PolyCut head for easy line advance, and a metal blade option for heavy brush. 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. Fuel & Lubrication 7. Starting & Operation 8. Cutting Attachments 9. Maintenance & Storage 10. Warranty & Support 11. Troubleshooting

Features at a Glance

Key components: 2-stroke engine, anti-vibration system, straight drive shaft, bicycle handles, PolyCut head, metal blade capability, and easy-start system.

Feature	Description
Engine	Powerful 2-stroke air-cooled gasoline engine
Anti-Vibration System	Reduces operator fatigue during extended use
Shaft	Straight solid drive shaft for durability
Handles	Bicycle-style handles with full controls for precise maneuvering
Cutting Head	STIHL PolyCut head for easy line advance (tap-and-go)
Blade Compatibility	Accepts metal blades for heavy brush and saplings
Throttle Trigger	With throttle trigger lockout for safety
Stop Switch	Easy-access stop switch
Fuel Tank	Integrated fuel tank with clear level indicator
Spark Arrestor	Muffler with USDA-approved spark arrestor screen

Safety Instructions

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

Personal Protective Equipment (PPE): Always wear safety glasses, hearing protection, sturdy gloves, long pants, and boots.
Clear Work Area: Remove stones, nails, wire, and other debris that could be thrown.
Fuel Safety: Refuel in a well-ventilated area with engine stopped and cool. Use approved fuel container.
Operation: Keep bystanders and pets at least 50 feet away. Hold unit firmly with both hands.
Hot Surfaces: Avoid contact with muffler and cylinder during/after operation.

WARNING! Failure to follow safety instructions can result in serious injury or death. This tool produces toxic exhaust fumes. Never operate in an enclosed area.

Assembly & Preparation

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

Handle Assembly: Attach bicycle handles to shaft as per diagram. Secure all bolts.
Cutting Attachment: Install PolyCut head or approved blade using appropriate guard and hardware.
Shaft Lock: Engage shaft lock before installing or removing cutting attachment.
Fuel Mix: Prepare fuel: Use 89+ octane gasoline mixed with STIHL HP Ultra 2-stroke oil at 50:1 ratio.
Fill: Fill fuel tank with proper mix. Check chain oil (if using blade) and fill as needed.

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

Specifications

Engine Displacement: 31.4 cc (1.9 cu. in.)
Power Output: 1.3 kW (1.8 hp)
Fuel/Oil Ratio: 50:1 (STIHL HP Ultra oil)
Fuel Tank Capacity: 0.53 L (18 fl. oz.)
Weight (without cutting attachment): Approx. 6.2 kg (13.7 lbs)
Shaft Type: Straight solid shaft
Handle System: Bicycle handles
Recommended Cutting Attachments: PolyCut head, circular metal blades, brush knife
Sound Pressure Level: Approx. 99 dB(A)
Vibration Level: Left/Right handles: < 2.5 m/s²

Controls and Components

Familiarize yourself with all controls before starting.

Right Handle: Throttle trigger, throttle trigger lockout, stop switch. Left Handle: Handle bracket, vibration dampeners. Shaft: Shaft lock button, clutch housing, gearbox. Engine Housing: Starter grip, choke lever, fuel tank cap, air filter cover, spark plug.

Operation: To operate, you must press the throttle trigger lockout with your right hand while squeezing the throttle trigger. Release throttle trigger to idle; press stop switch to stop engine.

Fuel & Lubrication

Use only high-quality, fresh fuel mixture.

Fuel Mix: 50:1 ratio. For 1 gallon (3.8L) of 89+ octane gasoline, add 2.6 fl. oz. (76ml) of STIHL HP Ultra oil.
Mixing: Mix in approved container. Shake well before filling.
Filling: Clean area around cap. Fill to level. Tighten cap securely.
Gearbox Lubrication: Lubricate gearbox with STIHL gear oil after every 25 hours of operation.
Blade Lubrication: If using a metal blade, ensure automatic chain oiler is filled with STIHL chain oil.

WARNING! Gasoline is extremely flammable. No smoking. Engine and muffler must be cool.

Starting & Operation

Starting procedure for cold and warm engine.

Cold Start: Place unit on clear, flat ground. Press fuel primer bulb 5-10 times. Move choke lever to closed position. Engage shaft lock. Pull starter rope sharply until engine fires (usually 1-3 pulls). Move choke lever to open position. Pull starter rope again to start. Allow 30-second warm-up.
Warm Start: Choke lever in open position. Pull starter rope to start.
Operation: Hold unit firmly. Use a smooth, sweeping motion from right to left. Do not force the tool; let the cutting attachment do the work.
Stopping: Release throttle trigger. Press and hold stop switch until engine stops.

Tip: For tall grass, work in layers. For brush, use a metal blade and swing in an arc.

Cutting Attachments

Select the correct attachment for the job.

PolyCut Head (Standard): For grass and light weeds. Tap head on ground while at high RPM to advance line.
Circular Metal Blades: For heavy weeds, brush, and saplings up to 4 inches. Ensure correct guard is installed.
Brush Knife: For very heavy, woody vegetation.
Installation: Engage shaft lock. Remove old attachment. Install new attachment with correct washer and nut. Tighten securely with wrench.
Line Replacement: For PolyCut head, use only STIHL .095" diameter line. Wind correctly as per diagram in head.

WARNING! Always use the correct guard for the cutting attachment. Never operate without a guard.

Maintenance & Storage

Regular maintenance ensures long life and safe operation.

Daily: Clean exterior. Check for loose fasteners. Inspect cutting attachment for damage.
Air Filter: Clean weekly or more often in dusty conditions. Replace if damaged.
Spark Plug: Check gap (0.020 in / 0.5 mm) monthly. Clean or replace as needed.
Fuel Filter: Replace fuel filter in tank annually or if clogged.
Storage (>30 days): Drain fuel tank and run engine until it stops. Remove spark plug, add 1 tsp of oil into cylinder, pull starter 2-3 times, reinstall plug. Store in a dry, secure place.

CAUTION! Perform maintenance only when engine is stopped and cool. Disconnect spark plug wire for safety.

Warranty & Support

STIHL Limited Warranty: 1 year commercial use, 2 year residential use from date of purchase. Proof of purchase required. Warranty covers defects in materials and workmanship. Does not cover normal wear, misuse, or improper maintenance. For technical support, genuine parts, and service, contact an authorized STIHL servicing dealer. Find a dealer at www.stihlusa.com.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No fuel, flooded, bad spark plug, choke position	Check fuel. Wait if flooded. Check/replace spark plug. Ensure correct choke setting.
Engine starts then dies	Clogged fuel filter, dirty air filter, carburetor adjustment	Replace fuel filter. Clean air filter. Have dealer adjust carburetor.
Loss of power	Dirty air filter, clogged spark arrestor, old fuel	Clean air filter. Clean spark arrestor screen. Use fresh fuel mix.
Excessive vibration	Damaged cutting attachment, loose fasteners, bent shaft	Inspect/replace cutting attachment. Tighten all bolts. Check shaft for damage.
Line not advancing	Line too short, head jammed, worn bump feed mechanism	Reload line with correct length. Clean head. Replace bump feed if worn.

Note: For repairs beyond basic maintenance, take your STIHL FS 90 to an authorized STIHL dealer.

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