STIHL FS 90 R Owner's Manual

The STIHL FS 90 R is a professional-grade, straight-shaft grass trimmer/brushcutter featuring a powerful 2-stroke engine, anti-vibration system, ErgoStart™ for easier starting, a loop handle with throttle trigger lockout, a QuickStop® inertia-activated chain brake (if equipped with a cutting blade), and a modular design allowing attachment of various cutting heads and blades. It is designed for clearing grass, weeds, and light brush. Below are key sections for safety, features, assembly, operation, maintenance, and troubleshooting.

Features at a Glance

Key components: 2-stroke engine, straight drive shaft, loop handle, ErgoStart™ system, anti-vibration elements, QuickStop® system (if equipped), and attachment interface for trimmer heads or blades.

Assembly & Preparation

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

1. Attach the handle: Slide onto shaft, align for operator comfort, and tighten clamping bolts securely.
2. Install the cutting attachment: Ensure engine is off and spark plug boot is disconnected. Fit the correct trimmer head or blade, securing with the appropriate nut and washer (use a STIHL combination wrench).
3. Install the shoulder harness: Adjust straps for balanced, comfortable operation.
4. Fill with fuel: Use only STIHL MotoMix® pre-mixed fuel or mix high-quality 2-stroke oil with unleaded gasoline (50:1 ratio).

WARNING! Never attach a metal cutting blade unless the machine is equipped with the QuickStop® system and you are trained and use proper protective equipment. Always disconnect the spark plug boot before assembly or maintenance.

First-Time Setup

Before first use, complete final checks and adjustments.

1. Check all fasteners: Ensure all screws, nuts, and bolts are tight.
2. Adjust harness: Fit and adjust the harness so the trimmer hangs comfortably with the cutting attachment clear of the ground.
3. Check throttle trigger: Ensure it moves freely and returns to idle automatically. Verify throttle trigger lockout functions.
4. Inspect cutting attachment: Ensure it is properly installed and secure. Check that the guard is correctly positioned.

CAUTION! Always wear appropriate Personal Protective Equipment (PPE): safety glasses, hearing protection, gloves, long pants, sturdy boots, and, when using a blade, chainsaw protection pants.

Specifications

• Engine Displacement: Approximately 27.2 cc (1.7 cu. in.).
• Powerhead Weight: Approximately 5.3 kg (11.7 lbs) without cutting attachment.
• Fuel/Oil Ratio: 50:1 (using STIHL 2-stroke oil).
• Fuel Tank Capacity: Approximately 0.55 liters (18.6 fl. oz.).
• Idle Speed: Factory set, approximately 2,800 rpm.
• Maximum Power Output Speed: Approximately 9,500 rpm.
• Drive System: Solid steel shaft.
• Sound Power Level (LWA): Declared per relevant directives.
• Vibration Levels (ah, h): Declared for handle positions.

Controls and Settings

Main controls are located on the handle and engine housing.

Handle: Throttle trigger, throttle trigger lockout, stop switch (often part of Master Control Lever).

Engine Housing: Master Control Lever (Stop/Choke/Run), fuel tank cap, primer bulb (if equipped), decompression valve (if equipped), spark plug boot.

Operation: Master Control Lever sets engine state. Throttle trigger controls engine speed. The lockout must be depressed before the throttle can be engaged. QuickStop® system activates automatically if kickback occurs (with blade).

Starting & Stopping the Engine

Follow the cold start procedure for a cold engine. For a warm engine, omit the choke step.

1. Place the unit on firm, clear ground. Ensure cutting attachment is clear and not touching anything.
2. Press the fuel pump bulb (if equipped) 5-10 times until fuel is visible in the bulb.
3. Set the Master Control Lever to the choke position (full left).
4. Hold the unit firmly: Place your right foot in the rear handle and hold the front handle with your left hand.
5. Pull the starter rope briskly until the engine fires (usually 1-3 pulls).
6. Immediately move the Master Control Lever to the half-throttle (run) position.
7. Pull the starter rope again to start the engine. It may run at high idle initially.
8. Allow to warm up: Briefly blip the throttle to return to normal idle.
9. To Stop: Move the Master Control Lever to the STOP (0) position.

Tip: Do not pull the starter rope to its full extent or let the starter handle snap back. Guide it back gently.

Operating Instructions

For trimming grass and light weeds.

1. Start the engine as described and allow it to warm up.
2. Put on the shoulder harness and hold the trimmer with both hands.
3. Use a smooth, sweeping motion from side to side, keeping the cutting head parallel to the ground.
4. Advance slowly, cutting a manageable swath. Let the tip of the line do the work.
5. For taller growth, use a slower, scything motion from the top down.
6. To advance line: Tap the cutting head on the ground (with throttle off) while at operating speed (bump feed).
7. For Brushcutting (with blade): Use a deliberate, controlled swinging stroke. Be aware of kickback zone (upper quadrant of blade rotation).

WARNING! Never operate the unit with one hand. Always be aware of your surroundings, especially people, animals, and obstacles like rocks and fences. Clear the work area of debris. Never use the machine above shoulder height.

Maintenance Schedule

Regular maintenance ensures safety and longevity. Disconnect spark plug boot before any maintenance.

Storage

For storage longer than 30 days, prepare the engine to prevent fuel system damage.

1. Run the engine to empty the carburetor of fuel (or use a fuel stabilizer).
2. Drain all fuel from the tank into an approved container.
3. Remove the spark plug, add a teaspoon of 2-stroke oil into the cylinder, pull starter rope slowly 2-3 times, reinstall plug.
4. Thoroughly clean the entire unit.
5. Store in a dry, clean, and secure place, out of reach of children.

CAUTION! Gasoline is highly flammable. Drain fuel in a well-ventilated area away from ignition sources. Dispose of old fuel properly.

Safety & Regulatory Symbols

Understand the symbols on your machine: Read and understand your manual warning symbols (safety alert), wear eye and hearing protection, danger of kickback, keep bystanders away, hot surface warning, and emissions compliance statements.

This machine complies with relevant EPA and CARB emissions regulations for handheld engines. The emissions compliance period is listed in the official documentation.

Troubleshooting

Service: For repairs beyond basic maintenance, contact your authorized STIHL servicing dealer. Use only genuine STIHL replacement parts.

Support: For the latest manual or support, visit www.stihlusa.com or contact STIHL Incorporated at 1-800-467-8445.