Stihl FS 420 Manual

The Stihl FS 420 is a professional-grade, high-performance brushcutter/string trimmer designed for demanding clearing and maintenance tasks. It features a powerful 2-stroke engine with anti-vibration system, ergonomic harness, straight shaft, versatile cutting attachments, and a comprehensive safety system. Below are key sections for safety, features, operation, maintenance, troubleshooting, and specifications.

Features at a Glance

Key components: Powerful 2-stroke engine, anti-vibration system, ErgoStart, straight drive shaft, QuickStop inertia chain brake, harness, and multiple cutting attachment options.

Feature	Description
Engine	2-stroke, single-cylinder, air-cooled
Displacement	40.2 cc
Power Output	High torque for demanding vegetation
Anti-Vibration System	Reduces operator fatigue
ErgoStart	Easier starting system
QuickStop Inertia Chain Brake	Stops cutting attachment rapidly in event of kickback
Drive Shaft	Straight, solid design for durability
Cutting Attachments	Accepts trimmer heads, metal blades, brush knives
Harness	Ergonomic harness for balanced operation
Fuel Tank Capacity	Approx. 0.95 liters (32 oz)

Safety Information

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

1. Personal Protective Equipment (PPE): Wear safety glasses, hearing protection, gloves, sturdy boots, long pants, and close-fitting clothing.
2. Work Area Safety: Clear area of bystanders, especially children and pets. Be aware of hidden obstacles.
3. Fuel Safety: Refuel in well-ventilated area, engine off and cool. Use approved fuel/oil mix. No smoking.
4. Operation: Keep firm footing. Do not operate above shoulder height. Be aware of kickback zone.
5. Transport & Storage: Secure unit. Empty fuel tank for long-term storage.

WARNING! Improper use can cause serious or fatal injury. The cutting tool continues to rotate after the throttle is released.

Assembly & Preparation

Unpack and inspect contents. Assemble handle, cutting attachment, and harness.

1. Attach Handle: Align handle halves on shaft and secure with provided screws.
2. Install Harness: Adjust harness straps for comfortable, balanced fit.
3. Install Cutting Attachment: Ensure engine is off. Use correct wrench to secure trimmer head or blade. Ensure guard is properly installed.
4. Fuel Mix: Prepare fuel mixture of 50:1 (Stihl HP Ultra oil and unleaded gasoline). Shake well.

CAUTION! Ensure all fasteners are tight before operation. Use only Stihl-approved attachments.

Specifications

• Model: FS 420
• Engine Type: 2-stroke, single cylinder, air-cooled
• Displacement: 40.2 cc
• Powerhead Weight: Approx. 6.8 kg (15 lbs) without cutting attachment
• Fuel/Oil Ratio: 50:1 (Stihl HP Ultra oil)
• Fuel Tank Capacity: 0.95 L (32 oz)
• Drive Shaft: Straight, solid
• Recommended Cutting Attachments: Trimmer heads, circular metal blades, brush knives
• Sound Power Level: High - hearing protection required
• Vibration Level: Reduced by anti-vibration system

Controls and Components

Main Controls: Throttle trigger with lockout, Stop switch, Choke lever, Primer bulb, Decompression valve (if equipped).

Components: Powerhead, Drive shaft, Handle with controls, Harness attachment points, Cutting attachment interface, Spark plug, Air filter cover, Fuel tank cap, Muffler.

Safety Features: QuickStop inertia chain brake, Throttle trigger lockout, Rear handle guard, Deflector/shield for cutting attachment.

Fuel & Lubrication

Use fresh, unleaded gasoline (min. 89 octane) mixed with Stihl HP Ultra 2-stroke oil at 50:1 ratio.

1. Mix fuel in approved, clean container. Shake thoroughly.
2. Fill fuel tank. Avoid spilling. Wipe any spills immediately.
3. Gearbox Lubrication: Lubricate gearbox at attachment end with Stihl grease before initial use and periodically.

Tip: Never use fuel older than 30 days. Do not use automotive oil or mixed fuel not intended for 2-stroke engines.

Starting & Operation

Starting Procedure (Cold Engine):

1. Place unit on clear, flat ground. Engage chain brake.
2. Set stop switch to "I" (Run).
3. Press primer bulb 5-10 times until fuel is visible.
4. Set choke lever to full choke position.
5. Hold unit securely. Pull starter rope briskly until engine fires (usually 2-5 pulls).
6. Move choke lever to half-choke or run position.
7. Pull starter rope again to start engine. Allow to warm up at idle.

Operation: Disengage chain brake. Squeeze throttle trigger to increase RPM. Use swinging motion from right to left for trimming. Do not force the tool.

Stopping: Release throttle, allow engine to idle. Move stop switch to "0" (Stop).

Cutting Attachments

Supports various attachments for different tasks.

1. Trimmer Head (Line): For grass and light weeds. Use correct diameter line. Bump feed to advance line.
2. Circular Metal Blades: For thick weeds, brush, and saplings. Ensure blade is sharp and balanced.
3. Brush Knife: For heavy brush and woody vegetation.
4. Installation: Ensure engine is off. Use wrench to secure attachment nut. Always install appropriate guard.

WARNING! Inspect cutting attachment for damage before each use. Replace damaged blades or heads immediately.

Maintenance Schedule

Regular maintenance ensures performance and longevity.

Interval	Task
Before each use	Check for loose fasteners, damage. Check fuel level.
Daily	Clean air filter. Check cutting attachment condition.
Every 25 hours	Clean spark plug, check electrode gap. Clean cooling fins.
Every 50 hours	Clean fuel filter. Check fuel lines. Grease gearbox.
Every 100 hours	Replace spark plug. Clean muffler screen (if equipped).
Annually or as needed	Professional servicing recommended.

Storage

For storage longer than 30 days:

1. Drain fuel tank completely and run engine until it stops to clear carburetor.
2. Remove spark plug, add a teaspoon of engine oil into cylinder, pull starter rope slowly to distribute, reinstall plug.
3. Clean entire unit thoroughly.
4. Store in a dry, secure place, out of reach of children.

CAUTION! Storing with fuel in the tank can cause gum deposits in the carburetor.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No fuel, flooded, spark plug faulty	Check fuel, use correct starting procedure, check/clean spark plug.
Engine starts but dies	Clogged air/fuel filter, carburetor issue	Clean/replace air filter, check fuel filter and lines.
Loss of power	Dirty air filter, clogged muffler screen	Clean air filter, clean spark arrestor screen.
Excessive vibration	Loose parts, unbalanced/damaged blade	Tighten all fasteners, inspect and balance/replace cutting attachment.
Cutting attachment does not rotate	Drive shaft/shear pin broken	Inspect drive shaft and shear pin (if equipped), replace if damaged.

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

Support: Stihl Incorporated, 1-800-467-8445, www.stihlusa.com