Stihl SR 420 Manual

The Stihl SR 420 is a professional-grade string trimmer designed for demanding clearing and trimming tasks. It features a powerful 2-stroke engine, the Easy2Start™ system for simplified starting, an anti-vibration system, an ErgoStart™ starter, a translucent fuel tank, a loop handle, a straight shaft, and a tap-activated cutting head. Below are key sections for safety, features, operation, maintenance, and troubleshooting.

Features at a Glance

Key components: 2-stroke engine, Easy2Start™ system, anti-vibration handles, ErgoStart™ starter, translucent fuel tank, loop handle, straight shaft, tap-activated cutting head.

Feature	Description
Engine	2-stroke, single-cylinder, air-cooled
Starting System	Easy2Start™ for reduced pulling force
Anti-Vibration	Integrated system in handles and shaft
Starter	ErgoStart™ for comfortable grip
Fuel Tank	Translucent for easy fuel level check
Handle	Loop handle with throttle trigger lockout
Shaft	Straight, metal drive shaft
Cutting Head	Tap-activated for easy line advance
Throttle Trigger	With lockout to prevent accidental acceleration
Stop Switch	Easy access for immediate engine shut-off

Safety Information

Read all instructions before use. Wear appropriate PPE.

1. Personal Protective Equipment: Always wear safety glasses, hearing protection, sturdy gloves, long pants, and closed-toe shoes.
2. Clear Work Area: Remove stones, sticks, wire, and other debris that could be thrown or become entangled.
3. Fuel Safety: Refuel in a well-ventilated area with engine off and cool. Use only STIHL MotoMix® or equivalent 50:1 pre-mix fuel.
4. Operation: Keep bystanders and pets at a safe distance. Hold unit firmly with both hands.

WARNING! Risk of severe injury or death from rotating cutting line. Never operate without proper guard installed. Do not operate in poor lighting or adverse weather conditions.

Assembly & First Use

Assemble handle, cutting attachment, and fill with fuel/oil mix.

1. Attach Handle: Align handle with shaft and secure with provided bolts.
2. Install Cutting Head: Thread cutting head onto drive shaft and tighten securely by hand.
3. Install Guard: Attach debris guard to shaft above cutting head.
4. Fuel: Fill tank with STIHL MotoMix® or a 50:1 mixture of high-quality 2-stroke oil and unleaded gasoline (min. 89 octane).
5. Starting: Place unit on clear, flat ground. Set stop switch to "I". Press primer bulb 5 times. Set choke lever to cold start position. Pull starter rope smoothly until engine fires, then move choke to run position and pull again to start.

CAUTION! Ensure all fasteners are tight before operation. Do not force parts during assembly.

Specifications

• Engine Displacement: 41.2 cc.
• Power Output: 1.8 kW / 2.4 hp.
• Fuel/Oil Ratio: 50:1 (STIHL MotoMix® recommended).
• Fuel Tank Volume: 0.71 L (24 fl. oz.).
• Sound Pressure Level: Approx. 96 dB(A).
• Vibration Level (left/right): Approx. 4.5/5.5 m/s².
• Weight (without cutting attachment): 6.2 kg (13.7 lbs).
• Shaft Type: Straight, solid drive shaft.
• Cutting Swath: Depends on line diameter and head type.
• Warranty: Limited warranty for personal/commercial use.

Controls and Components

Main controls located on the handle and engine housing.

Handle Area: Throttle trigger, throttle trigger lockout, stop switch (I/0).

Engine Housing: Starter grip, choke lever (cold start/run), primer bulb, air filter cover, spark plug.

Shaft/Head: Debris guard, cutting head (tap-activated), line spool.

Operation: Squeeze throttle trigger lockout and throttle trigger simultaneously to accelerate. Release to idle. Press stop switch to "0" to stop engine.

Operation

Use a smooth, sweeping motion from right to left.

1. Starting: Follow "First Use" procedure. For warm start, set choke to run position.
2. Trimming Technique: Hold trimmer at a slight angle so the cutting line contacts vegetation at full speed. Use a sweeping motion, cutting from right to left for best debris discharge.
3. Edging: Rotate unit so cutting head is vertical. Use line tip for precise edging along sidewalks or driveways.
4. Stopping: Release throttle, allow engine to idle for a few seconds, then press stop switch to "0".

Tip: Do not force the tool. Let the cutting line speed do the work. Avoid cutting near fences, walls, or trees to prevent line wear.

Maintenance & Storage

Regular maintenance ensures performance and longevity.

1. Daily: Clean exterior, check for loose fasteners, inspect cutting head and line.
2. Air Filter: Clean weekly or more often in dusty conditions. Remove cover, tap out debris, or wash in soapy water (let dry completely).
3. Spark Plug: Check and clean electrode gap (0.5 mm / 0.020 in) every 50 hours. Replace if worn.
4. Fuel Filter: Replace annually or if fuel flow is restricted.
5. Storage: For long-term storage (over 30 days), run engine until fuel tank is empty. Remove spark plug and add a few drops of oil into cylinder, then pull starter cord slowly to distribute. Store in a clean, dry place.

WARNING! Always perform maintenance with engine off and spark plug wire disconnected. Use only STIHL approved parts.

Cutting Head & Line

Uses a tap-activated head with .095" or .105" diameter line.

1. Line Advance: Bump cutting head firmly on ground (with engine at high idle) to advance fresh line.
2. Replacing Line: Stop engine. Remove spool by depressing tabs and turning. Wind new line onto spool following direction arrows. Leave approx. 15 cm (6 in) of line unwound. Reinstall spool, threading ends through eyelets.
3. Line Length: Do not extend line beyond cutting guard. Trim ends evenly for balanced operation.
4. Spool Replacement: Replace entire spool when line is depleted or if spool is damaged.

WARNING! Use only STIHL approved trimmer line. Incorrect line can damage the cutting head or cause imbalance.

Carburetor Adjustment

Factory pre-set. Adjustment may be needed for altitude or temperature changes.

Adjustment screws (may be limited by caps): L (low speed), H (high speed), LA (idle speed).

1. Warm up engine for 5 minutes.
2. Idle Speed (LA): Turn clockwise to increase, counter-clockwise to decrease. Set so cutting head does not rotate at idle.
3. Low Speed (L): For smooth acceleration from idle. Turn clockwise until engine runs poorly, then counter-clockwise to find smoothest acceleration.
4. High Speed (H): For maximum power without overspeed. Recommended to be set by authorized STIHL dealer.

CAUTION! Improper adjustment can cause engine damage or unsafe operation. If unsure, consult a STIHL servicing dealer.

Warranty & Support

STIHL Limited Warranty. Proof of purchase required.

For personal use: Warranty period as defined by region. For commercial use: Different warranty terms may apply. Warranty does not cover damage from misuse, lack of maintenance, or unapproved modifications. For support, contact an authorized STIHL dealer or visit www.stihlusa.com.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine will not start	No fuel, flooded, spark plug faulty	Check fuel; set choke to run and pull with throttle open; check/replace spark plug.
Engine starts then dies	Clogged fuel filter, dirty air filter, carburetor issue	Replace fuel filter; clean air filter; have carburetor serviced.
Loss of power	Clogged air filter, old fuel, exhaust screen blocked	Clean/replace air filter; use fresh fuel; clean spark arrestor screen.
Excessive vibration	Bent shaft, unbalanced cutting head, loose parts	Inspect shaft for damage; check head balance; tighten all fasteners.
Line does not advance	Empty spool, tangled line, worn head	Replace line spool; rewind line correctly; replace cutting head if worn.

Dealer Service: For complex repairs, contact an authorized STIHL dealer.

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