stihl sr420 mist blower manual

The STIHL SR420 is a professional-grade, backpack-mounted mist blower designed for applying pesticides, herbicides, and liquid fertilizers over large areas. It features a powerful 4.2 HP engine, adjustable airflow, variable flow rate control, ergonomic harness, corrosion-resistant tank, and multiple nozzle options for precise application. Below are key sections for safety, features, assembly, operation, maintenance, and troubleshooting.

Features at a Glance

Key components: 4.2 HP 2-stroke engine, 4-gallon polyethylene tank, adjustable harness, throttle control, airflow control knob, chemical flow control, nozzle assembly, and priming bulb.

Assembly & Preparation

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

1. Attach harness: Connect harness brackets to tank mounting points.
2. Install blower tube: Secure tube to engine outlet with provided clamp.
3. Connect hoses: Attach chemical delivery hose to pump outlet and nozzle assembly.
4. Install nozzle: Select desired nozzle type and screw onto blower tube end.
5. Add fuel mix: Fill fuel tank with 50:1 gasoline and 2-stroke oil mixture.

WARNING! Always wear personal protective equipment (PPE) including gloves, goggles, and respirator when handling chemicals. Assemble in a well-ventilated area.

First-Time Operation

Perform pre-operation checks before starting.

1. Check all connections: Ensure hoses, clamps, and nozzles are secure.
2. Fill chemical tank: Add mixed solution to tank, not exceeding 4 gallons.
3. Prime engine: Press priming bulb 5-7 times until fuel is visible.
4. Start engine: Set choke, pull starter cord firmly until engine starts.
5. Adjust settings: Set airflow and flow rate to minimum for initial test.
6. Test spray: Direct nozzle away from people/animals and test spray pattern.

CAUTION! Never point nozzle toward yourself or others. Keep bystanders at least 50 feet away during operation.

Specifications

• Engine Type: 2-stroke, single cylinder
• Engine Power: 4.2 HP at 7,000 RPM
• Fuel Capacity: 1.1 quarts (1.05 liters)
• Chemical Tank: 4 gallons (15 liters)
• Airflow Rate: Up to 700 CFM adjustable
• Flow Rate: 0 to 1.5 GPM adjustable
• Weight (empty): 25.3 lbs (11.5 kg)
• Sound Power Level: 105 dB(A) - hearing protection required
• Vibration Level: 5.2 m/s² (front handle)
• Warranty: 1 year commercial, 2 year residential

Controls and Settings

Main controls located on harness frame and blower tube.

Harness Frame: Airflow control knob, Flow rate control valve, Tank shut-off valve, Primer bulb.

Blower Tube: Throttle lever, Nozzle selector (if equipped).

Airflow Control: Adjust from minimum to maximum airflow for different droplet sizes. Flow Rate Control: Regulates chemical output from off to full flow. Throttle: Controls engine speed - low for fine mist, high for long-range application. Nozzle Selection: Cone nozzle for general broadcast, flat fan for targeted application, adjustable for variable patterns.

Connecting Accessories

Compatible with STIHL accessory nozzles and extension tubes.

1. Turn off engine and ensure chemical flow is stopped.
2. Remove current nozzle by unscrewing counterclockwise.
3. Apply thread seal tape to new accessory threads.
4. Screw accessory clockwise until hand-tight.
5. Check for leaks by running water through system before chemical use.

Tip: Always clean accessories immediately after use to prevent chemical buildup and corrosion.

Applying Chemicals

Follow chemical manufacturer instructions for mixing and application rates.

1. Calculate area: Measure treatment area to determine chemical quantity needed.
2. Mix solution: Combine chemical with water in separate container before adding to tank.
3. Filter solution: Use built-in tank filter or pour through mesh screen.
4. Set application rate: Adjust flow control based on walking speed and desired coverage.
5. Apply evenly: Walk at steady pace, swinging blower tube in smooth arc pattern.
6. Overlap passes: Overlap each pass by 30-50% for uniform coverage.
7. Clean after use: Triple rinse tank and run clean water through system for 2 minutes.

WARNING! Always follow chemical label directions. Never eat, drink, or smoke while handling chemicals. Wash hands thoroughly after use.

Safety Features

Quick-release harness, Deadman throttle (returns to idle when released), Chemical-resistant materials, Spark arrestor muffler, Tank pressure relief valve.

Care & Maintenance

Clean after each use. Store in dry, covered area.

1. Daily: Clean exterior, check for leaks, inspect hoses and connections.
2. Weekly: Clean tank thoroughly, inspect nozzle for clogs, check harness straps.
3. Monthly: Clean air filter, check spark plug, inspect fuel lines.
4. Seasonally: Change fuel filter, lubricate moving parts, store with empty tank.

CAUTION! Never store with chemical in tank. Always dispose of leftover chemicals according to local regulations.

Regulatory Information

EPA certified for pesticide application. Meets ANSI B175.2 standards for handheld power equipment. CARB compliant for California emissions. Copyright © STIHL Incorporated. All rights reserved. For technical support, visit www.stihlusa.com or contact 1-800-467-8445.

Troubleshooting

Service Intervals: Refer to maintenance schedule in manual. Major service recommended every 100 operating hours.

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