How to Start a Stihl Br 420 Leaf Blower?

Introduction

The Stihl BR 420 is one of the most popular backpack leaf blowers on the market, favored by both residential homeowners and commercial landscaping crews for its powerful 63.7cc engine, 500 CFM airflow, and ergonomic design. Despite its reputation for reliability, many new users struggle with starting the BR 420, especially if they are unfamiliar with two-stroke engine operation. Unlike four-stroke lawn mowers or trimmers, two-stroke engines require a specific fuel mix, priming steps, and choke adjustment to start properly, which can be confusing for first-time users. If you’re wondering how to start a Stihl BR 420 leaf blower, this comprehensive guide walks you through every step, from pre-start safety checks to troubleshooting common issues, so you can get to work clearing leaves, debris, and grass clippings in no time. We’ll also cover maintenance tips to prevent starting problems down the line, plus a comparison of the BR 420’s starting features to competing models to help you understand what makes it unique.

• Introduction
• Understanding Your Stihl BR 420
• Pre-Start Safety Checks
• Fuel Preparation for Cold Starts
• Step-by-Step Cold Start Guide
• Step-by-Step Warm Start Guide
• Troubleshooting Common Starting Issues
• Maintenance Tips to Prevent Starting Issues
• Stihl BR 420 vs. Competitor Starting Comparison
• Best Practices for Long-Term Reliability
• Frequently Asked Questions
• Conclusion

Understanding Your Stihl BR 420

The Stihl BR 420 is a professional-grade backpack leaf blower designed for heavy-duty use, with a 63.7cc two-stroke engine that delivers 500 cubic feet per minute (CFM) of airflow and a maximum airspeed of 200 mph. This power makes it ideal for clearing large piles of wet leaves, thick debris, and even light snow. Unlike electric blowers, the BR 420 runs on a gas and oil mix, which requires a few extra steps to start, but also delivers far more power and runtime for large properties.

Key Components of the BR 420

Before you learn how to start a Stihl BR 420 leaf blower, it’s important to familiarize yourself with the key components that control the starting process. These parts are located on the right side of the engine housing, within easy reach of the operator:

• Stop Switch: A small toggle switch marked “I” (on) and “O” (off). This cuts power to the spark plug when in the “O” position, preventing the engine from starting.
• Primer Bulb: A soft, squeezable rubber bulb located next to the fuel tank. Pressing this bulb draws fuel from the tank into the carburetor, priming the engine for starting.
• Choke Lever: A small metal or plastic lever that controls the air-to-fuel ratio. When closed, it restricts air flow to create a richer fuel mixture for cold starts.
• Throttle Trigger: A lever on the handle that controls engine speed. For warm starts, you may need to engage the fast idle position by squeezing the trigger slightly before pulling the starter cord.
• Starter Cord: A pull cord located on the top of the engine housing. Pulling this cord spins the engine’s crankshaft to ignite the fuel mixture.

Familiarizing yourself with these parts will make the starting process much smoother, and help you troubleshoot issues if the blower fails to start.

Pre-Start Safety Checks

Skipping pre-start safety checks can lead to injury, damage to the blower, or starting failures. Taking 2-3 minutes to complete these checks before each use will ensure safe, reliable operation.

Personal Protective Equipment

Always wear the following PPE before operating the BR 420:

• ANSI-rated safety glasses or goggles to protect your eyes from flying debris
• Hearing protection (earplugs or earmuffs) to prevent hearing damage from the blower’s 103 dB noise level
• Heavy-duty work gloves to protect your hands from vibration and sharp debris
• Non-slip, closed-toe boots to prevent slips on wet or uneven surfaces
• Long pants and a long-sleeved shirt to protect your skin from flying sticks and rocks

Workspace Safety

Before starting the blower, ensure your work area is safe:

• Place the blower on a flat, stable surface free of debris, wet grass, or oil that could cause slips.
• Clear the area of bystanders, pets, and loose objects that could become projectiles when the blower is running.
• Check that the fuel tank cap is tightened securely to prevent spills.
• Inspect the fuel line for cracks or leaks, which can cause fire hazards or starting issues.

Fuel Preparation for Cold Starts

Proper fuel preparation is the most critical factor in successfully starting a Stihl BR 420 leaf blower. Two-stroke engines require a precise mix of gasoline and oil to lubricate internal components, and using the wrong mix or stale fuel is the most common cause of starting failures.

Correct Fuel Mix Ratio

Stihl recommends a 50:1 fuel-to-oil ratio for the BR 420, meaning 50 parts gasoline to 1 part Stihl-approved two-stroke oil. For example, mix 1 gallon of 89 octane or higher unleaded gasoline with 2.6 ounces of Stihl two-stroke oil. Never use automotive motor oil, as it does not contain the additives needed to lubricate two-stroke engines properly, and will cause carbon buildup and starting issues over time. Always mix fuel in a clean, approved fuel container, and shake the container thoroughly before each use to ensure the oil and gas are fully combined.

Fresh Fuel Importance

Ethanol-blended gasoline (most standard pump gas) begins to degrade after 30 days, separating into gas and alcohol layers that clog the carburetor jets and prevent starting. For best results, use ethanol-free gasoline if available, and always mix no more fuel than you will use in a 30-day period. If you have leftover fuel at the end of the month, pour it into your car’s gas tank (if it’s compatible with ethanol) to avoid storing stale fuel. Adding a fuel stabilizer to your mix can extend fuel life up to 6 months, but it is still best to use fresh fuel for optimal starting performance.

Step-by-Step Cold Start Guide

A cold start is required when the engine has not been run for more than 30 minutes, or if the blower has been stored for an extended period. Follow these steps to start a cold Stihl BR 420:

1. Place the blower on a flat, stable surface. Ensure the throttle trigger is not engaged, and the stop switch is in the “I” (on) position. If the switch is in the “O” (off) position, the engine will not receive spark and will not start.
2. Press the primer bulb 3 to 5 times, holding it down for 1 second each time. You should see fuel begin to fill the clear fuel line connected to the carburetor. If no fuel appears in the line after 5 presses, check for a clogged fuel line or empty fuel tank.
3. Locate the choke lever on the side of the engine housing, and set it to the fully closed position. This is usually marked with a solid line or a “cold start” symbol. Closing the choke restricts air flow into the carburetor, creating a richer fuel mixture that is easier to ignite when the engine is cold.
4. Pull the starter cord slowly 2 to 3 times. Do not pull the cord at full speed, as this can flood the engine with too much fuel. These slow pulls draw fuel into the combustion chamber without overloading it.
5. Set the choke lever to the half-open position. Pull the starter cord firmly and quickly 3 to 5 times. You should hear the engine sputter or fire briefly, but it will not run continuously yet.
6. Once the engine fires, let it run for 10 to 15 seconds, then gradually open the choke lever fully. If the engine dies when you open the choke, close it slightly and pull the cord 1 to 2 more times to adjust the air-fuel ratio.
7. Once the engine runs smoothly at idle, squeeze the throttle trigger briefly to rev the engine to 3000-4000 RPM, then release. The blower is now fully started and ready for use. Let the engine idle for 30 seconds before engaging full throttle to allow the oil to circulate through all internal components.

Step-by-Step Warm Start Guide

A warm start is required if the engine has been running within the last 30 minutes, and the engine is still at operating temperature. Warm starts require fewer steps, as the engine already has a rich fuel mixture and does not need priming or choke adjustment.

1. Place the blower on a flat, stable surface, and ensure the stop switch is in the “I” (on) position.
2. If your BR 420 model has a fast idle lever, engage it by moving the lever to the “fast idle” position. If not, squeeze the throttle trigger slightly to set the engine to a higher idle speed for starting.
3. Pull the starter cord firmly 1 to 3 times. The engine should start immediately, as the warm combustion chamber ignites the fuel mixture easily.
4. Once the engine is running, release the throttle trigger (or disengage the fast idle lever) and let the engine idle for 30 seconds before using full throttle. This allows the engine to reach optimal operating temperature and ensures proper lubrication.

Troubleshooting Common Starting Issues

Even with proper starting technique, you may encounter starting issues from time to time. Below are the most common problems and their solutions:

Engine Won’t Crank

If pulling the starter cord feels loose and the engine does not turn over, first check that the stop switch is in the “I” (on) position. If the switch is on, the spark plug may be fouled, damaged, or have an incorrect gap. Remove the spark plug, clean it with a wire brush, and check the gap with a spark plug gauge (it should be 0.02 inches). If the spark plug is damaged, replace it with a new Stihl-approved spark plug. If the engine still won’t crank, the starter recoil spring may be broken, which requires professional repair.

Engine Cranks But Won’t Start

If the starter cord pulls normally and the engine turns over but does not fire, first check the choke position. For cold starts, the choke should be fully closed for the first few pulls, then half-open. If the choke is fully open during a cold start, the air-fuel mixture will be too lean to ignite. Next, check that the primer bulb was pressed enough times to fill the fuel line, and that the fuel tank has fresh, properly mixed fuel. If the fuel is stale or the mix ratio is incorrect, drain the tank and refill with fresh 50:1 mix. A clogged air filter can also prevent starting, so clean or replace the air filter if it is dirty.

Engine Starts But Dies Immediately

If the engine fires but dies within a few seconds of starting, the choke is likely closed too much, creating an overly rich fuel mixture that smothers the engine. Gradually open the choke lever while pulling the starter cord to find the correct air-fuel ratio. If the engine still dies, check for a clogged fuel line or carburetor jet, which can block fuel flow to the engine. A dirty air filter can also cause the engine to die, as it restricts air flow needed to keep the engine running.

Maintenance Tips to Prevent Starting Problems

Regular maintenance is the best way to avoid starting issues and extend the life of your Stihl BR 420. Follow these tips to keep your blower running reliably:

• Clean the air filter after every 10 hours of use: A dirty air filter restricts air flow, causing a lean fuel mixture that makes starting difficult. Tap the filter gently to remove loose debris, and wash it with soap and water if it is heavily soiled. Replace the filter every 6 months or if it is torn.
• Replace the spark plug every 100 hours of use: A worn or fouled spark plug will not produce a strong enough spark to ignite the fuel mixture. Check the spark plug gap every 25 hours, and clean or replace it as needed.
• Empty the fuel tank before long-term storage: If you will not use the blower for more than 30 days, drain the fuel tank and run the engine until it stalls to clear fuel from the carburetor. This prevents stale fuel from clogging the carburetor over time.
• Clean the carburetor annually: Carbon buildup in the carburetor jets can block fuel flow and cause starting issues. Use a carburetor cleaner spray to clean the jets every year, or take the blower to a Stihl dealer for a professional tune-up.
• Inspect fuel lines and O-rings every 25 hours: Cracked fuel lines or worn O-rings can cause fuel leaks, which not only create fire hazards but also prevent proper priming and starting. Replace damaged parts immediately.

Stihl BR 420 vs. Competitor Models: Starting Comparison

If you are comparing the Stihl BR 420 to other backpack leaf blowers, it’s helpful to understand how its starting features stack up against popular competitors. Below is a comparison of starting features for the BR 420, Echo PB-580T, and Husqvarna 350BT, three of the most popular models in its class:

As the table shows, the BR 420’s manual choke and primer system are simple and reliable, with fewer electronic components that can fail compared to the Husqvarna’s automatic choke. While the Echo’s automatic primer reduces the number of steps for cold starts, all three models require the same 50:1 fuel mix and regular maintenance to ensure reliable starting. The BR 420’s manual controls also give experienced users more precise control over the air-fuel ratio, which can be helpful for starting in extremely cold temperatures.

Best Practices for Long-Term Reliability

Following these best practices will ensure your Stihl BR 420 starts reliably for years to come:

• Use fuel stabilizer for long-term storage: If you need to store fuel for more than 30 days, add a fuel stabilizer to the mix to prevent ethanol separation and degradation. This is especially important if you use the blower only seasonally.
• Store the blower in a dry, sheltered location: Moisture can cause rust on internal engine components and clog the carburetor. Store the blower in a garage or shed, covered with a breathable tarp to protect it from dust and moisture.
• Run the engine dry before long-term storage: Before storing the blower for more than 30 days, drain the fuel tank, then start the engine and let it run until it stalls. This clears all fuel from the carburetor and fuel lines, preventing stale fuel buildup.
• Clean the blower after each use: Wipe down the engine housing, air filter, and cooling vents with a dry cloth after each use to remove grass, leaves, and debris. Buildup in the cooling vents can cause the engine to overheat, which can damage internal components and affect starting performance.
• Schedule an annual professional tune-up: Take the blower to a Stihl authorized dealer once a year for a full tune-up, including carburetor adjustment, spark plug replacement, and fuel system cleaning. This will catch small issues before they cause starting failures.

Frequently Asked Questions

Can I use regular gas in my Stihl BR 420?

No, the Stihl BR 420 requires 89 octane or higher unleaded gasoline mixed with Stihl-approved two-stroke oil at a 50:1 ratio. Regular 87 octane gas can cause engine knock, and gasoline with more than 10% ethanol degrades quickly, clogging the carburetor and causing starting issues. Ethanol-free gas is recommended for best performance.

Why do I have to pull the starter cord so many times?

Pulling the starter cord 3-5 times for a cold start is normal for the BR 420, as it takes a few pulls to draw fuel into the combustion chamber. If you need more than 10 pulls, check the primer bulb, choke position, fuel freshness, and spark plug condition. A clogged air filter or stale fuel mix are the most common causes of excessive pulling.

Can I start the BR 420 without the choke?

You can only start the BR 420 without the choke if the engine is warm, meaning it has been running within the last 30 minutes. For cold starts, the choke is required to create the rich fuel mixture needed to ignite the engine at ambient temperature. A cold start without the choke will result in a lean mixture that will not ignite.

How do I know if the fuel mix is correct?

A correct 50:1 mix has a uniform light amber color with no separated oil at the bottom of the container. If the mix looks cloudy or separated, it is mixed incorrectly. Always mix fuel in a clean approved container and shake it thoroughly for 10-15 seconds before use to ensure proper combination.

What should I do if the engine starts but dies right away?

If the engine starts but dies immediately, first check that the choke is not fully closed. Gradually open the choke while pulling the starter cord to adjust the air-fuel ratio. If the issue persists, check for a clogged air filter, stale fuel, or a fouled spark plug that needs cleaning or replacement.

Conclusion

Learning how to start a Stihl BR 420 leaf blower is simple once you understand the basics of two-stroke engine operation and follow the correct steps for cold and warm starts. By completing pre-start safety checks, using fresh 50:1 fuel mix, and adjusting the choke and primer correctly, you can start the blower in just a few pulls every time. Regular maintenance, including air filter cleaning, spark plug replacement, and carburetor cleaning, will prevent most starting issues and extend the life of your blower. If you do encounter problems, the troubleshooting steps outlined in this guide will help you identify and fix the issue quickly, without needing to take the blower to a repair shop. With proper care, your Stihl BR 420 will provide reliable, powerful performance for years of leaf and debris clearing.