Stihl FS46 String Trimmer (5 Expert Fixes For Engine Start Failure)

As someone who’s spent countless hours in the woods, felling trees and preparing firewood, I understand the frustration of a tool that won’t cooperate. The Stihl FS46 string trimmer is a reliable workhorse, but like any small engine, it can sometimes refuse to start. I’ve wrestled with this issue myself more times than I care to admit, often in the chilly dawn before a long day of clearing brush. Over the years, I’ve developed a systematic approach to diagnosing and fixing these problems. I will share my top 5 fixes for a Stihl FS46 string trimmer that refuses to start. This guide is designed to help you get back to work quickly and efficiently.

Stihl FS46 String Trimmer: 5 Expert Fixes for Engine Start Failure

A non-starting string trimmer can halt your yard work in its tracks. These fixes are based on my experience and aim to address the most common causes of starting problems with the Stihl FS46. We’ll cover fuel issues, spark problems, air intake blockages, and even delve into carburetor adjustments. Let’s get started.

1. Fuel System Troubleshooting: Ensuring a Clean and Consistent Fuel Supply

The fuel system is the lifeline of any small engine. Without a clean and consistent supply of fuel, your Stihl FS46 simply won’t start. I’ve seen firsthand how neglected fuel can cause headaches. I once spent an entire morning trying to start a trimmer only to discover the fuel filter was completely clogged with debris. Let’s examine the fuel system step-by-step.

Fuel Mixture: The Foundation of Reliable Operation

The Stihl FS46 requires a specific fuel-to-oil mixture. Stihl typically recommends a 50:1 ratio using high-quality two-stroke oil. This means 50 parts gasoline to 1 part oil. Using the wrong mixture can cause serious engine damage, including scoring of the cylinder walls due to insufficient lubrication.

Why it matters: The oil in the mixture lubricates the engine’s internal components. Too little oil, and you risk engine seizure. Too much oil, and you’ll experience excessive smoke and potential fouling of the spark plug.
My experience: I always pre-mix my fuel in a separate container and use a graduated measuring cup to ensure accuracy. I stick to Stihl’s two-stroke oil, as I’ve found it provides excellent lubrication and burns cleanly.
Actionable step: Double-check your fuel mixture. If you suspect it’s incorrect, drain the fuel tank and refill it with the correct ratio.

Fuel Quality: The Importance of Fresh Fuel

Gasoline degrades over time, especially when mixed with oil. Old fuel can become gummy and varnish-like, clogging fuel lines, filters, and carburetor jets.

Why it matters: Stale fuel loses its volatility, making it difficult to ignite. This is especially true in colder weather.
My experience: I make it a rule to only mix enough fuel for a few weeks of use. I also add a fuel stabilizer to my fuel can, which helps prevent degradation.
Actionable step: If your fuel has been sitting for more than a month, discard it properly and replace it with fresh fuel.

Fuel Filter Inspection and Replacement: Preventing Blockages

The fuel filter prevents debris from entering the carburetor. A clogged filter restricts fuel flow, leading to starting problems or poor engine performance.

Why it matters: The fuel filter is your engine’s first line of defense against contaminants.
My experience: I routinely check the fuel filter on my FS46 every few months. I simply remove it from the fuel tank using a bent piece of wire and inspect it for debris.
Actionable step: Locate the fuel filter inside the fuel tank (it’s usually attached to the fuel line). Remove it and inspect it. If it’s dirty or clogged, replace it with a new one. Stihl part number is typically printed on the filter itself.

Fuel Line Inspection: Checking for Cracks and Leaks

Fuel lines can become brittle and cracked over time, leading to fuel leaks and air leaks. Air leaks in the fuel system can prevent the engine from drawing fuel properly.

Why it matters: Even a small crack in a fuel line can disrupt the fuel supply.
My experience: I’ve had fuel lines crack due to exposure to sunlight and extreme temperatures.
Actionable step: Carefully inspect all fuel lines for cracks, leaks, or kinks. Replace any damaged lines with new ones of the correct size and material.

Fuel Tank Vent: Ensuring Proper Airflow

The fuel tank vent allows air to enter the tank as fuel is used. A clogged vent creates a vacuum in the tank, preventing fuel from flowing to the carburetor.

Why it matters: Without proper venting, the engine will starve for fuel.
My experience: I’ve seen fuel tank vents become clogged with dirt and debris.
Actionable step: Locate the fuel tank vent (it’s usually a small hole in the fuel cap or a separate vent line). Clean it with a piece of wire or compressed air.

Key Takeaway: A clean and properly functioning fuel system is crucial for starting your Stihl FS46. Regularly inspect and maintain the fuel filter, fuel lines, and fuel tank vent.

2. Spark Plug Diagnosis and Maintenance: Igniting the Combustion Process

The spark plug ignites the fuel-air mixture in the engine cylinder. A faulty spark plug is a common cause of starting problems. I’ve often found that simply replacing a spark plug can solve the issue.

Spark Plug Inspection: Visual Assessment of Condition

A visual inspection of the spark plug can reveal a lot about the engine’s condition. Look for signs of fouling, wear, or damage.

Why it matters: The spark plug’s condition reflects the engine’s overall health.
My experience: I’ve seen spark plugs fouled with carbon deposits, oil, and even small pieces of debris.
Actionable step: Remove the spark plug and inspect it. A healthy spark plug will have a tan or light brown color. Black, oily, or wet spark plugs indicate a problem.

Spark Plug Cleaning: Removing Fouling Deposits

If the spark plug is fouled with carbon deposits, you can try cleaning it.

Why it matters: Cleaning can restore the spark plug’s performance.
My experience: I’ve used a wire brush and carburetor cleaner to remove carbon deposits from spark plugs.
Actionable step: Use a wire brush to clean the electrodes of the spark plug. You can also use carburetor cleaner to dissolve stubborn deposits. Be careful not to damage the electrodes.

Spark Plug Gap Adjustment: Ensuring Optimal Spark

The spark plug gap is the distance between the center electrode and the ground electrode. The correct gap is crucial for optimal spark.

Why it matters: An incorrect gap can result in a weak or nonexistent spark.
My experience: I always use a spark plug gapping tool to ensure the correct gap.
Actionable step: Use a spark plug gapping tool to adjust the gap to the specification in your Stihl FS46 owner’s manual (typically around 0.020 inches or 0.5 mm).

Spark Plug Testing: Verifying Spark Delivery

Even if the spark plug looks good, it’s essential to test it to ensure it’s delivering a strong spark.

Why it matters: A weak spark may not be sufficient to ignite the fuel-air mixture.
My experience: I use an inline spark tester to visually confirm spark delivery.
Actionable step: Reconnect the spark plug to the spark plug wire. Hold the spark plug against the engine cylinder (grounding it). Pull the starter cord and observe the spark plug. You should see a strong, blue spark. If the spark is weak, yellow, or nonexistent, replace the spark plug.

Spark Plug Replacement: When to Replace

If the spark plug is damaged, severely fouled, or fails the spark test, it’s time to replace it.

Why it matters: A new spark plug ensures reliable ignition.
My experience: I typically replace the spark plug on my FS46 every year, regardless of its condition.
Actionable step: Replace the spark plug with a new one of the correct type (refer to your owner’s manual for the correct spark plug type). Stihl typically uses NGK or Bosch spark plugs.

Key Takeaway: A properly functioning spark plug is essential for starting your Stihl FS46. Regularly inspect, clean, and replace the spark plug as needed.

3. Air Filter Maintenance: Promoting Clean Air Intake

The air filter prevents dirt and debris from entering the engine. A clogged air filter restricts airflow, leading to starting problems, poor performance, and engine damage. I once neglected the air filter on my trimmer, and it cost me a costly repair bill when dirt entered the engine and scored the cylinder walls.

Air Filter Inspection: Assessing Cleanliness

Regularly inspect the air filter for dirt and debris.

Why it matters: A clean air filter allows the engine to breathe properly.
My experience: I check the air filter on my FS46 every time I refuel.
Actionable step: Remove the air filter cover and inspect the air filter. If it’s dirty, clean or replace it.

Air Filter Cleaning: Removing Dust and Debris

You can clean most air filters by tapping them gently or using compressed air.

Why it matters: Cleaning can extend the life of the air filter.
My experience: I use compressed air to blow out the dirt and debris from my air filter.
Actionable step: Tap the air filter gently to remove loose dirt. If necessary, use compressed air to blow out the remaining dirt. Be careful not to damage the filter element.

Air Filter Replacement: When Cleaning Isn’t Enough

If the air filter is heavily soiled or damaged, it’s time to replace it.

Why it matters: A new air filter ensures optimal airflow.
My experience: I replace the air filter on my FS46 every year.
Actionable step: Replace the air filter with a new one of the correct type (refer to your owner’s manual).

Air Intake Inspection: Checking for Obstructions

Even with a clean air filter, the air intake can become blocked by debris.

Why it matters: A blocked air intake restricts airflow.
My experience: I’ve found leaves, twigs, and even spiderwebs blocking the air intake on my trimmer.
Actionable step: Inspect the air intake area for any obstructions. Remove any debris that you find.

Key Takeaway: A clean and unobstructed air intake is crucial for starting and running your Stihl FS46. Regularly inspect and maintain the air filter and air intake area.

4. Recoil Starter Mechanism: Examining Pull Cord Functionality

The recoil starter mechanism is responsible for turning the engine over to start it. A faulty recoil starter can prevent the engine from starting. I’ve had recoil starters fail due to broken springs, frayed ropes, and worn pawls.

Recoil Starter Inspection: Checking for Damage

Inspect the recoil starter for any signs of damage, such as a broken rope, a damaged pulley, or a loose spring.

Why it matters: A damaged recoil starter can prevent the engine from turning over.
My experience: I’ve had the recoil starter rope break on my trimmer in the middle of a job.
Actionable step: Inspect the recoil starter for any visible damage.

Recoil Starter Rope Replacement: Addressing Fraying or Breaks

If the recoil starter rope is frayed or broken, replace it with a new one.

Why it matters: A frayed or broken rope can prevent the engine from starting.
My experience: Replacing the recoil starter rope is a relatively simple task.
Actionable step: Replace the recoil starter rope with a new one of the correct length and diameter. Follow the instructions in your owner’s manual.

Recoil Starter Spring Tension: Restoring Proper Retraction

If the recoil starter rope doesn’t retract properly, the spring tension may be too weak.

Why it matters: Weak spring tension can prevent the engine from starting.
My experience: I’ve had to adjust the spring tension on my recoil starter several times.
Actionable step: Adjust the spring tension according to the instructions in your owner’s manual. Be careful not to over-tighten the spring.

Recoil Starter Pawl Inspection: Ensuring Engagement

The pawls are small levers that engage the engine flywheel when you pull the starter rope. If the pawls are worn or damaged, they may not engage properly.

Why it matters: Worn pawls can prevent the engine from turning over.
My experience: I’ve had to replace the pawls on my recoil starter due to wear.
Actionable step: Inspect the pawls for wear or damage. Replace them if necessary.

Key Takeaway: A properly functioning recoil starter mechanism is essential for starting your Stihl FS46. Regularly inspect and maintain the recoil starter components.

5. Carburetor Adjustments: Fine-Tuning Fuel-Air Mixture

The carburetor regulates the mixture of fuel and air that enters the engine. Incorrect carburetor settings can lead to starting problems, poor performance, and excessive emissions. Adjusting the carburetor should be done with caution, as incorrect adjustments can damage the engine.

Identifying Carburetor Adjustment Screws

The Stihl FS46 carburetor typically has two or three adjustment screws:

Low-speed (L) screw: Controls the fuel-air mixture at idle and low speeds.
High-speed (H) screw: Controls the fuel-air mixture at high speeds.
Idle speed (LA) screw: Controls the engine’s idle speed.
Why it matters: Knowing the function of each screw is crucial for making accurate adjustments.
My experience: I always refer to the owner’s manual before making any carburetor adjustments.
Actionable step: Locate the carburetor adjustment screws on your Stihl FS46. Refer to your owner’s manual for their specific locations.

Initial Carburetor Settings: Establishing a Baseline

Before making any adjustments, it’s essential to know the initial settings of the carburetor screws.

Why it matters: The initial settings provide a starting point for fine-tuning.
My experience: I typically start by turning each screw clockwise until it’s lightly seated, then backing it out to the factory setting (usually specified in the owner’s manual).
Actionable step: Turn each adjustment screw clockwise until it’s lightly seated (do not overtighten). Then, back it out to the factory setting specified in your owner’s manual.

Low-Speed (L) Adjustment: Optimizing Idle Performance

The low-speed screw affects the engine’s idle and low-speed performance.

Why it matters: Proper low-speed adjustment ensures smooth idling and good throttle response.
My experience: I adjust the low-speed screw until the engine idles smoothly without stalling.
Actionable step: Start the engine and let it warm up. Turn the low-speed screw clockwise to lean the mixture (less fuel) or counterclockwise to richen the mixture (more fuel). Adjust the screw until the engine idles smoothly and responds quickly to throttle input.

High-Speed (H) Adjustment: Maximizing Power Output

The high-speed screw affects the engine’s performance at high speeds. Caution: Incorrect high-speed adjustments can damage the engine.

Why it matters: Proper high-speed adjustment ensures maximum power output without risking engine damage.
My experience: I typically adjust the high-speed screw by ear, listening for the engine to run smoothly and powerfully without any signs of hesitation or misfire.
Actionable step: With the engine running at full throttle, turn the high-speed screw clockwise to lean the mixture or counterclockwise to richen the mixture. Adjust the screw until the engine runs smoothly and powerfully without any signs of hesitation or misfire. Important: Do not run the engine lean for extended periods, as this can cause engine damage.

Idle Speed (LA) Adjustment: Setting the Correct Idle Speed

The idle speed screw controls the engine’s idle speed.

Why it matters: Proper idle speed prevents the engine from stalling or running too fast at idle.
My experience: I adjust the idle speed screw until the engine idles smoothly without the trimmer head engaging.
Actionable step: Turn the idle speed screw clockwise to increase the idle speed or counterclockwise to decrease the idle speed. Adjust the screw until the engine idles smoothly without the trimmer head engaging.

Key Takeaway: Carburetor adjustments can fine-tune your Stihl FS46’s performance. However, proceed with caution and always refer to your owner’s manual. If you’re unsure about making carburetor adjustments, consult a qualified service technician.

Additional Tips and Considerations

Beyond these five key fixes, here are some additional tips and considerations that can help you troubleshoot starting problems with your Stihl FS46:

Choke Position: Ensure the choke is in the correct position for starting. The choke enriches the fuel-air mixture, which is often necessary for cold starting.
Primer Bulb: Prime the carburetor by pressing the primer bulb several times before starting. This helps draw fuel into the carburetor.
Compression Test: If you’ve tried all the above fixes and your engine still won’t start, consider performing a compression test. Low compression can indicate worn piston rings or valves.
Professional Service: If you’re not comfortable performing these repairs yourself, or if you’ve tried everything and your engine still won’t start, take it to a qualified service technician.

Safety Precautions

When working on your Stihl FS46, always follow these safety precautions:

Disconnect the spark plug wire: This prevents accidental starting of the engine.
Wear safety glasses: Protect your eyes from flying debris.
Work in a well-ventilated area: Avoid breathing in gasoline fumes.
Use caution when handling fuel: Gasoline is flammable.
Refer to your owner’s manual: Always consult your owner’s manual for specific instructions and safety information.

By following these steps, you should be able to diagnose and fix most starting problems with your Stihl FS46 string trimmer. Remember to be patient, methodical, and always prioritize safety. With a little effort, you can get your trimmer running smoothly and get back to tackling your yard work. Good luck, and happy trimming!