Carburetor For Stihl FS 55 (5 Pro Tips For Smooth Trimming)

Okay, let’s dive deep into the heart of small engine mechanics and get your Stihl FS 55 trimmer humming like new.

Introduction: The Frustration of a Faulty Fuel System

There’s nothing quite as frustrating as gearing up for a satisfying trimming session, only to be met with a sputtering, stalling engine. I’ve been there countless times, wrestling with stubborn equipment that refuses to cooperate. Often, the culprit behind this headache is the carburetor. It’s the heart of your engine’s fuel system, and when it malfunctions, your trimmer becomes about as useful as a screen door on a submarine.

The Stihl FS 55 is a workhorse, a reliable tool I’ve often relied on for tackling overgrown edges and maintaining a tidy yard. But even the most dependable machines can fall victim to the inevitable wear and tear. A dirty or damaged carburetor can lead to a host of problems: difficult starting, rough idling, loss of power, and even complete engine failure.

The good news? Replacing or properly tuning a carburetor on a Stihl FS 55 is a task that’s well within the reach of the average DIY enthusiast. It doesn’t require a degree in mechanical engineering, just a bit of patience, the right tools, and a solid understanding of the process.

In this guide, I’m going to share five pro tips that will help you get your Stihl FS 55 carburetor sorted out and your trimmer running smoothly again. I’ll walk you through the process step-by-step, drawing on my own experiences and insights gained from years of working with small engines. Let’s get started!

Key Takeaways:

Understanding the Carburetor’s Role: Recognize the carburetor’s critical function in delivering the correct fuel-air mixture for optimal engine performance.
Identifying Carburetor Issues: Learn to diagnose common symptoms of a faulty carburetor, such as starting problems, poor idling, and power loss.
Essential Tools and Materials: Gather the necessary tools and replacement parts before beginning the carburetor replacement or tuning process.
Step-by-Step Replacement Guide: Follow a detailed, easy-to-understand guide for safely removing the old carburetor and installing a new one.
Tuning and Adjustment Tips: Master the art of carburetor tuning to achieve optimal engine performance and fuel efficiency.

The Importance of a Well-Functioning Carburetor

The carburetor’s job is simple in theory: to mix air and fuel in the correct proportion so the engine can combust it and generate power. In practice, it’s a delicate balancing act. Too much fuel (a “rich” mixture) can cause the engine to run poorly, emit excessive smoke, and waste fuel. Too little fuel (a “lean” mixture) can lead to overheating, engine damage, and a lack of power.

The Stihl FS 55 carburetor is a relatively simple design, but it’s still a precision instrument. Over time, it can become clogged with dirt, varnish, and other contaminants. The tiny passages and jets within the carburetor are particularly vulnerable to blockage. Additionally, the carburetor’s internal components, such as the diaphragm and needle valve, can wear out or become damaged, leading to improper fuel metering.

1. Diagnosing Carburetor Problems: Is It Really the Carburetor?

Before you jump to replacing the carburetor, it’s important to make sure that’s actually the problem. Many engine issues can mimic carburetor problems, so a little bit of troubleshooting can save you time and money.

Here are some common symptoms of a faulty carburetor on a Stihl FS 55:

Difficult Starting: The engine is hard to start, or it requires multiple attempts to get it running.
Rough Idling: The engine idles erratically, stalls frequently, or runs very rough.
Loss of Power: The engine lacks power, especially when under load.
Hesitation: The engine hesitates or stumbles when you accelerate.
Excessive Smoke: The engine emits excessive black smoke, indicating a rich fuel mixture.
Fuel Leakage: Fuel leaks from the carburetor or surrounding area.

Ruling Out Other Potential Causes

Before you condemn the carburetor, check these other potential culprits:

Fuel Quality: Old or contaminated fuel can cause a variety of engine problems. Always use fresh, high-quality fuel mixed with the correct ratio of two-stroke oil. I personally prefer using fuel stabilizers, especially if the trimmer sits unused for extended periods.
Spark Plug: A fouled or worn spark plug can prevent the engine from starting or running properly. Remove the spark plug and inspect it. If it’s dirty or damaged, replace it with a new one. Ensure the spark plug gap is correct.
Air Filter: A dirty air filter restricts airflow to the engine, leading to a rich fuel mixture. Remove the air filter and clean it or replace it if necessary.
Fuel Lines: Check the fuel lines for cracks, leaks, or blockages. Replace any damaged fuel lines.
Fuel Filter: The fuel filter prevents debris from entering the carburetor. A clogged fuel filter can starve the engine of fuel. Replace the fuel filter if it’s dirty or blocked.
Ignition System: A faulty ignition coil or other ignition system components can prevent the engine from starting or running properly. This is less common, but it’s worth checking if you’ve ruled out other potential causes.

A Personal Anecdote: The Case of the Mysterious Misfire

I once spent an entire afternoon troubleshooting a Stihl FS 55 that was running terribly. I was convinced it was the carburetor, so I took it apart, cleaned it meticulously, and reassembled it. The problem persisted. Frustrated, I was about to order a new carburetor when I decided to check the spark plug one last time. To my surprise, the spark plug was cracked and barely firing. A quick spark plug replacement solved the problem instantly. The moral of the story? Always check the simple things first!

2. Gathering Your Tools and Materials: Preparation is Key

Before you start tearing into your Stihl FS 55, it’s essential to gather all the necessary tools and materials. Having everything on hand will make the process much smoother and prevent you from having to stop mid-job to run to the hardware store.

Here’s a list of the tools and materials you’ll need:

New Carburetor: Purchase a replacement carburetor that is specifically designed for the Stihl FS 55. Make sure you buy a high-quality carburetor from a reputable supplier. I’ve found that aftermarket carburetors can vary greatly in quality, so it’s worth spending a little extra to get a good one.
Screwdrivers: You’ll need a variety of screwdrivers, including Phillips-head and flat-head screwdrivers, in different sizes.
Pliers: Pliers are useful for removing fuel lines and other small parts.
Socket Set: A socket set can be helpful for removing the air filter housing and other components.
Carburetor Cleaner: Carburetor cleaner is essential for cleaning the carburetor and removing dirt, varnish, and other deposits.
Small Brushes: Small brushes, such as toothbrushes or carburetor cleaning brushes, are useful for cleaning the carburetor’s tiny passages and jets.
Compressed Air: Compressed air can be used to blow out the carburetor’s passages and jets after cleaning.
Fuel Line Clamp: A fuel line clamp can be used to prevent fuel from leaking when you disconnect the fuel lines.
New Fuel Lines (Optional): If your fuel lines are cracked, brittle, or leaking, replace them with new fuel lines.
New Fuel Filter (Optional): It’s a good idea to replace the fuel filter when you replace the carburetor.
Spark Plug Wrench: You’ll need a spark plug wrench to remove and install the spark plug.
Shop Rags: Shop rags are essential for cleaning up spills and wiping off dirty parts.
Safety Glasses: Always wear safety glasses when working with tools and chemicals.
Gloves: Gloves can protect your hands from dirt, grease, and chemicals.
Carburetor Adjustment Tool (Screwdriver type): A special “D” shaped screwdriver is often needed to adjust the high and low speed mixture screws.

Finding the Right Carburetor

When purchasing a replacement carburetor, make sure it’s compatible with your specific Stihl FS 55 model. There are different versions of the FS 55, so it’s important to check the model number and serial number of your trimmer to ensure you get the correct carburetor. Many online retailers and local small engine shops can help you identify the correct carburetor for your machine.

3. Step-by-Step Carburetor Replacement: A Hands-On Guide

Now that you’ve gathered your tools and materials, it’s time to get down to business. Follow these step-by-step instructions to replace the carburetor on your Stihl FS 55:

Step 1: Preparation and Safety

Disconnect the Spark Plug Wire: This is crucial for safety. Disconnecting the spark plug wire prevents the engine from accidentally starting while you’re working on it.
Drain the Fuel Tank: Empty the fuel tank to prevent fuel from spilling when you disconnect the fuel lines. You can drain the fuel into a suitable container.
Work in a Well-Ventilated Area: Carburetor cleaner and gasoline fumes can be harmful, so work in a well-ventilated area.

Step 2: Removing the Air Filter Housing

Remove the Air Filter Cover: Remove the air filter cover by unscrewing the retaining screws or clips.
Remove the Air Filter: Remove the air filter from the air filter housing.
Remove the Air Filter Housing: Remove the air filter housing by unscrewing the retaining screws.

Step 3: Disconnecting the Fuel Lines

Locate the Fuel Lines: Identify the fuel lines that connect to the carburetor. There are typically two fuel lines: a fuel supply line and a fuel return line.
Use a Fuel Line Clamp: Use a fuel line clamp to pinch off the fuel lines to prevent fuel from leaking when you disconnect them.
Disconnect the Fuel Lines: Carefully disconnect the fuel lines from the carburetor. Use pliers if necessary to loosen the fuel lines. Note which fuel line goes where! Take a picture if you need to.

Step 4: Removing the Old Carburetor

Locate the Carburetor Mounting Screws: Identify the screws that hold the carburetor to the engine.
Remove the Carburetor Mounting Screws: Remove the carburetor mounting screws.
Remove the Old Carburetor: Carefully remove the old carburetor from the engine. Be sure to remove any gaskets or spacers that may be attached to the carburetor or engine.

Step 5: Installing the New Carburetor

Install New Gaskets and Spacers: Install new gaskets and spacers (if applicable) onto the engine. This will ensure a good seal between the carburetor and the engine.
Mount the New Carburetor: Carefully mount the new carburetor onto the engine.
Install the Carburetor Mounting Screws: Install the carburetor mounting screws and tighten them securely.

Step 6: Reconnecting the Fuel Lines

Reconnect the Fuel Lines: Reconnect the fuel lines to the carburetor. Make sure you connect the fuel lines to the correct ports. Refer to the picture you took earlier if needed.
Remove the Fuel Line Clamp: Remove the fuel line clamp.

Step 7: Reinstalling the Air Filter Housing

Reinstall the Air Filter Housing: Reinstall the air filter housing and secure it with the retaining screws.
Reinstall the Air Filter: Reinstall the air filter into the air filter housing.
Reinstall the Air Filter Cover: Reinstall the air filter cover and secure it with the retaining screws or clips.

Step 8: Reconnecting the Spark Plug Wire

Reconnect the Spark Plug Wire: Reconnect the spark plug wire to the spark plug.

Step 9: Starting the Engine

Fill the Fuel Tank: Fill the fuel tank with fresh fuel mixed with the correct ratio of two-stroke oil.
Prime the Carburetor: Prime the carburetor by pressing the primer bulb several times until fuel is visible in the bulb.
Start the Engine: Start the engine and let it run for a few minutes to warm up.

A Word of Caution: The Importance of Cleanliness

When working with carburetors, cleanliness is paramount. Even a tiny speck of dirt can clog the carburetor’s passages and cause problems. Before installing the new carburetor, make sure the mounting surface on the engine is clean and free of debris. Use a clean shop rag to wipe it down. Also, be careful not to introduce any dirt into the fuel lines when you’re connecting them to the carburetor.

4. Carburetor Tuning and Adjustment: Fine-Tuning for Optimal Performance

Once you’ve installed the new carburetor, it’s important to tune it properly to achieve optimal engine performance and fuel efficiency. Carburetor tuning involves adjusting the fuel-air mixture to ensure the engine is running smoothly and efficiently.

The Stihl FS 55 carburetor typically has two adjustment screws:

High-Speed Adjustment Screw (H): This screw controls the fuel-air mixture at high engine speeds.
Low-Speed Adjustment Screw (L): This screw controls the fuel-air mixture at low engine speeds and idle.

The Tuning Process

Warm Up the Engine: Start the engine and let it run for a few minutes to warm up.
Locate the Adjustment Screws: Locate the high-speed (H) and low-speed (L) adjustment screws on the carburetor.
Initial Settings: As a starting point, gently turn both screws clockwise until they seat lightly. Then, back them out (counter-clockwise) to the factory recommended settings. These settings can usually be found in your owner’s manual or online. A common starting point is 1 to 1.5 turns out.
Adjust the Low-Speed Screw (L): Turn the low-speed screw (L) clockwise to lean the mixture (less fuel) or counter-clockwise to richen the mixture (more fuel). Adjust the screw until the engine idles smoothly without stalling.
Adjust the High-Speed Screw (H): With the engine at full throttle, adjust the high-speed screw (H) to achieve the best performance. You want the engine to run smoothly without hesitation or excessive smoke.
Fine-Tuning: Make small adjustments to the high-speed and low-speed screws until you achieve the desired engine performance.
Idle Speed Adjustment: Some carburetors also have an idle speed adjustment screw. If your carburetor has one, adjust it to set the desired idle speed.

Important Considerations

Use a Tachometer (Optional): A tachometer can be helpful for setting the correct idle speed and high-speed RPM.
Listen to the Engine: Pay close attention to the engine’s sound and performance when you’re adjusting the carburetor. The engine should run smoothly without hesitation, excessive smoke, or stalling.
Don’t Force the Screws: Be careful not to force the adjustment screws. If you encounter resistance, stop and check the threads for damage.
Small Adjustments: Make small adjustments to the carburetor and let the engine run for a few seconds to see how it responds.
Altitude: Carburetor tuning can be affected by altitude. If you’re working at a high altitude, you may need to lean out the fuel mixture.

Dealing with “Tamper-Proof” Carburetors

Many newer Stihl FS 55 trimmers come equipped with carburetors that have “tamper-proof” adjustment screws. These screws are designed to prevent users from easily adjusting the carburetor. However, it is still possible to adjust these carburetors with the right tools. You’ll typically need a special “D” shaped screwdriver or a similar tool to access the adjustment screws. These tools are readily available online or at small engine repair shops.

5. Maintenance and Prevention: Keeping Your Carburetor Clean

Once you’ve got your Stihl FS 55 carburetor running smoothly, it’s important to take steps to prevent future problems. Regular maintenance and proper storage can help keep your carburetor clean and functioning properly for years to come.

Here are some tips for maintaining your carburetor:

Use Fresh Fuel: Always use fresh, high-quality fuel mixed with the correct ratio of two-stroke oil. Old or contaminated fuel can cause a variety of engine problems.
Use Fuel Stabilizer: Add fuel stabilizer to the fuel tank, especially if you’re going to be storing the trimmer for an extended period of time. Fuel stabilizer prevents the fuel from breaking down and forming varnish deposits in the carburetor.
Drain the Fuel Tank: If you’re going to be storing the trimmer for more than a few months, drain the fuel tank completely. This will prevent the fuel from gumming up the carburetor.
Run the Engine Dry: After draining the fuel tank, start the engine and let it run until it stalls. This will ensure that the carburetor is completely empty of fuel.
Clean the Air Filter Regularly: A dirty air filter restricts airflow to the engine, leading to a rich fuel mixture. Clean the air filter regularly or replace it if necessary.
Inspect the Fuel Lines: Inspect the fuel lines regularly for cracks, leaks, or blockages. Replace any damaged fuel lines.
Replace the Fuel Filter: Replace the fuel filter regularly to prevent debris from entering the carburetor.
Clean the Carburetor Periodically: If you notice any signs of carburetor problems, such as difficult starting, rough idling, or loss of power, clean the carburetor. You can use carburetor cleaner and compressed air to clean the carburetor’s passages and jets.

Long-Term Storage: My Personal Method

When I’m storing my Stihl FS 55 for the winter, I go the extra mile to protect the carburetor. After draining the fuel tank and running the engine dry, I remove the carburetor and spray it thoroughly with carburetor cleaner. Then, I store the carburetor in a sealed plastic bag with a desiccant pack to prevent moisture from entering. This helps to keep the carburetor clean and prevents corrosion.

Troubleshooting Common Issues After Carburetor Replacement

Even after following all the steps carefully, you might encounter some issues after replacing the carburetor. Here are some common problems and how to troubleshoot them:

Engine Won’t Start:
- Check the Spark Plug: Make sure the spark plug is clean and properly gapped.
- Check the Fuel Lines: Ensure the fuel lines are connected correctly and are not blocked.
- Prime the Carburetor: Make sure you’ve primed the carburetor by pressing the primer bulb several times.
Engine Runs Rough or Stalls:
- Adjust the Carburetor: Fine-tune the high-speed and low-speed adjustment screws.
- Check for Air Leaks: Inspect the carburetor mounting surface and fuel lines for air leaks.
- Clean the Carburetor: If the problem persists, clean the carburetor again.
Engine Lacks Power:
- Adjust the High-Speed Screw: Adjust the high-speed screw to richen the fuel mixture.
- Check the Air Filter: Make sure the air filter is clean.
- Check the Exhaust Port: A clogged exhaust port can restrict engine power.

Conclusion: A Smooth-Running Stihl FS 55 is Within Your Reach

Replacing or tuning the carburetor on your Stihl FS 55 can seem daunting at first, but with a little patience and the right guidance, it’s a task that’s well within your reach. By following the pro tips I’ve shared in this guide, you can diagnose carburetor problems, replace the carburetor safely and effectively, and tune it for optimal performance.

Remember, a well-maintained carburetor is essential for a smooth-running Stihl FS 55. By taking the time to properly care for your carburetor, you can extend the life of your trimmer and enjoy years of reliable performance.

Now, get out there and get trimming!