Stihl FS55R Carburetor Replacement (5 Pro Tips For Smooth Running)

Stihl FS55R Carburetor Replacement: 5 Pro Tips for Smooth Running

Replacing a carburetor might seem daunting, but with the right approach and a little know-how, it’s a task you can tackle yourself. I’ve rebuilt countless small engines over the years, and the Stihl FS55R is a common visitor in my workshop. I’ve learned that a well-tuned trimmer not only saves you time and frustration but also extends the life of your equipment. This guide is designed to be your trusted companion, walking you through the process with clear instructions, pro tips, and a dash of my personal experience.

Understanding the Need for Carburetor Replacement

Before diving in, let’s understand why carburetors need replacing in the first place. Carburetors are responsible for mixing air and fuel in the correct ratio for combustion. Over time, they can become clogged with debris, corroded by old fuel, or simply wear out.

Common Symptoms Indicating Carburetor Problems:

Difficulty Starting: The engine refuses to start or requires excessive cranking.
Rough Idling: The engine idles unevenly or stalls frequently.
Loss of Power: The engine lacks power, especially under load.
Black Smoke: Excessive black smoke from the exhaust indicates a rich fuel mixture (too much fuel).
Hesitation: The engine hesitates or stumbles when accelerating.

In my experience, neglecting these symptoms can lead to more significant engine damage, so addressing carburetor issues promptly is crucial. I once had a client who ignored a rough idle for months, only to discover that the carburetor was so corroded that it had damaged the intake manifold. A simple carburetor replacement could have saved him a costly repair.

Pro Tip #1: Gathering Your Tools and Parts

Preparation is key to a successful carburetor replacement. Before you even think about touching your trimmer, make sure you have everything you need.

Essential Tools:

Screwdrivers: A set of flathead and Phillips head screwdrivers. I recommend having a magnetic screwdriver to prevent losing screws in hard-to-reach places.
Socket Set: A small socket set can be handy for removing certain components.
Pliers: A pair of needle-nose pliers for removing fuel lines and other small parts.
Carburetor Adjustment Tool: This is crucial for fine-tuning the carburetor after installation. Stihl carburetors often require a specific “D” shaped adjustment tool. (More on this later.)
Clean Rags: For wiping up spills and keeping your workspace clean.
Carburetor Cleaner: To clean any remaining parts or the intake manifold.
Compressed Air: To blow out any debris from the intake manifold.
Fuel Line Clamps (Optional): To prevent fuel leakage when disconnecting fuel lines.
Gloves: To protect your hands from fuel and chemicals.

Replacement Parts:

New Carburetor: Ensure you order the correct carburetor for your Stihl FS55R. Check the part number in your owner’s manual or on the old carburetor.
New Fuel Filter: It’s always a good idea to replace the fuel filter when replacing the carburetor. A clogged fuel filter can restrict fuel flow and cause similar symptoms to a bad carburetor.
New Air Filter: A clean air filter ensures proper airflow to the carburetor.
New Fuel Lines (Optional): If your fuel lines are cracked or brittle, replace them.
Gaskets (Optional): Check the condition of the gaskets between the carburetor and the intake manifold. Replace them if they are damaged.

Data Point: According to Stihl’s service manual, using genuine Stihl replacement parts ensures optimal performance and longevity of your equipment. Aftermarket parts may not meet the same quality standards.

My Experience: I once tried to save a few bucks by using an aftermarket carburetor on a Stihl FS55R. It fit fine, but the engine never ran quite right. The fuel mixture was always off, and I ended up replacing it with a genuine Stihl carburetor. Lesson learned: stick with genuine parts whenever possible.

Pro Tip #2: Step-by-Step Carburetor Replacement

Now, let’s get down to the actual replacement process.

Step 1: Safety First!

Disconnect the spark plug wire to prevent accidental starting.
Drain the fuel tank or use a fuel line clamp to prevent fuel leakage.

Step 2: Remove the Air Filter Housing

Remove the air filter cover by loosening the screw or clip.
Remove the air filter.
Remove the air filter housing by loosening the screws or clips that hold it in place.

Step 3: Disconnect the Fuel Lines and Throttle Linkage

Carefully disconnect the fuel lines from the carburetor. Use pliers to gently remove the fuel lines. Note the position of each fuel line for reassembly. Typically, one line is the fuel supply from the tank, and the other is the return line.
Disconnect the throttle linkage from the carburetor. The throttle linkage is a small rod or cable that connects the throttle trigger to the carburetor.

Step 4: Remove the Old Carburetor

Loosen the screws or bolts that hold the carburetor to the intake manifold.
Carefully remove the carburetor.
Inspect the intake manifold for any damage or debris. Clean the intake manifold with carburetor cleaner and compressed air.

Step 5: Install the New Carburetor

Install the new carburetor onto the intake manifold.
Tighten the screws or bolts that hold the carburetor in place.
Reconnect the fuel lines to the carburetor. Make sure you connect them in the correct order.
Reconnect the throttle linkage to the carburetor.

Step 6: Reassemble the Air Filter Housing

Install the air filter housing.
Install the air filter.
Install the air filter cover.

Step 7: Reconnect the Spark Plug Wire

Reconnect the spark plug wire.

Step 8: Initial Start-Up

Prime the carburetor by pressing the primer bulb several times until fuel is visible in the bulb.
Start the engine. It may take a few tries to start the engine after replacing the carburetor.

Technical Requirement: When reconnecting fuel lines, ensure they are securely attached to prevent fuel leaks. Use fuel line clamps if necessary.

Visual Example: (Imagine a diagram here showing the fuel line connections to the carburetor, clearly indicating the fuel supply and return lines.)

Pro Tip #3: Carburetor Adjustment: The Art of Fine-Tuning

Once you’ve installed the new carburetor, it’s crucial to adjust it for optimal performance. Carburetor adjustment involves fine-tuning the fuel-air mixture to ensure the engine runs smoothly and efficiently.

Understanding Carburetor Adjustment Screws:

Most Stihl FS55R carburetors have two or three adjustment screws:

L (Low-Speed) Screw: This screw controls the fuel-air mixture at idle and low speeds.
H (High-Speed) Screw: This screw controls the fuel-air mixture at high speeds.
LA (Idle Speed) Screw: This screw adjusts the engine’s idle speed.

The Adjustment Process:

Start the Engine: Let the engine warm up for a few minutes.
Adjust the Low-Speed (L) Screw: Turn the L screw clockwise until the engine starts to stumble or stall. Then, turn it counterclockwise until the engine runs smoothly at idle.
Adjust the High-Speed (H) Screw: This screw is a bit trickier to adjust without a tachometer. The goal is to find the setting where the engine runs smoothly at full throttle without bogging down or sounding strained. Start by turning the H screw clockwise until the engine starts to lose power. Then, turn it counterclockwise until the engine runs smoothly at full throttle.
Adjust the Idle Speed (LA) Screw: Turn the LA screw until the engine idles smoothly without stalling. The ideal idle speed for the Stihl FS55R is around 2800-3200 RPM.

The “D” Shaped Adjustment Tool:

As I mentioned earlier, many Stihl carburetors use a unique “D” shaped adjustment screw. You’ll need a special tool to adjust these screws. These tools are readily available online or at your local Stihl dealer.

Data Point: According to Stihl’s service manual, incorrect carburetor adjustment can lead to poor performance, engine damage, and increased emissions.

My Experience: I remember spending hours trying to adjust a carburetor without the correct tool. I ended up stripping the adjustment screw and causing more damage. Investing in the right tools is always worth it in the long run.

Technical Tip: When adjusting the high-speed screw, listen carefully to the engine. A lean mixture (too little fuel) can cause the engine to overheat and damage the piston. A rich mixture (too much fuel) can cause the engine to bog down and produce excessive smoke.

Pro Tip #4: Troubleshooting Common Issues

Even with a new carburetor and proper adjustment, you might encounter some issues. Here are some common problems and their solutions:

Engine Still Won’t Start:
- Check the spark plug. Is it clean and properly gapped?
- Check the fuel lines for kinks or clogs.
- Make sure the fuel filter is clean.
- Ensure the carburetor is properly primed.
Engine Runs Rough or Stalls:
- Re-adjust the carburetor.
- Check for air leaks around the carburetor and intake manifold.
- Clean the air filter.
- Check the fuel tank vent. A clogged vent can prevent fuel from flowing properly.
Engine Lacks Power:
- Adjust the high-speed (H) screw.
- Check the spark arrestor screen. A clogged screen can restrict exhaust flow and reduce power.
- Make sure the cutting head is not binding or overloaded.

Case Study: I once worked on a Stihl FS55R that was consistently stalling, even after a carburetor replacement. After checking all the usual suspects, I discovered that the fuel tank vent was clogged with dirt. Cleaning the vent solved the problem immediately.

Technical Limitation: The Stihl FS55R is designed for light-duty trimming. Overloading the engine with heavy brush can cause it to overheat and stall.

Pro Tip #5: Maintenance for Longevity

Replacing the carburetor is just one step in maintaining your Stihl FS55R. Regular maintenance is essential for keeping your trimmer running smoothly for years to come.

Essential Maintenance Tasks:

Clean the Air Filter Regularly: A dirty air filter restricts airflow and can cause the engine to run poorly. Clean the air filter every 25 hours of use, or more often in dusty conditions.
Replace the Fuel Filter Regularly: A clogged fuel filter restricts fuel flow and can damage the carburetor. Replace the fuel filter every year, or more often if you use your trimmer frequently.
Use Fresh Fuel: Old fuel can gum up the carburetor and cause starting problems. Always use fresh fuel and add a fuel stabilizer to prevent fuel from going bad. I recommend using ethanol-free fuel whenever possible, as ethanol can damage small engine components.
Inspect Fuel Lines Regularly: Check the fuel lines for cracks or leaks. Replace them if necessary.
Sharpen the Cutting Head Regularly: A dull cutting head requires more power and can strain the engine. Sharpen the cutting head every few hours of use.
Store Your Trimmer Properly: When storing your trimmer for extended periods, drain the fuel tank and run the engine until it stalls. This will prevent fuel from gumming up the carburetor.

Data Point: According to the Outdoor Power Equipment Institute (OPEI), regular maintenance can extend the life of your outdoor power equipment by up to 50%.

Material Specification: When storing your Stihl FS55R, keep it in a dry, well-ventilated area to prevent rust and corrosion.

My Personal Story: I’ve seen countless trimmers ruined by neglect. One of my neighbors left his Stihl FS55R sitting in his shed for years without draining the fuel. When he finally tried to start it, the carburetor was completely gummed up and beyond repair. A little bit of maintenance could have saved him a lot of money and frustration.

Additional Tips for Smooth Running:

Use the Correct Fuel-Oil Mixture: The Stihl FS55R requires a 50:1 fuel-oil mixture. Use a high-quality two-stroke oil specifically designed for air-cooled engines.
Check the Spark Plug Gap: The correct spark plug gap for the Stihl FS55R is 0.020 inches.
Inspect the Spark Arrestor Screen: A clogged spark arrestor screen can restrict exhaust flow and reduce power. Clean the screen regularly.

By following these pro tips and performing regular maintenance, you can keep your Stihl FS55R running smoothly for years to come. Replacing the carburetor is a manageable task with the right knowledge and tools. Remember to prioritize safety, use genuine parts, and fine-tune the carburetor for optimal performance. Happy trimming!

Stihl FS55R Carburetor Replacement (5 Pro Tips for Smooth Running)