Stihl Weedeater FS38 Troubleshooting (5 Pro Fixes)

I love the feel of rough-hewn wood, the way the fibers catch on my gloves as I handle a freshly split log. There’s a primal satisfaction in transforming raw timber into neatly stacked firewood, ready to warm a home. But sometimes, the tools that help us achieve this connection to nature can be temperamental. The Stihl FS38 weed eater, a trusty companion for many homeowners, is no exception. When it sputters, refuses to start, or just doesn’t perform as expected, it can be frustrating. I’ve spent countless hours troubleshooting small engine problems, and I’m here to share my experience and guide you through some common issues and fixes for your Stihl FS38.

Stihl Weedeater FS38 Troubleshooting: 5 Pro Fixes

The Stihl FS38 is a lightweight, gas-powered string trimmer designed for homeowners. Its simplicity is usually its strength, but when problems arise, understanding the basic mechanics is key to getting it running smoothly again. These five fixes cover the most common issues I’ve encountered over the years.

1. Fuel System Issues: The Lifeblood of Your FS38

The fuel system is the heart of any gasoline-powered engine. If the fuel isn’t flowing correctly, your FS38 won’t run. This is the first place I usually check.

Diagnosing Fuel Problems

Symptoms: Hard starting, stalling, lack of power, or the engine not starting at all.
My Experience: I once had an FS38 that refused to start after sitting for a few months. The fuel had turned to varnish, clogging the carburetor.
Tools Needed: Screwdrivers (flathead and Phillips), fuel line pliers (optional), carburetor cleaner.

The Fix: Step-by-Step

Check the Fuel: Drain the old fuel into a suitable container. Look for signs of contamination, water, or a varnish-like consistency. Fresh fuel is crucial. I recommend using premium gasoline mixed with a high-quality two-stroke oil at the correct ratio (usually 50:1, but always check your owner’s manual).
Fuel Filter Inspection: Locate the fuel filter inside the fuel tank. It’s usually attached to the end of the fuel line. Use a small hooked wire or needle-nose pliers to carefully pull it out. Examine the filter for clogs or debris. If it’s dirty, replace it. A clogged fuel filter restricts fuel flow, starving the engine.
Fuel Line Check: Inspect the fuel lines for cracks, leaks, or kinks. Brittle or damaged fuel lines can cause air leaks, disrupting the fuel supply. Replace any damaged sections. I’ve found that using fuel line pliers makes this task much easier, especially when dealing with tight spaces.
Carburetor Cleaning: This is often the culprit. The carburetor mixes air and fuel to create the combustible mixture that powers the engine. Over time, small passages within the carburetor can become clogged with fuel residue.
- Disassembly: Carefully remove the air filter housing and the carburetor. Take pictures as you disassemble it to help with reassembly.
- Cleaning: Use carburetor cleaner to thoroughly clean all parts of the carburetor, paying special attention to the jets and small passages. Use compressed air to blow out any remaining debris. I use a set of carburetor cleaning brushes to get into the tiny orifices.
- Reassembly: Reassemble the carburetor, ensuring all gaskets and seals are in good condition. Replace any worn or damaged parts.
- Fuel Tank Vent: A clogged fuel tank vent can create a vacuum in the tank, preventing fuel from flowing. Locate the vent (usually a small hole in the fuel cap) and make sure it’s clear. A small wire or needle can be used to clear any blockage.

Pro Tip

When cleaning the carburetor, pay close attention to the idle jet. This small jet is responsible for providing fuel at idle speed, and it’s often the first to become clogged.

Measurement and Data

Fuel Ratio: Always mix your fuel and oil at the correct ratio. Using the wrong ratio can lead to engine damage. Stihl recommends a 50:1 ratio for most of their two-stroke engines. This means 50 parts gasoline to 1 part two-stroke oil.
Fuel Filter Replacement: Replace your fuel filter at least once a year, or more often if you use your FS38 frequently.

Expected Outcome

Improved starting, smoother idling, and increased power.

Next Steps

If the fuel system checks out, move on to the ignition system.

2. Ignition System Issues: Sparking the Flame

The ignition system provides the spark that ignites the fuel-air mixture in the engine cylinder. A weak or non-existent spark will prevent the engine from starting.

Diagnosing Ignition Problems

Symptoms: Engine won’t start, weak spark, or intermittent starting.
My Experience: I once spent an entire afternoon trying to start an FS38, only to discover that the spark plug was fouled. A simple replacement solved the problem.
Tools Needed: Spark plug wrench, spark plug tester (optional), small wire brush.

The Fix: Step-by-Step

Spark Plug Inspection: Remove the spark plug using a spark plug wrench. Examine the spark plug for signs of damage, fouling (carbon buildup), or wear. A healthy spark plug should have a clean, dry electrode.
- Cleaning: If the spark plug is fouled, clean it with a wire brush or replace it. I prefer to replace spark plugs annually for optimal performance.
- Gap Check: Check the spark plug gap using a spark plug gap tool. The correct gap for the Stihl FS38 is typically around 0.5 mm (0.020 inches). Adjust the gap if necessary.
- Spark Test: Reconnect the spark plug to the spark plug wire and ground the spark plug against the engine cylinder. Pull the starter cord and observe the spark plug. A strong, blue spark indicates a healthy ignition system. A weak, yellow spark or no spark indicates a problem.
- Ignition Coil Inspection: If the spark is weak or non-existent, the ignition coil may be faulty. The ignition coil generates the high voltage needed to create a spark.
- Testing: Use a multimeter to test the ignition coil for continuity. Consult your service manual for the correct resistance values.
- Replacement: If the ignition coil is faulty, replace it. This usually involves removing the flywheel and disconnecting the wires from the coil.
- Flywheel Key: The flywheel key is a small metal key that aligns the flywheel with the crankshaft. If the flywheel key is sheared, the timing of the engine will be off, preventing it from starting. Inspect the flywheel key for damage. If it’s sheared, replace it.

Pro Tip

When installing a new spark plug, tighten it to the manufacturer’s recommended torque specification. Over-tightening can damage the cylinder head.

Measurement and Data

Spark Plug Gap: The correct spark plug gap is crucial for optimal engine performance. Always consult your owner’s manual for the recommended gap.
Ignition Coil Resistance: Use a multimeter to measure the resistance of the ignition coil. The resistance should be within the manufacturer’s specified range.

Expected Outcome

Strong, consistent spark and improved starting.

Next Steps

If the ignition system is functioning properly, move on to the compression test.

3. Compression Issues: Sealing the Deal

Compression is the measure of how tightly the engine seals the combustion chamber. Low compression means that the fuel-air mixture is leaking out, preventing the engine from starting or running properly.

Diagnosing Compression Problems

Symptoms: Hard starting, lack of power, engine won’t start, or a “weak” feeling when pulling the starter cord.
My Experience: I once encountered an FS38 with a worn piston ring that caused low compression. The engine would start, but it lacked power and stalled frequently.
Tools Needed: Compression tester, spark plug wrench.

The Fix: Step-by-Step

Compression Test: Remove the spark plug and screw the compression tester into the spark plug hole. Pull the starter cord several times and observe the reading on the compression tester.
- Reading Interpretation: The compression reading should be within the manufacturer’s specified range. A reading below the minimum indicates low compression. The FS38 should ideally be above 100 PSI. Anything below 80 PSI will likely cause issues.
- Possible Causes of Low Compression:
- Worn Piston Rings: Piston rings seal the gap between the piston and the cylinder wall. Worn or damaged piston rings can allow combustion gases to leak past the piston, resulting in low compression.
- Leaking Valves: (Less common on a 2-stroke) Valves seal the intake and exhaust ports. Leaking valves can allow combustion gases to escape, resulting in low compression.
- Damaged Cylinder: Scratches or damage to the cylinder wall can also cause low compression.
- Blown Head Gasket: (Less Common but possible) A damaged head gasket can leak compression.
- Remediation:
- Piston Ring Replacement: Replacing piston rings requires disassembling the engine. This is a more complex repair that may be best left to a professional.
- Cylinder Inspection: Inspect the cylinder for damage. Minor scratches can sometimes be honed out. Severe damage may require cylinder replacement.
- Head Gasket Replacement: If you suspect a blown head gasket, replace it.

Pro Tip

Before performing a compression test, make sure the engine is warm. This will help to ensure an accurate reading.

Measurement and Data

Compression Reading: The compression reading should be within the manufacturer’s specified range. Consult your service manual for the correct value.
Leakdown Test: A leakdown test can help to pinpoint the source of compression leaks.

Expected Outcome

Improved compression and increased power.

Next Steps

If the compression is good, move on to checking the exhaust system.

4. Exhaust System Issues: Letting it Breathe

A clogged exhaust system can restrict the flow of exhaust gases, causing the engine to run poorly or not at all.

Diagnosing Exhaust Problems

Symptoms: Lack of power, overheating, engine won’t start, or a “muffled” sound.
My Experience: I once had an FS38 that was running poorly due to a clogged spark arrestor screen. Cleaning the screen restored the engine’s power.
Tools Needed: Screwdrivers, wire brush.

The Fix: Step-by-Step

Spark Arrestor Inspection: The spark arrestor is a small screen located in the muffler that prevents sparks from exiting the exhaust system. Over time, the spark arrestor can become clogged with carbon deposits.
- Removal: Remove the spark arrestor screen from the muffler. The location and removal method may vary depending on the model.
- Cleaning: Clean the spark arrestor screen with a wire brush. You can also soak it in carburetor cleaner to loosen stubborn deposits.
- Reinstallation: Reinstall the spark arrestor screen.
- Muffler Inspection: Inspect the muffler for damage or blockages. A damaged muffler can restrict exhaust flow.
- Removal: Remove the muffler from the engine.
- Inspection: Inspect the muffler for cracks, dents, or other damage. Check for any blockages inside the muffler.
- Replacement: If the muffler is damaged or blocked, replace it.
- Exhaust Port Inspection: The exhaust port is the opening in the cylinder where exhaust gases exit the engine. Over time, carbon deposits can build up in the exhaust port, restricting exhaust flow.
- Removal: Remove the muffler from the engine.
- Inspection: Inspect the exhaust port for carbon deposits.
- Cleaning: Use a scraper or wire brush to remove carbon deposits from the exhaust port. Be careful not to damage the cylinder wall.

Pro Tip

Regularly clean the spark arrestor screen to prevent it from becoming clogged.

Measurement and Data

Exhaust Backpressure: Use an exhaust backpressure gauge to measure the backpressure in the exhaust system. Excessive backpressure indicates a blockage.

Expected Outcome

Improved exhaust flow and increased power.

Next Steps

If the exhaust system is clear, move on to checking the air filter.

5. Air Filter Issues: Breathing Easy

A clean air filter is essential for proper engine performance. A dirty air filter restricts airflow, causing the engine to run rich (too much fuel) and lose power.

Diagnosing Air Filter Problems

Symptoms: Lack of power, black smoke, engine won’t start, or a “bogging” feeling.
My Experience: I’ve seen many FS38 trimmers running poorly simply because the air filter was completely clogged with dirt and debris.
Tools Needed: Screwdriver, cleaning solution (soap and water or air filter cleaner).

The Fix: Step-by-Step

Air Filter Inspection: Locate the air filter housing and remove the air filter. Examine the air filter for dirt, dust, or debris.
- Types of Air Filters: The FS38 typically uses a foam air filter.
- Air Filter Cleaning:
- Foam Air Filter: Wash the foam air filter with warm, soapy water. Rinse thoroughly and allow it to dry completely. You can also use a dedicated air filter cleaner. After drying, lightly oil the filter with air filter oil. This helps to trap dirt and debris. I use a dedicated foam filter oil.
- Air Filter Housing Cleaning: Clean the air filter housing to remove any dirt or debris that may have accumulated.
- Air Filter Replacement: If the air filter is damaged or excessively dirty, replace it.
- Reinstallation: Reinstall the air filter and air filter housing.

Pro Tip

Clean your air filter regularly, especially if you operate your FS38 in dusty conditions.

Measurement and Data

Air Filter Restriction: Use an air filter restriction gauge to measure the amount of restriction in the air filter.

Expected Outcome

Improved airflow and increased power.

Additional Troubleshooting Tips: Beyond the Basics

While the five fixes above address the most common issues, here are a few additional things to consider:

Primer Bulb: The primer bulb is a small rubber bulb that pumps fuel into the carburetor. If the primer bulb is cracked or damaged, it can leak air, preventing the engine from starting. Inspect the primer bulb for damage and replace it if necessary.
Choke: The choke restricts airflow to the carburetor, creating a richer fuel-air mixture for starting a cold engine. Make sure the choke is functioning properly and that it’s in the correct position for starting.
Idle Adjustment: The idle speed can be adjusted using the idle adjustment screw on the carburetor. If the engine is idling too fast or too slow, adjust the idle speed accordingly.
High-Speed Adjustment: (Use Caution) The high-speed mixture can be adjusted using the high-speed adjustment screw on the carburetor. This adjustment should only be made by experienced technicians, as improper adjustment can damage the engine.
Recoil Starter: If the starter cord is difficult to pull or won’t retract, the recoil starter mechanism may be faulty. Inspect the recoil starter for damage and lubricate it if necessary.
String Head: Problems with the string head can also cause issues. Make sure the string is properly loaded and that the head is not damaged.

Preventing Future Problems: Maintenance is Key

Regular maintenance is the best way to prevent problems with your Stihl FS38. Here are a few tips:

Use Fresh Fuel: Always use fresh fuel mixed with the correct ratio of two-stroke oil.
Clean Air Filter Regularly: Clean the air filter regularly, especially if you operate your FS38 in dusty conditions.
Replace Spark Plug Annually: Replace the spark plug annually for optimal performance.
Clean Spark Arrestor Screen Regularly: Clean the spark arrestor screen regularly to prevent it from becoming clogged.
Store Properly: Store your FS38 in a dry place when not in use. Drain the fuel tank before storing it for extended periods.
Sharpen Cutting Blades: Keep your cutting blades sharp for optimal performance and safety.
Check and Tighten Bolts: Regularly check and tighten all bolts and screws on your FS38.

Case Study: Reviving a Neglected FS38

I once acquired an FS38 from a neighbor who was moving. It had been sitting in his shed for years and was in rough shape. The fuel tank was full of old, gummy fuel, the air filter was completely clogged, and the spark plug was fouled.

I started by draining the old fuel and cleaning the fuel tank. I then replaced the fuel filter and fuel lines. Next, I cleaned the carburetor thoroughly. I replaced the spark plug and cleaned the air filter. Finally, I cleaned the spark arrestor screen.

After performing these repairs, the FS38 started on the first pull and ran like new. This case study highlights the importance of regular maintenance and the potential for reviving even neglected equipment.

Logging Tools and Firewood Preparation: A Synergistic Relationship

While the Stihl FS38 is primarily a weed eater, it’s often used in conjunction with other logging tools and firewood preparation equipment. For example, after felling a tree with a chainsaw, an FS38 can be used to clear brush and debris around the work area. It can also be used to trim around firewood stacks to keep them neat and tidy.

Understanding how different tools work together can improve efficiency and safety in wood processing and firewood preparation.

Safety First: Protecting Yourself

Always wear appropriate safety gear when operating any power equipment, including a Stihl FS38. This includes:

Eye Protection: Wear safety glasses or goggles to protect your eyes from flying debris.
Hearing Protection: Wear earplugs or earmuffs to protect your hearing from the loud engine noise.
Gloves: Wear gloves to protect your hands from cuts and abrasions.
Long Pants and Sleeves: Wear long pants and sleeves to protect your skin from scratches and insect bites.
Steel-Toed Boots: Wear steel-toed boots to protect your feet from falling objects.

Conclusion: Empowering You to Fix Your FS38

Troubleshooting a Stihl FS38 can seem daunting, but with a little knowledge and the right tools, you can often diagnose and fix common problems yourself. By following the steps outlined in this guide, you can keep your FS38 running smoothly and efficiently for years to come. Remember to prioritize safety and consult your owner’s manual for specific instructions and recommendations. And always wear gloves.