Why Won't My Stihl Weed Eater Start? (5 Pro Troubleshooting Tips)

As a seasoned woodcutter and firewood enthusiast, I know firsthand the frustration of a stubborn weed eater. Wear and tear is inevitable, and like any hardworking tool, your Stihl weed eater will eventually face starting issues. I’ve spent years troubleshooting these machines, and I’m here to share my expertise. In this guide, I’ll walk you through the most common reasons why your Stihl weed eater won’t start and provide pro troubleshooting tips to get it running again.

Why Won’t My Stihl Weed Eater Start? (5 Pro Troubleshooting Tips)

A Stihl weed eater that refuses to start can be a real headache, especially when you’re eager to tackle your lawn. I’ve been there, staring at a silent machine on a hot summer day, wondering where I went wrong. More often than not, it’s a simple fix, but finding that fix requires a systematic approach. Let’s dive into the most common culprits.

1. Fuel System Issues: The Heart of the Problem

The fuel system is the lifeline of your weed eater. If it’s not delivering the right fuel mixture, your engine simply won’t fire up. I can’t stress enough how crucial it is to start here.

a. Old or Contaminated Fuel

Old fuel is the number one reason why weed eaters refuse to start. Gasoline degrades over time, especially when mixed with oil in two-stroke engines. This degradation leads to a gummy residue that clogs the carburetor and fuel lines.

My Experience: I once left a Stihl FS 56 RC sitting in my shed for about six months with fuel still in the tank. When I tried to start it, nothing happened. I drained the old fuel, cleaned the carburetor, and filled it with fresh fuel, and it fired up on the second pull.
Actionable Steps:
- Always use fresh fuel. I recommend using fuel that’s less than 30 days old.
- Use a fuel stabilizer if you plan to store your weed eater for more than a month.
- Drain the fuel tank and run the engine until it stalls before storing it for extended periods.
- Dispose of old fuel properly at a local recycling center or hazardous waste facility.
Relevant Statistics: According to a study by the Outdoor Power Equipment Institute (OPEI), over 70% of small engine starting problems are related to fuel issues.

b. Clogged Fuel Filter

The fuel filter prevents debris from entering the carburetor. Over time, this filter can become clogged, restricting fuel flow.

My Experience: I had a Stihl FS 90 that would start but quickly die. I suspected the fuel filter, so I replaced it with a new one. The difference was night and day. The engine ran smoothly and didn’t stall.
Actionable Steps:
- Locate the fuel filter inside the fuel tank. It’s usually attached to the end of the fuel line.
- Remove the filter and inspect it for clogs.
- If the filter is clogged, replace it with a new one. Stihl fuel filters are relatively inexpensive.
- I recommend replacing the fuel filter annually as part of your routine maintenance.
Tool List:
- Small pliers or fuel line clamp
- New fuel filter (Stihl part number varies by model)

c. Blocked Fuel Lines

Fuel lines can become brittle and cracked over time, leading to air leaks or fuel blockages.

My Experience: I once had a Stihl FS 45 that was incredibly difficult to start. After checking the fuel filter and carburetor, I noticed a small crack in the fuel line. Replacing the fuel line solved the problem instantly.
Actionable Steps:
- Inspect the fuel lines for cracks, kinks, or leaks.
- If you find any damage, replace the fuel lines.
- Ensure the fuel lines are properly connected to the fuel tank, carburetor, and primer bulb.
- Use fuel line pliers to avoid damaging the lines during removal and installation.
Tool List:
- Fuel line pliers
- New fuel lines (Stihl part number varies by model)

d. Carburetor Problems

The carburetor mixes fuel and air to create a combustible mixture. If the carburetor is clogged or improperly adjusted, your weed eater won’t start.

My Experience: I’ve encountered countless instances where a dirty carburetor was the culprit. A Stihl FS 70 R I worked on recently wouldn’t start no matter what I did. I removed the carburetor, cleaned it thoroughly with carburetor cleaner, and reassembled it. It started on the first pull.
Actionable Steps:
- Cleaning the Carburetor:
  - Remove the air filter and inspect the carburetor intake.
  - Spray carburetor cleaner into the intake while pulling the starter cord. This can sometimes dislodge minor clogs.
  - If the problem persists, remove the carburetor and disassemble it.
  - Clean all the jets and passages with carburetor cleaner and small brushes.
  - Reassemble the carburetor and install it back on the weed eater.
- Adjusting the Carburetor:
  - Locate the high (H) and low (L) speed adjustment screws on the carburetor.
  - Consult your Stihl weed eater’s manual for the recommended starting settings.
  - Adjust the screws slightly until the engine starts and runs smoothly.
  - A general rule of thumb is to turn both screws clockwise until they are lightly seated, then back them out the recommended number of turns.
Tool List:
- Screwdrivers (flathead and Phillips)
- Carburetor cleaner
- Small brushes or pipe cleaners
- Carburetor adjustment tool (if needed for your model)
Case Study: A local landscaping company was having trouble with several of their Stihl FS 56 RC weed eaters. They brought them to me, and I found that the carburetors were clogged with residue from old fuel. After cleaning and adjusting the carburetors, all the weed eaters were back in service.

Takeaway: The fuel system is the most common source of starting problems in Stihl weed eaters. Always start your troubleshooting here.

2. Ignition System: Sparking the Engine to Life

The ignition system creates the spark that ignites the fuel-air mixture in the engine. If there’s no spark, your weed eater won’t start.

a. Spark Plug Inspection

The spark plug ignites the fuel-air mixture. A fouled, cracked, or improperly gapped spark plug can prevent the engine from starting.

My Experience: I had a Stihl FS 38 that wouldn’t start. I pulled the spark plug, and it was covered in black carbon deposits. I cleaned the spark plug with a wire brush, gapped it correctly, and reinstalled it. The weed eater started right up.
Actionable Steps:
- Remove the spark plug using a spark plug wrench.
- Inspect the spark plug for cracks, damage, or excessive carbon buildup.
- Clean the spark plug with a wire brush or spark plug cleaner.
- Check the spark plug gap using a spark plug gapping tool. The correct gap is usually around 0.020-0.025 inches (0.5-0.6 mm), but consult your owner’s manual for the exact specification.
- If the spark plug is damaged or heavily fouled, replace it with a new one. Stihl recommends using the correct spark plug for your model.
Tool List:
- Spark plug wrench
- Wire brush or spark plug cleaner
- Spark plug gapping tool
- New spark plug (Stihl part number varies by model)

b. Testing for Spark

Even if the spark plug looks good, it may not be producing a spark.

My Experience: I once had a Stihl FS 55 R that wouldn’t start, even after replacing the spark plug. I tested for spark and found none. The problem turned out to be a faulty ignition coil.
Actionable Steps:
- Remove the spark plug and reattach it to the spark plug wire.
- Hold the spark plug against the engine cylinder head.
- Pull the starter cord.
- Look for a bright blue spark jumping across the spark plug gap.
- If there is no spark, the ignition coil may be faulty.
Safety Note: Be careful when testing for spark. Avoid touching the spark plug wire while pulling the starter cord, as this can result in an electric shock.

c. Ignition Coil Problems

The ignition coil generates the high voltage needed to create a spark. A faulty ignition coil can prevent the engine from starting.

My Experience: Replacing an ignition coil can be a bit more involved, but it’s a common fix for no-start issues. I replaced the ignition coil on a Stihl FS 80 once, and it solved the problem immediately.
Actionable Steps:
- Test the ignition coil using a multimeter to check for continuity.
- If the ignition coil is faulty, replace it with a new one.
- Ensure the air gap between the ignition coil and the flywheel is correct. Consult your owner’s manual for the proper specification.
- I usually use a business card or a specialized air gap tool to set the gap.
Tool List:
- Multimeter
- Screwdrivers
- New ignition coil (Stihl part number varies by model)
- Air gap tool or business card

Takeaway: A healthy spark is essential for your weed eater to start. If you’re not getting a spark, focus on the spark plug and ignition coil.

3. Compression Issues: The Engine’s Power Stroke

Compression is the pressure created in the cylinder when the piston moves up. Without adequate compression, the fuel-air mixture won’t ignite.

a. Checking Compression

Low compression can be caused by worn piston rings, a damaged cylinder, or a leaky head gasket.

My Experience: I once worked on a Stihl FS 100 RX that had very low compression. The engine was difficult to start and lacked power. After disassembling the engine, I found that the piston rings were worn and the cylinder was scored.
Actionable Steps:
- Use a compression tester to measure the compression in the cylinder.
- Remove the spark plug and screw the compression tester into the spark plug hole.
- Pull the starter cord several times and record the highest reading on the tester.
- Consult your Stihl weed eater’s manual for the minimum compression specification.
- If the compression is below the minimum specification, the engine may need to be rebuilt or replaced.
Tool List:
- Compression tester

b. Common Causes of Low Compression

Several factors can lead to low compression, including:

Worn Piston Rings: These rings seal the gap between the piston and the cylinder wall. Over time, they can wear down, allowing compression to escape.
Damaged Cylinder: Scratches or scoring on the cylinder wall can also cause compression loss.
Leaky Head Gasket: The head gasket seals the cylinder head to the engine block. If it’s damaged, it can leak compression.
Loose Spark Plug: Ensure the spark plug is properly tightened. A loose spark plug can cause compression to leak out.
Preventive Measures:
- Always use the correct fuel-oil mixture. Using too little oil can cause excessive wear on the piston rings and cylinder.
- Avoid running the engine at high RPMs for extended periods.
- Keep the air filter clean to prevent dirt and debris from entering the engine.

Takeaway: Compression is crucial for a healthy engine. If you suspect low compression, have it checked by a qualified mechanic.

4. Air Filter Problems: Breathing Easy

The air filter prevents dirt and debris from entering the engine. A clogged air filter restricts airflow, causing the engine to run poorly or not start at all.

a. Inspecting the Air Filter

A dirty air filter can choke the engine and prevent it from starting.

My Experience: I’ve seen many weed eaters that wouldn’t start simply because the air filter was completely clogged. A Stihl FS 36 I worked on recently had an air filter so dirty it looked like it had been dipped in mud.
Actionable Steps:
- Locate the air filter housing on your Stihl weed eater.
- Remove the air filter and inspect it for dirt and debris.
- If the air filter is dirty, clean it with warm soapy water.
- Rinse the air filter thoroughly and allow it to dry completely before reinstalling it.
- If the air filter is damaged or excessively dirty, replace it with a new one.
Tool List:
- Warm soapy water
- Clean cloth
- New air filter (Stihl part number varies by model)

b. Air Filter Maintenance

Regular air filter maintenance is essential for keeping your weed eater running smoothly.

Maintenance Schedule:
- Inspect the air filter before each use.
- Clean the air filter every 25 hours of use.
- Replace the air filter annually or more often if needed.
Pro Tip: I often recommend using a foam air filter oil to help trap dirt and debris. Apply a thin coat of oil to the clean air filter before reinstalling it.

Takeaway: A clean air filter is essential for proper engine performance. Make air filter maintenance a regular part of your weed eater routine.

5. Starter Mechanism Issues: Getting the Engine Turning

The starter mechanism engages the engine and gets it turning. If the starter mechanism is damaged or malfunctioning, you won’t be able to start the weed eater.

a. Inspecting the Starter Cord

A broken or tangled starter cord can prevent you from starting the engine.

My Experience: I had a Stihl FS 40 C that wouldn’t start because the starter cord was frayed and wouldn’t retract properly. Replacing the starter cord was a simple fix that got the weed eater back in action.
Actionable Steps:
- Inspect the starter cord for fraying, breaks, or tangles.
- If the starter cord is damaged, replace it with a new one.
- Ensure the starter cord retracts smoothly and completely.
- Lubricate the starter mechanism with a silicone-based lubricant to keep it working smoothly.
Tool List:
- Screwdrivers
- New starter cord (Stihl part number varies by model)
- Silicone-based lubricant

b. Recoil Starter Problems

The recoil starter mechanism can fail due to wear and tear or damage.

My Experience: I’ve seen recoil starters fail for various reasons, from broken springs to damaged pawls. A Stihl FS 56 RC I worked on had a recoil starter that wouldn’t engage. The pawls were worn, preventing them from catching the flywheel.
Actionable Steps:
- Remove the recoil starter assembly from the weed eater.
- Inspect the recoil spring, pawls, and other components for damage.
- Replace any damaged parts.
- Reassemble the recoil starter and install it back on the weed eater.
- Ensure the recoil starter engages the flywheel properly.
Tool List:
- Screwdrivers
- Pliers
- New recoil starter parts (Stihl part number varies by model)

c. Flywheel Key

The flywheel key is a small part that connects the flywheel to the crankshaft. If the flywheel key is sheared, the engine timing will be off, and the weed eater won’t start.

My Experience: A sheared flywheel key is less common, but it can happen if the engine experiences a sudden stop. I once had a Stihl FS 85 that wouldn’t start after hitting a rock. The flywheel key was sheared, causing the timing to be off.
Actionable Steps:
- Remove the flywheel cover and inspect the flywheel key.
- If the flywheel key is sheared, replace it with a new one.
- Ensure the flywheel is properly aligned with the crankshaft.
Tool List:
- Screwdrivers
- Pliers
- New flywheel key (Stihl part number varies by model)

Takeaway: A properly functioning starter mechanism is essential for getting the engine turning. If you’re having trouble with the starter, inspect the cord, recoil starter, and flywheel key.

Additional Tips and Considerations

Primer Bulb: Check the primer bulb for cracks or damage. If the primer bulb is cracked, it won’t properly prime the engine. Replace it if necessary.
Exhaust System: A clogged exhaust port or spark arrestor screen can restrict exhaust flow and prevent the engine from starting. Clean the exhaust port and spark arrestor screen regularly.
Choke: Make sure the choke is properly engaged when starting a cold engine. The choke restricts airflow, enriching the fuel-air mixture and making it easier to start.
Professional Help: If you’ve tried all these troubleshooting steps and your Stihl weed eater still won’t start, it may be time to take it to a qualified repair shop.

Preventative Maintenance: Keeping Your Stihl Weed Eater Running Strong

Preventative maintenance is the key to keeping your Stihl weed eater running smoothly and avoiding starting problems.

Regular Cleaning: Clean your weed eater regularly to remove dirt, debris, and grass clippings.
Proper Storage: Store your weed eater in a dry, protected location.
Fuel Stabilization: Use a fuel stabilizer if you plan to store your weed eater for more than a month.
Scheduled Maintenance: Follow the maintenance schedule in your owner’s manual.

Conclusion

Troubleshooting a Stihl weed eater that won’t start can be frustrating, but with a systematic approach, you can usually find the problem and fix it yourself. Remember to start with the fuel system, check the ignition system, and inspect the air filter and starter mechanism. By following these pro troubleshooting tips and performing regular maintenance, you can keep your Stihl weed eater running strong for years to come. Now, get out there and conquer that lawn!

Why Won’t My Stihl Weed Eater Start? (5 Pro Troubleshooting Tips)