Stihl FS 45 Trimmer Repair Tips (5 Essential Ignition Fixes)
Have you ever been ready to tackle that overgrown patch of weeds, only to find your Stihl FS 45 trimmer refusing to start? It’s frustrating, isn’t it? Especially when you’ve got a yard waiting to be tamed, or a firewood project that needs clearing. I’ve been there countless times, and more often than not, the culprit is the ignition system. Don’t despair! In this guide, I’ll walk you through five essential ignition fixes that can breathe life back into your trusty Stihl FS 45 trimmer.
Stihl FS 45 Trimmer Repair Tips: 5 Essential Ignition Fixes
The Stihl FS 45 is a workhorse. It’s lightweight, easy to handle, and perfect for trimming around the house or prepping areas for larger wood processing tasks. But like any small engine, its ignition system is crucial, and problems here can bring your work to a screeching halt. These fixes are designed to be straightforward, even if you’re not a seasoned mechanic. I’ve broken down each step with clear instructions and practical advice gleaned from years of experience in maintaining and repairing my own equipment.
Why Ignition Matters: A Foundation for Understanding
Before diving into the fixes, it’s essential to understand why the ignition system is so crucial. The ignition system’s primary job is to create the spark that ignites the fuel-air mixture in the engine cylinder. Without a strong, consistent spark, the engine simply won’t run. Think of it like trying to light a campfire with damp matches – no matter how much wood you have, you’re not getting a fire without that initial spark.
Key Components of the Ignition System:
- Spark Plug: This component delivers the high-voltage spark to ignite the fuel-air mixture.
- Ignition Coil: The coil transforms low-voltage electricity from the magneto into the high voltage needed for the spark plug.
- Flywheel: The flywheel contains magnets that pass by the ignition coil, generating the electrical current.
- Ignition Module: This component controls the timing and duration of the spark.
- Kill Switch: This switch grounds the ignition system, stopping the engine.
Understanding these components will help you troubleshoot problems more effectively.
Fix #1: The Spark Plug Inspection and Cleaning
The spark plug is the most accessible and often the most problematic part of the ignition system. Over time, it can become fouled with carbon deposits, oil, or fuel residue, preventing it from firing correctly.
Step-by-Step Guide:
- Safety First: Disconnect the spark plug wire from the spark plug. This prevents accidental starting.
- Removal: Use a spark plug wrench (typically a 19mm or 3/4 inch socket) to carefully unscrew the spark plug. Turn it counterclockwise.
- Inspection: Examine the spark plug. Look for:
- Carbon Buildup: Black, sooty deposits indicate a rich fuel mixture or excessive oil burning.
- Oil Fouling: Wet, oily deposits suggest worn piston rings or valve seals.
- Cracked or Damaged Insulator: Any cracks or chips in the white ceramic insulator can cause misfires.
- Worn Electrode: A rounded or excessively worn electrode gap indicates that the spark plug needs replacement.
- Cleaning (if applicable): If the spark plug is only slightly fouled with carbon, you can try cleaning it.
- Use a wire brush or spark plug cleaner to remove the deposits.
- Be careful not to damage the electrode or insulator.
- Alternatively, you can use a solvent like carburetor cleaner to soak the spark plug and then scrub it clean.
- Gap Adjustment: The electrode gap is crucial for proper spark. Refer to your Stihl FS 45 owner’s manual for the correct gap specification (usually around 0.5mm or 0.020 inches). Use a spark plug gap tool to adjust the gap by carefully bending the ground electrode.
- Reinstallation: Carefully screw the spark plug back into the cylinder head. Tighten it to the specified torque (usually around 15-20 Nm or 11-15 ft-lbs). If you don’t have a torque wrench, tighten it snugly but don’t overtighten it, as this can damage the cylinder head threads.
- Reconnection: Reattach the spark plug wire securely.
My Experience: I once had a Stihl FS 45 that refused to start after sitting idle for several months. The spark plug was completely covered in carbon. A thorough cleaning and gap adjustment were all it took to get it running like new. I now make it a habit to inspect and clean the spark plug at least once a year, especially before storing the trimmer for the winter.
Troubleshooting Tips:
- If the spark plug is heavily fouled, cracked, or has a worn electrode, it’s best to replace it. Spark plugs are relatively inexpensive, and a new one can make a significant difference in performance.
- Always use the correct spark plug type recommended by Stihl. Using the wrong spark plug can damage the engine.
- When reinstalling the spark plug, make sure the seating surface is clean to ensure a good seal.
Fix #2: Testing the Spark Plug for Spark
Even if the spark plug looks clean, it might not be producing a spark. This test confirms whether the ignition system is delivering the necessary voltage to the spark plug.
Step-by-Step Guide:
- Safety First: Disconnect the spark plug wire from the spark plug.
- Grounding: Reattach the spark plug to the spark plug wire. Hold the spark plug against a metal part of the engine block (the cylinder head fins work well). This grounds the spark plug.
- Starting: With the spark plug grounded, pull the starter cord briskly.
- Observation: Watch for a spark jumping across the electrode gap of the spark plug.
- Interpretation:
- Strong, Blue Spark: This indicates a healthy ignition system. The problem likely lies elsewhere (fuel, compression, etc.).
- Weak, Yellow Spark: This suggests a weak ignition system. The ignition coil, flywheel, or ignition module might be faulty.
- No Spark: This indicates a complete failure of the ignition system.
My Experience: I had a Stihl FS 45 that would start intermittently. The spark test revealed a weak, yellow spark. After replacing the ignition coil, the trimmer ran perfectly. This experience taught me the importance of a strong spark for reliable starting.
Troubleshooting Tips:
- Make sure the spark plug is making good contact with the engine block for grounding.
- If you’re having trouble seeing the spark, try performing the test in a dimly lit area.
- If you suspect a weak spark, try testing the spark plug with a known good ignition coil. This will help you isolate the problem.
Fix #3: Checking the Ignition Coil Air Gap
The ignition coil generates the high voltage needed for the spark plug. It works by creating a magnetic field that collapses when the flywheel magnets pass by. The air gap between the ignition coil and the flywheel is critical for proper operation. If the gap is too large or too small, the ignition coil won’t generate enough voltage.
Step-by-Step Guide:
- Safety First: Disconnect the spark plug wire from the spark plug.
- Location: Locate the ignition coil. It’s usually mounted near the flywheel.
- Inspection: Visually inspect the ignition coil and flywheel for damage. Look for cracks, loose wires, or corrosion.
- Air Gap Measurement: Use a feeler gauge to measure the air gap between the ignition coil and the flywheel magnets. Refer to your Stihl FS 45 owner’s manual for the correct air gap specification (usually around 0.3mm or 0.012 inches).
- Adjustment: If the air gap is incorrect, loosen the mounting screws that hold the ignition coil in place. Use the feeler gauge to set the correct air gap. Tighten the mounting screws securely.
My Experience: I once bought a used Stihl FS 45 that wouldn’t start. After checking the spark and fuel, I discovered that the ignition coil air gap was way too large. Someone had apparently removed the ignition coil and didn’t reinstall it correctly. Setting the correct air gap solved the problem instantly.
Troubleshooting Tips:
- A business card or a folded piece of paper can be used as a makeshift feeler gauge if you don’t have a proper feeler gauge set.
- Make sure the flywheel magnets are clean and free of debris.
- If the ignition coil is damaged or corroded, it’s best to replace it.
Fix #4: Inspecting the Kill Switch and Wiring
The kill switch is a simple but essential component that grounds the ignition system to stop the engine. If the kill switch or its wiring is faulty, it can prevent the engine from starting or cause it to stall intermittently.
Step-by-Step Guide:
- Safety First: Disconnect the spark plug wire from the spark plug.
- Location: Locate the kill switch. It’s usually a small switch located on the handle of the trimmer.
- Inspection: Visually inspect the kill switch and its wiring for damage. Look for loose wires, corroded connections, or cracked insulation.
- Testing: Use a multimeter to test the continuity of the kill switch.
- Disconnect the wires from the kill switch.
- Set the multimeter to the continuity setting.
- Touch the multimeter probes to the terminals of the kill switch.
- With the kill switch in the “run” position, the multimeter should show continuity (a reading of zero ohms).
- With the kill switch in the “stop” position, the multimeter should show no continuity (an infinite reading).
- Repair or Replacement: If the kill switch is faulty, replace it with a new one. If the wiring is damaged, repair it or replace the wiring harness.
My Experience: I had a Stihl FS 45 that would start and then immediately stall. After checking everything else, I discovered that the kill switch wiring was frayed and shorting out. Replacing the wiring harness solved the problem.
Troubleshooting Tips:
- If you don’t have a multimeter, you can try disconnecting the kill switch wires completely. If the engine starts and runs normally, the kill switch is likely the problem.
- Make sure the kill switch is properly grounded. A poor ground connection can cause intermittent problems.
- When replacing the wiring harness, make sure to use the correct type of wire and connectors.
Fix #5: Checking the Flywheel Key
The flywheel key is a small metal piece that aligns the flywheel with the crankshaft. If the flywheel key is sheared or damaged, the flywheel will be out of time, and the engine won’t start. This is a less common problem, but it’s worth checking if you’ve ruled out other causes.
Step-by-Step Guide:
- Safety First: Disconnect the spark plug wire from the spark plug.
- Removal: Remove the starter housing and any other components that are blocking access to the flywheel.
- Flywheel Removal: Use a flywheel puller to remove the flywheel from the crankshaft. A flywheel puller is a specialized tool that prevents damage to the flywheel and crankshaft.
- Inspection: Inspect the flywheel key for damage. It should be a small, rectangular piece of metal that fits snugly into a slot in the crankshaft and the flywheel.
- Replacement: If the flywheel key is sheared or damaged, replace it with a new one.
- Reinstallation: Reinstall the flywheel, making sure the flywheel key is properly aligned. Tighten the flywheel nut or bolt to the specified torque.
- Reassembly: Reinstall the starter housing and any other components that were removed.
My Experience: I once had a Stihl FS 45 that had been dropped hard. It wouldn’t start, and I suspected internal damage. After removing the flywheel, I found that the flywheel key was completely sheared. Replacing the flywheel key and properly torquing the flywheel nut restored the engine’s timing and allowed it to start.
Troubleshooting Tips:
- A sheared flywheel key is often a sign of a sudden stop or impact.
- When reinstalling the flywheel, make sure to use the correct torque specification. Overtightening the flywheel nut can damage the crankshaft.
- If you’re not comfortable removing the flywheel, it’s best to take the trimmer to a qualified mechanic.
Beyond Ignition: A Holistic Approach to Trimmer Health
While these five fixes focus on the ignition system, it’s important to remember that other factors can affect your Stihl FS 45’s performance.
- Fuel System: A clogged fuel filter, dirty carburetor, or stale fuel can prevent the engine from starting. Regularly clean or replace the fuel filter and carburetor, and always use fresh fuel. I prefer using premium fuel with a fuel stabilizer, especially when storing the trimmer for extended periods.
- Compression: Low compression can also prevent the engine from starting. If you suspect a compression problem, have a mechanic perform a compression test.
- Air Filter: A dirty air filter restricts airflow to the engine, causing it to run poorly. Clean or replace the air filter regularly.
- Maintenance: Regular maintenance is key to keeping your Stihl FS 45 running smoothly. Follow the manufacturer’s recommendations for oil changes, spark plug replacement, and other maintenance tasks.
The Importance of Proper Storage
Proper storage is crucial for extending the life of your Stihl FS 45 trimmer. Before storing the trimmer for the winter, drain the fuel tank, clean the carburetor, and remove the spark plug. Store the trimmer in a dry, protected location. I also like to add a fuel stabilizer to the fuel before storing it to prevent fuel from going stale.
Safety First: A Constant Reminder
Working with small engines can be dangerous. Always follow these safety precautions:
- Wear safety glasses to protect your eyes from debris.
- Wear gloves to protect your hands from cuts and burns.
- Work in a well-ventilated area to avoid inhaling fumes.
- Disconnect the spark plug wire before working on the ignition system.
- Use the correct tools for the job.
- If you’re not comfortable performing a repair, take the trimmer to a qualified mechanic.
Case Study: Reviving a Neglected Stihl FS 45
I once acquired a Stihl FS 45 that had been neglected for years. It was covered in dirt and grime, and it wouldn’t start. After a thorough cleaning and inspection, I discovered the following problems:
- The spark plug was heavily fouled with carbon.
- The fuel filter was clogged.
- The carburetor was dirty.
- The air filter was completely blocked.
I cleaned the spark plug, replaced the fuel filter and air filter, and cleaned the carburetor. After these repairs, the trimmer started on the first pull and ran like new. This experience reinforced the importance of regular maintenance and proper storage.
Strategic Advantages of DIY Repair
Performing your own repairs on your Stihl FS 45 trimmer offers several strategic advantages:
- Cost Savings: You can save a significant amount of money by performing your own repairs.
- Time Savings: You can avoid the hassle of taking the trimmer to a mechanic and waiting for it to be repaired.
- Increased Knowledge: You’ll gain a better understanding of how your trimmer works, which will help you troubleshoot problems in the future.
- Self-Reliance: You’ll be able to keep your trimmer running smoothly without relying on others.
Next Steps: Putting Knowledge into Action
Now that you’ve learned about these essential ignition fixes, it’s time to put your knowledge into action. Start by inspecting your Stihl FS 45 trimmer and identifying any potential problems. Gather the necessary tools and supplies, and follow the step-by-step guides to perform the repairs. Don’t be afraid to ask for help if you get stuck. With a little patience and effort, you can keep your Stihl FS 45 trimmer running smoothly for years to come.
I hope this guide has been helpful. Remember, a little maintenance goes a long way in keeping your Stihl FS 45 trimmer running strong. Happy trimming!
