Stihl MS250 Coil Troubleshooting (5 Proven Fixes for No Spark)

“My Stihl MS250 just won’t start! I’ve checked the fuel, the air filter… everything seems fine. I suspect it’s the coil, but I’m not sure how to troubleshoot it. Any ideas?” – Mark J., Firewood Enthusiast

That’s a familiar cry from many Stihl MS250 owners. There’s nothing more frustrating than a chainsaw that refuses to fire up, especially when you’re ready to tackle a pile of logs. A faulty ignition coil is often the culprit, and while it might seem intimidating, troubleshooting it is something you can definitely handle. I’ve spent years felling trees, processing firewood, and generally wrestling with chainsaws, so I’ve seen this problem more times than I care to admit. Let me walk you through five proven fixes for a Stihl MS250 with no spark. We’ll cover everything from basic checks to more in-depth troubleshooting, giving you the knowledge to get your saw roaring again.

Stihl MS250 Coil Troubleshooting: 5 Proven Fixes for No Spark

Before we dive in, let’s get one thing straight: safety first! Disconnect the spark plug wire before doing any work near the spark plug or coil. This prevents accidental starting and potential electric shock. Wear gloves, especially when handling electrical components. And, if you’re not comfortable with electrical troubleshooting, it’s always best to consult a qualified mechanic.

1. The Obvious First: Spark Plug Inspection and Cleaning

It sounds simple, but a faulty spark plug is the most common reason for a no-spark condition. Don’t underestimate the power of a visual inspection!

• The Process: Remove the spark plug using a spark plug wrench. Carefully examine the electrode (the metal tip inside the plug). Is it cracked, broken, or heavily fouled with carbon deposits? A dark, oily residue indicates a rich fuel mixture, while a light, ashy deposit suggests a lean mixture.

• The Fix: If the plug is simply dirty, try cleaning it with a wire brush. You can also use a specialized spark plug cleaner. However, if the plug is damaged or heavily fouled, replace it. Spark plugs are relatively inexpensive, and a new one can often solve the problem immediately.

• Data Point: Did you know that using the correct spark plug gap is crucial for optimal performance? The Stihl MS250 typically requires a spark plug gap of 0.020 inches (0.5 mm). Use a spark plug gap tool to ensure the gap is correct. I’ve seen many instances where an incorrect gap caused a weak or non-existent spark.

• Personal Story: I once spent hours troubleshooting a stubborn MS250, convinced it was a major issue. Turns out, the spark plug was simply cracked! A five-dollar replacement saved me a lot of time and frustration.

2. The Ground Connection Check: Ensuring a Proper Circuit

The ignition coil needs a good ground connection to function correctly. A loose or corroded ground can interrupt the electrical circuit, preventing the coil from generating a spark.

• The Process: Locate the ground wire connected to the ignition coil. It’s usually a black wire attached to the engine block or the saw’s frame. Check the connection point for corrosion or looseness.

• The Fix: Clean the connection point with a wire brush or sandpaper to remove any corrosion. Ensure the ground wire is securely attached to the engine block. If the wire is damaged, replace it.

• Data Point: A poor ground connection can also lead to intermittent spark issues, making troubleshooting even more challenging. I recommend using a multimeter to test the continuity of the ground wire to ensure a solid connection. Set your multimeter to the continuity setting (usually indicated by a diode symbol or a sound wave symbol). Touch one probe to the ground wire and the other to a clean metal part of the engine block. You should get a reading of near zero ohms or a beep, indicating a good connection.

• Unique Insight: When dealing with ground connections, a little dielectric grease can go a long way. Applying a thin layer of dielectric grease to the connection point helps prevent corrosion and ensures a reliable electrical connection.

3. The Air Gap Adjustment: Optimizing Coil Proximity

The air gap is the distance between the ignition coil and the flywheel. This gap is crucial for the coil to generate a spark. If the gap is too wide or too narrow, the coil won’t function correctly.

• The Process: Locate the ignition coil on the engine. You’ll see a small gap between the coil and the flywheel (the rotating part with magnets). You’ll need a feeler gauge to accurately measure this gap.

• The Fix: The Stihl MS250 typically requires an air gap of 0.010-0.014 inches (0.25-0.35 mm). Loosen the screws that hold the coil in place. Insert the feeler gauge between the coil and the flywheel. Gently tighten the screws while ensuring the feeler gauge remains in place. Remove the feeler gauge after tightening the screws.

• Data Point: Using a business card as a makeshift feeler gauge is a common trick, but it’s not always accurate. A proper feeler gauge provides a more precise measurement, ensuring optimal coil performance.

• Case Study: I worked on a logging project in Oregon where we were felling Douglas fir. One of our MS250s kept cutting out intermittently. After checking everything else, I discovered the air gap was too wide. Adjusting it to the correct specification solved the problem immediately, and we were back in business. Douglas fir, by the way, is a fantastic wood for construction – strong, relatively lightweight, and readily available. But, like any job, it requires reliable equipment!

4. The Coil Resistance Test: Diagnosing Internal Failure

If the spark plug, ground connection, and air gap are all good, the ignition coil itself might be faulty. A multimeter can help you diagnose this.

• The Process: Disconnect the ignition coil from the spark plug wire and the ground wire. Set your multimeter to the resistance setting (Ohms – Ω). You’ll need to consult the Stihl MS250 service manual to find the correct resistance values for the primary and secondary windings of the coil.

• The Fix: Touch one probe of the multimeter to the primary terminal of the coil (the terminal connected to the ground wire) and the other probe to the coil body. Compare the reading to the specification in the service manual. Then, touch one probe to the spark plug wire terminal and the other probe to the coil body. Again, compare the reading to the specification. If the resistance values are significantly different from the specifications, the coil is likely faulty and needs to be replaced.

• Data Point: The resistance values can vary slightly depending on the coil manufacturer and the age of the coil. However, a reading that is significantly outside the specified range indicates a problem.

• Professional Tip: When testing the coil, make sure the multimeter is properly calibrated. A faulty multimeter can give inaccurate readings, leading to misdiagnosis.

5. The Flywheel Key Inspection: Addressing Timing Issues

While less common, a sheared flywheel key can also cause a no-spark condition. The flywheel key is a small metal piece that aligns the flywheel with the crankshaft. If it shears, the flywheel can rotate out of sync, disrupting the ignition timing and preventing the coil from generating a spark at the right time.

• The Process: Remove the flywheel cover. You’ll likely need a special flywheel puller tool to remove the flywheel itself. Once the flywheel is off, inspect the flywheel key (a small, often semi-circular piece of metal) for damage. Is it sheared, bent, or missing?

• The Fix: If the flywheel key is damaged, replace it. Reinstall the flywheel, ensuring it’s properly aligned with the crankshaft. Tighten the flywheel nut to the manufacturer’s specified torque.

• Data Point: A sheared flywheel key is often caused by a sudden impact or a hard stop. It’s more common on older chainsaws or those that have been subjected to heavy use.

• Unique Insight: When reassembling the flywheel, be sure to clean the surfaces of the flywheel and the crankshaft. A clean, dry surface ensures a good connection and prevents slippage. I recommend using a brake cleaner to remove any oil or grease.

Beyond the Spark: Addressing Other Potential Issues

While these five fixes address the most common causes of a no-spark condition, it’s important to remember that other factors can also contribute to the problem.

• The Kill Switch: A faulty kill switch can ground the ignition circuit, preventing the coil from generating a spark. Disconnect the kill switch wire from the coil to see if that resolves the issue. If it does, replace the kill switch.

• The Wiring Harness: Damaged or frayed wires in the wiring harness can also cause a no-spark condition. Inspect the wiring harness for any signs of damage and repair or replace it as needed.

• The Fuel System: While we’ve focused on the ignition system, a lack of fuel can also mimic a no-spark condition. Ensure the fuel tank is full, the fuel filter is clean, and the carburetor is functioning correctly.

Wood Species and Processing Considerations

Now, let’s shift gears and talk about wood species and how they relate to chainsaw maintenance and performance. Different wood species have different densities and resin contents, which can affect how your chainsaw performs and how frequently you need to sharpen the chain or clean the spark arrestor.

• Hardwoods vs. Softwoods: Hardwoods like oak, maple, and hickory are denser than softwoods like pine, fir, and cedar. Hardwoods require more power to cut and can dull the chain faster. Softwoods, on the other hand, tend to be more resinous, which can clog the carburetor and spark arrestor.

• Resin Content: Wood species with high resin content, such as pine and fir, can leave sticky deposits on the chainsaw’s components. Regular cleaning is essential to prevent these deposits from affecting performance.

• Processing Techniques: The way you process wood can also affect chainsaw maintenance. Bucking (cutting logs into shorter lengths) generates more sawdust than felling trees. Sawdust can clog the air filter and the cooling fins, leading to overheating.

• Data Point: A study by the USDA Forest Service found that processing hardwoods can reduce chainsaw chain life by up to 30% compared to processing softwoods. This highlights the importance of using the correct chain type and sharpening the chain regularly when working with hardwoods.

Safety Standards and Best Practices

No discussion of chainsaw maintenance is complete without addressing safety standards and best practices. Chainsaws are powerful tools that can cause serious injuries if used improperly.

• Personal Protective Equipment (PPE): Always wear appropriate PPE, including a helmet, eye protection, hearing protection, gloves, chaps, and sturdy boots.

• Safe Operating Procedures: Follow the manufacturer’s instructions for safe chainsaw operation. Never operate a chainsaw when you’re tired or under the influence of drugs or alcohol.

• Kickback Prevention: Be aware of the risk of kickback, which occurs when the tip of the chainsaw bar contacts a solid object. Use a chainsaw with a chain brake and practice safe cutting techniques to minimize the risk of kickback.

• Data Point: According to the Consumer Product Safety Commission, chainsaws cause approximately 30,000 injuries each year in the United States. Many of these injuries are preventable by following safe operating procedures and wearing appropriate PPE.

Cost-Effectiveness and Efficiency

Firewood preparation and wood processing can be time-consuming and expensive. Here are some tips for maximizing cost-effectiveness and efficiency:

• Tool Selection: Choose the right tools for the job. A chainsaw is essential for felling trees and bucking logs, but a log splitter can significantly reduce the time and effort required to split firewood.

• Wood Selection: Choose wood species that are readily available and relatively easy to process. Softwoods like pine and fir are often less expensive and easier to split than hardwoods like oak and maple.

• Processing Methods: Optimize your processing methods to minimize waste and maximize efficiency. Use a firewood processor to cut and split firewood in a single operation.

• Data Point: A study by the University of Minnesota Extension found that using a firewood processor can reduce firewood production time by up to 50% compared to manual methods.

Final Thoughts

Troubleshooting a Stihl MS250 with no spark can seem daunting, but by following these five proven fixes, you can systematically diagnose the problem and get your saw back in action. Remember to prioritize safety, use the correct tools, and consult the service manual for specific instructions. And, don’t underestimate the value of a clean spark plug and a good ground connection! With a little patience and perseverance, you’ll be back to processing wood and preparing firewood in no time. Happy sawing!

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