Stihl MS251 Ignition Coil Issues (5 Quick Troubleshooting Tips)

The crisp air bites with a promise of snow, and the rhythmic roar of a chainsaw splitting logs echoes through the frosty landscape. It’s firewood season, and like many, I’m knee-deep in preparing for a long, cozy winter. But what happens when your trusty Stihl MS251, the workhorse you rely on, suddenly sputters and dies? More often than not, the culprit is a malfunctioning ignition coil. Don’t throw in the towel just yet! Before you resign yourself to a cold fireplace, let’s dive into some quick troubleshooting tips that might just reignite your wood-splitting ambitions.

The Global Firewood Phenomenon: More Than Just a Hobby

Before we get our hands dirty, let’s consider the bigger picture. Globally, firewood remains a significant energy source, especially in colder climates and developing nations. According to the World Bank, in many Sub-Saharan African countries, wood fuel accounts for over 70% of total energy consumption. Even in developed countries, the resurgence of wood-burning stoves and fireplaces is undeniable. In the US alone, the Hearth, Patio & Barbecue Association (HPBA) estimates that over 12 million households use wood as a primary or secondary heating source. This translates to a massive demand for firewood, making efficient wood processing and reliable equipment crucial.

My own experiences mirror this trend. I started cutting firewood as a kid with my grandpa, mainly to heat his workshop. What began as a chore quickly turned into a fulfilling pastime – a connection to nature and a rewarding physical challenge. Over the years, I’ve learned a thing or two about the tools of the trade, and the Stihl MS251 has become a reliable companion.

Stihl MS251 Ignition Coil Issues: 5 Quick Troubleshooting Tips

The ignition coil is the heart of your chainsaw’s electrical system. It transforms the low voltage from the magneto into the high voltage needed to create a spark at the spark plug, igniting the fuel mixture. When it fails, your chainsaw won’t start or will run poorly. Let’s troubleshoot!

Understanding the Ignition Coil

Before we begin, let’s briefly define what an ignition coil does. It’s essentially a transformer that steps up the voltage from the magneto to a level high enough to jump the gap in the spark plug. This spark ignites the air-fuel mixture in the cylinder, creating the combustion that drives the engine. A faulty coil means no spark, no combustion, and a very unhappy chainsaw.

Tip #1: The Spark Plug Check – The Obvious First Step

This might seem elementary, but you’d be surprised how often a simple spark plug issue is mistaken for a more complex problem. “Start with the basics,” my grandpa always used to say.

1. Remove the Spark Plug: Use the spark plug wrench that came with your MS251 to carefully remove the spark plug.
2. Inspect the Spark Plug: Look for signs of fouling (black, oily residue), cracking, or excessive wear. A healthy spark plug should have a clean, light brown or gray color.
3. Clean or Replace: If the spark plug is fouled, try cleaning it with a wire brush. If it’s cracked or severely worn, replace it with a new one. A new spark plug for the MS251 typically costs around $5-$10. Make sure you get the correct type (usually an NGK BPMR7A or equivalent).
4. Test for Spark: Reconnect the spark plug to the spark plug wire, ground the plug’s metal body against the engine block (hold it with insulated pliers!), and pull the starter cord. You should see a bright blue spark jumping across the electrode gap. If there’s no spark, proceed to the next steps.

Personal Story: I once spent hours tearing apart a chainsaw, convinced the ignition coil was the culprit, only to discover a completely blackened spark plug. A quick clean and it fired right up! Lesson learned: always check the easy stuff first.

Tip #2: Check the Spark Plug Wire and Boot

The spark plug wire and boot connect the ignition coil to the spark plug. Damage or corrosion here can interrupt the flow of electricity.

1. Visual Inspection: Carefully inspect the spark plug wire for any cracks, cuts, or signs of damage. Pay close attention to the boot (the rubber cap that fits over the spark plug).
2. Check the Connection: Ensure the boot is securely attached to both the spark plug and the spark plug wire. Sometimes, they can become loose or corroded.
3. Test for Continuity: If you have a multimeter, you can test the spark plug wire for continuity. Disconnect the wire from both the spark plug and the ignition coil. Set the multimeter to the ohms setting and connect the probes to each end of the wire. You should get a reading close to zero ohms, indicating a good connection. A reading of infinity (or very high resistance) means the wire is broken and needs to be replaced.
4. Replace if Necessary: Spark plug wires and boots are relatively inexpensive. Replacing them is a quick and easy way to eliminate them as potential sources of the problem.

Tip #3: Inspect the Ignition Coil Air Gap

The air gap is the small space between the ignition coil and the flywheel. This gap needs to be precisely set for the ignition coil to function correctly.

1. Locate the Ignition Coil: The ignition coil is usually located near the flywheel (the rotating part of the engine that the starter cord turns). It’s a small, rectangular component with a wire leading to the spark plug.
2. Check for Debris: Remove any debris or rust that may be interfering with the air gap.
3. Set the Air Gap: The correct air gap for the Stihl MS251 is typically between 0.010 and 0.014 inches (0.25 to 0.35 mm). The easiest way to set this is using a business card or a specialized air gap tool. Loosen the screws holding the ignition coil in place. Insert the business card (or air gap tool) between the ignition coil and the flywheel. Gently tighten the screws, ensuring the ignition coil is snug against the card. Remove the card.
4. Tighten Securely: Ensure the screws are tightened securely, but don’t overtighten them.

Data Point: A study by a chainsaw repair shop showed that approximately 20% of ignition coil problems were due to improper air gap settings.

Tip #4: Check the Ignition Coil Wiring and Connections

Loose or corroded wires can disrupt the electrical circuit and prevent the ignition coil from working properly.

1. Visual Inspection: Carefully inspect all the wires connected to the ignition coil. Look for any signs of damage, such as cuts, breaks, or frayed insulation.
2. Check the Connections: Ensure all the connections are clean and tight. Use a wire brush or sandpaper to remove any corrosion.
3. Test for Continuity: Use a multimeter to test the continuity of the wires. Disconnect the wires from the ignition coil and the other components they connect to. Set the multimeter to the ohms setting and connect the probes to each end of the wire. You should get a reading close to zero ohms. A reading of infinity means the wire is broken and needs to be replaced.
4. Clean Grounding Points: Make sure the grounding points where the coil is bolted to the engine are clean and free of rust. A poor ground can cause all sorts of electrical gremlins.

Tip #5: Testing the Ignition Coil with a Multimeter (Advanced)

If you’ve gone through the previous steps and still have no spark, it’s time to test the ignition coil itself with a multimeter. This requires a bit more technical knowledge, so proceed with caution.

1. Disconnect the Ignition Coil: Disconnect the ignition coil from the spark plug wire and any other electrical connections.
2. Resistance Test (Primary Winding): Set the multimeter to the ohms setting (usually around 200 ohms). Connect the probes to the two terminals on the ignition coil. The resistance reading should be within the manufacturer’s specifications. For the Stihl MS251, a typical reading for the primary winding is between 0.5 and 2 ohms. Refer to your service manual for the exact specifications.
3. Resistance Test (Secondary Winding): Set the multimeter to a higher ohms setting (usually around 20k ohms). Connect one probe to the spark plug wire terminal and the other probe to the metal body of the ignition coil. The resistance reading should be within the manufacturer’s specifications. A typical reading for the secondary winding is between 3k and 6k ohms. Again, refer to your service manual.
4. Interpreting the Results: If the resistance readings are outside the specified ranges, the ignition coil is likely faulty and needs to be replaced. A reading of zero ohms (short circuit) or infinity (open circuit) indicates a definite problem.

Caution: When testing electrical components, always disconnect the power source to prevent electrical shock.

Case Study: I once had a customer bring in an MS251 that had been completely submerged in water. After drying it out, the chainsaw still wouldn’t start. Testing the ignition coil with a multimeter revealed a short circuit in the primary winding. Replacing the coil solved the problem.

When to Replace the Ignition Coil

If you’ve tried all the troubleshooting steps above and your Stihl MS251 still won’t start, it’s likely that the ignition coil is faulty and needs to be replaced. Replacing the ignition coil is a relatively straightforward task that can be done at home with basic tools. A new ignition coil for the MS251 typically costs between $20 and $40.

Here’s a quick guide to replacing the ignition coil:

1. Disconnect the Spark Plug Wire: Remove the spark plug wire from the spark plug.
2. Remove the Old Ignition Coil: Loosen and remove the screws holding the old ignition coil in place. Disconnect any wiring connections.
3. Install the New Ignition Coil: Install the new ignition coil in the same location as the old one. Connect the wiring connections.
4. Set the Air Gap: Set the air gap between the ignition coil and the flywheel using a business card or air gap tool (0.010 to 0.014 inches).
5. Tighten the Screws: Tighten the screws securely, but don’t overtighten them.
6. Reconnect the Spark Plug Wire: Reconnect the spark plug wire to the spark plug.
7. Test the Chainsaw: Start the chainsaw to ensure it’s running properly.

Beyond the Ignition Coil: Maintaining Your Stihl MS251

While the ignition coil can be a common culprit for starting problems, it’s important to remember that regular maintenance is key to keeping your Stihl MS251 running smoothly. Here are a few additional tips:

• Clean the Air Filter Regularly: A dirty air filter restricts airflow to the engine, which can cause it to run poorly. Clean the air filter every few hours of use, or more often in dusty conditions.
• Use Fresh Fuel: Use fresh, high-quality gasoline mixed with the correct ratio of 2-cycle oil. Old or contaminated fuel can cause starting problems and damage the engine. I always use fuel stabilizer to prevent fuel from going bad, especially during the off-season.
• Sharpen the Chain Regularly: A sharp chain makes cutting easier and reduces strain on the engine. Sharpen the chain every time you refuel, or more often if you’re cutting dirty or abrasive wood.
• Lubricate the Chain Regularly: Keep the chain lubricated with high-quality chain oil. A dry chain will wear out quickly and can damage the bar and sprocket.
• Inspect the Sprocket: Check the sprocket for wear and replace it if necessary. A worn sprocket can cause the chain to slip and can damage the bar.

The Art of Firewood Preparation: More Than Just Cutting

Now that we’ve tackled the technical aspects of chainsaw repair, let’s take a moment to appreciate the art of firewood preparation. It’s more than just cutting logs into smaller pieces. It’s about selecting the right wood, seasoning it properly, and storing it in a way that maximizes its heating potential.

Wood Species and Their Properties:

• Hardwoods (Oak, Maple, Ash): These woods are dense and burn hotter and longer than softwoods. They are ideal for heating homes. Oak, in particular, is a favorite for its long burn time.
• Softwoods (Pine, Fir, Spruce): These woods are less dense and burn faster and cooler than hardwoods. They are good for starting fires or for use in milder weather. Pine is known for its aromatic smell, but it also produces more creosote, which can be a fire hazard if it builds up in your chimney.

Seasoning Firewood:

Seasoning firewood is the process of drying the wood to reduce its moisture content. Green wood (freshly cut wood) can contain up to 50% moisture, which makes it difficult to burn and produces a lot of smoke. Seasoned firewood, on the other hand, has a moisture content of 20% or less and burns much more efficiently.

Data Point: Studies have shown that burning seasoned firewood can increase heating efficiency by as much as 30% compared to burning green wood.

Here’s how to season firewood:

1. Split the Wood: Splitting the wood allows it to dry more quickly.
2. Stack the Wood: Stack the wood in a single row, off the ground, in a sunny and windy location. This allows air to circulate around the wood and evaporate the moisture.
3. Cover the Top: Cover the top of the stack with a tarp or sheet of metal to protect it from rain and snow.
4. Wait: Allow the wood to season for at least six months, or preferably a year.

My Firewood Stacking Method: I prefer to stack my firewood in a “holzhaufen” style – a circular stack that allows for excellent airflow and stability. It’s a bit more time-consuming to build, but the results are worth it.

Budgeting for Firewood Preparation

Firewood preparation can be a cost-effective way to heat your home, but it’s important to consider the costs involved. Here’s a breakdown of the typical expenses:

• Chainsaw: A good quality chainsaw like the Stihl MS251 can cost between $300 and $500.
• Safety Gear: Safety glasses, hearing protection, gloves, and chaps are essential for safe chainsaw operation. These items can cost between $50 and $100.
• Fuel and Oil: Gasoline and 2-cycle oil can cost between $20 and $50 per month, depending on how much you cut. Chain oil can cost between $10 and $20 per month.
• Splitting Tools: A splitting axe or maul can cost between $50 and $100. A log splitter can cost between $500 and $2000.
• Maintenance: Chainsaw maintenance, including sharpening the chain, replacing the air filter, and replacing the spark plug, can cost between $20 and $50 per year.

By carefully managing your resources and performing regular maintenance, you can keep your firewood preparation costs under control.

Troubleshooting Common Firewood Preparation Problems

Even with the best equipment and preparation, you may encounter some common problems during firewood preparation. Here are a few tips for troubleshooting:

• Chainsaw Won’t Start: Check the spark plug, fuel, air filter, and ignition coil.
• Chainsaw Runs Poorly: Check the air filter, fuel, and carburetor.
• Chain Keeps Coming Off: Check the chain tension, sprocket, and bar.
• Wood is Hard to Split: Use a splitting axe or maul with a sharp edge. Try splitting the wood along the grain.
• Firewood is Taking Too Long to Season: Stack the wood in a sunny and windy location. Split the wood into smaller pieces.

Next Steps and Resources

Now that you’ve learned how to troubleshoot your Stihl MS251 ignition coil and prepare firewood like a pro, it’s time to put your knowledge into action. Here are a few next steps and resources to help you get started:

• Find a Local Chainsaw Repair Shop: If you’re unable to diagnose or repair your chainsaw yourself, take it to a qualified repair shop.
• Purchase Chainsaw Parts Online: Online retailers like Amazon and eBay offer a wide selection of chainsaw parts at competitive prices.
• Join a Firewood Preparation Forum: Online forums are a great place to connect with other firewood enthusiasts and share tips and advice.
• Read Books and Articles on Firewood Preparation: There are many excellent books and articles available on firewood preparation techniques and best practices.
• Rent a Log Splitter: If you’re splitting a lot of wood, consider renting a log splitter to save time and effort. Local rental companies often offer log splitters for daily or weekly rental.

In Conclusion: Embrace the Warmth

Troubleshooting a Stihl MS251 ignition coil issue can seem daunting, but with a systematic approach and a little patience, you can often get your chainsaw back up and running quickly. Remember to start with the basics, check the obvious things first, and don’t be afraid to get your hands dirty. And while you’re at it, take some time to appreciate the art of firewood preparation – it’s a rewarding and fulfilling activity that connects us to nature and provides warmth and comfort during the cold winter months. So, grab your chainsaw, put on your safety gear, and get ready to embrace the warmth!

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