Coil for MS250 Stihl Chainsaw (5 Pro Tips to Fix No Spark)

The world of small-engine repair is seeing a surge in interest, fueled by a desire for self-sufficiency and cost savings. More and more people are tackling repairs on their own, from lawnmowers to chainsaws. But what happens when your trusty MS250 suddenly refuses to roar to life? Often, the culprit is a faulty ignition coil, the unsung hero responsible for creating the spark that ignites the fuel mixture. I’m here to guide you through diagnosing and fixing this common issue. I will share my years of experience, learned the hard way through trial, error, and more than a few frustrating afternoons. We’ll dive deep into the ignition coil of the MS250, arming you with the knowledge and pro tips to get your chainsaw back in action.

Coil for MS250 Stihl Chainsaw: 5 Pro Tips to Fix No Spark

Understanding the Ignition Coil: The Heart of the Spark

The ignition coil is, in essence, a transformer. It takes the low-voltage electricity from the chainsaw’s magneto and amplifies it into the high-voltage jolt needed to create a spark at the spark plug. This spark ignites the air-fuel mixture in the cylinder, driving the piston and turning the chain. Without a properly functioning coil, you’re dead in the water.

Think of it like this: I once spent a whole weekend trying to fell a rather large oak tree, only to be thwarted by a failing coil. I was convinced it was a fuel issue, endlessly cleaning the carburetor. The lesson? Knowing your system is crucial.

Pro Tip #1: Diagnosing the “No Spark” Situation

Before tearing into your MS250, let’s confirm the coil is indeed the problem. Here’s my systematic approach:

• Visual Inspection: Start with a close look. Are there any obvious cracks, burns, or melted plastic on the coil? Disconnected or frayed wires? These are immediate red flags. I once found a coil housing cracked due to prolonged exposure to heat and vibration.
• Spark Plug Check: Remove the spark plug. Is it fouled with carbon buildup, wet with fuel, or damaged? A healthy spark plug should be clean and dry. Clean or replace the plug before proceeding. A spark plug gap gauge helps ensure the gap is within the Stihl specified range (typically 0.5mm or 0.020 inches for the MS250). An incorrect gap can prevent a spark, even with a good coil.

• Spark Test: Reconnect the spark plug to the spark plug wire. Hold the plug against the engine cylinder (grounding it) and pull the starter cord. Look for a strong, blue spark jumping across the spark plug gap. If you see a weak, intermittent spark, or no spark at all, the coil is suspect. Important Safety Note: Avoid touching the spark plug while pulling the starter cord, as you will receive a shock.

  • Data Point: A healthy spark should be a bright blue color. A weak or yellow spark often indicates a failing coil.

  • Kill Switch Test: Disconnect the kill switch wire from the coil. Sometimes, a faulty kill switch can short to ground, preventing the coil from firing. Repeat the spark test. If you now have a spark, the kill switch is the culprit.

  • Case Study: I had a client who swore his coil was bad. Turns out, a small rodent had chewed through the kill switch wire, causing it to ground against the engine block. A simple wire repair saved him the cost of a new coil.

Pro Tip #2: Air Gap Adjustment: The Critical Clearance

The air gap is the tiny space between the coil’s core and the flywheel magnets. This gap is absolutely crucial for proper coil function. If the gap is too wide, the magnetic field won’t be strong enough to induce the necessary voltage in the coil. If it’s too narrow, the coil can rub against the flywheel, causing damage.

• Stihl Specification: The air gap for the MS250 is typically 0.3mm (0.012 inches). Always consult your service manual for the exact specification.
• Setting the Gap:
  1. Loosen the coil mounting screws.
  2. Insert a business card or a feeler gauge of the correct thickness (0.3mm) between the coil and the flywheel magnet.
  3. Gently push the coil against the flywheel, ensuring the business card or feeler gauge remains in place.
  4. Tighten the coil mounting screws.
  5. Remove the business card or feeler gauge.

• My Experience: I’ve seen countless coils misdiagnosed as faulty simply because the air gap was incorrect. A properly set air gap can often revive a seemingly dead coil.

  • Insight: Using a non-magnetic feeler gauge is preferable to avoid accidentally attracting the coil to the flywheel during adjustment.

Pro Tip #3: Testing the Coil with a Multimeter: Digging Deeper

A multimeter can help you determine if the coil is truly bad. While this test isn’t foolproof, it can provide valuable clues.

• Resistance Test:
  1. Set your multimeter to the ohms setting (resistance).
  2. Disconnect the spark plug wire from the spark plug.
  3. Touch one multimeter probe to the spark plug wire terminal on the coil and the other probe to the coil’s ground terminal (usually the mounting point).
  4. Compare the reading to the specification in your service manual. Typically, a good coil will have a primary resistance of around 0.5-2 ohms.
  5. Next, test the secondary resistance. Connect one probe to the spark plug wire terminal and the other to the engine ground. The reading should be several thousand ohms (typically 2,000-4,000 ohms).

• Interpreting the Results:

  • Zero or very low resistance: Indicates a short circuit within the coil.
  • Infinite resistance: Indicates an open circuit within the coil.

  • Readings significantly outside the specified range: Suggests a faulty coil.

  • Caution: Multimeter readings can vary slightly depending on the meter and the coil’s temperature. Use the service manual specifications as a guideline, not an absolute rule.

  • Personal Tip: I always perform this test at room temperature. Extreme temperatures can affect resistance readings and lead to inaccurate conclusions.

Pro Tip #4: Wiring and Connections: The Devil is in the Details

Even a brand-new coil won’t work if the wiring is faulty. Carefully inspect all wiring connections related to the ignition system.

• Check for Corrosion: Corrosion can build up on the coil terminals, spark plug wire connection, and ground connections, impeding electrical flow. Clean these connections with a wire brush or electrical contact cleaner.
• Inspect the Spark Plug Wire: Look for cracks, cuts, or brittleness in the spark plug wire. A damaged wire can leak voltage to ground, preventing a strong spark. If in doubt, replace the wire.
• Ground Connection: Ensure the coil has a good, clean ground connection to the engine. A loose or corroded ground can cause intermittent or no spark. I often add a dab of dielectric grease to the ground connection to prevent future corrosion.
• Kill Switch Wiring: As mentioned earlier, the kill switch wiring can cause problems. Make sure the wire is properly connected and not grounding against the engine block.
• Data Point: A poor ground connection can reduce spark intensity by as much as 50%.

Pro Tip #5: Coil Replacement: Choosing the Right Part and Installation

If all other troubleshooting steps fail, it’s time to replace the coil. Here’s how to do it right:

• Part Selection: Use only a high-quality replacement coil specifically designed for the Stihl MS250. Aftermarket coils are available, but their quality can vary significantly. I generally recommend sticking with a genuine Stihl coil or a reputable OEM (Original Equipment Manufacturer) brand.
  • Caution: Avoid generic, unbranded coils, as they may not meet the required specifications and could fail prematurely.
• Installation:
  1. Disconnect the spark plug wire from the spark plug.
  2. Remove the old coil.
  3. Install the new coil, ensuring the wiring is properly connected.
  4. Set the air gap as described in Pro Tip #2.
  5. Reconnect the spark plug wire.
  6. Test for spark.
• Breaking In the New Coil It’s a good idea to run the chainsaw at various speeds for the first hour to ensure the new coil is properly seated and functioning optimally.
• Documentation: Keep records of maintenance and parts replacements, including the date and part number. This can be helpful for future troubleshooting.

Important Considerations

• Safety First: Always disconnect the spark plug wire before working on the ignition system to prevent accidental shocks.
• Service Manual: Consult your Stihl MS250 service manual for specific instructions and torque specifications.
• Fuel Quality: While not directly related to the coil, using fresh, high-quality fuel is essential for proper engine operation. Stale fuel can cause starting problems and engine damage.
• Environmental Factors: Extreme heat or cold can affect coil performance. If you’re experiencing problems in extreme weather, try warming up the coil (in cold weather) or allowing it to cool down (in hot weather) before testing.

Conclusion

Troubleshooting a “no spark” issue on your Stihl MS250 can be frustrating, but with a systematic approach and these pro tips, you can diagnose and fix the problem yourself. Remember to start with the basics, check the simple things first, and don’t be afraid to get your hands dirty. With a little patience and persistence, you’ll have your chainsaw roaring back to life in no time. And who knows, you might even learn a thing or two along the way. I know I have, every single time I pick up a tool.

Specifications and Technical Requirements Recap

Here’s a summary of the key specifications and technical requirements mentioned in this guide:

Remember to always consult your Stihl MS250 service manual for the most accurate and up-to-date specifications.

Learn more