Stihl MS250 No Spark: 7 Causes & Fixes (Pro Logger Checklist)

It’s happened to all of us. You’re ready to tackle that pile of firewood, the crisp air is calling, and then… nothing. You pull the starter cord on your Stihl MS250, and it’s just dead air. No sputter, no pop, just the frustrating silence of a chainsaw refusing to cooperate. And the first thought that flashes through your mind? “No spark. Seriously?”

I’ve been there countless times, wrestling with uncooperative machinery in the middle of the woods. Over the years, I’ve learned that a no-spark situation on a Stihl MS250, while infuriating, is often a problem you can diagnose and fix yourself. This guide is your pro logger checklist to troubleshoot and get your saw roaring again. I’ll walk you through the common culprits behind a lack of spark and provide step-by-step fixes based on my own experiences and the best practices I’ve picked up from seasoned pros.

Stihl MS250 No Spark: 7 Causes & Fixes (Pro Logger Checklist)

Before we dive in, remember safety first. Disconnect the spark plug wire before doing any work on the ignition system. And always refer to your Stihl MS250 owner’s manual for specific instructions and safety precautions.

1. The Humble Spark Plug: A Prime Suspect

The spark plug is the unsung hero of the internal combustion engine. It’s responsible for igniting the fuel-air mixture and starting the whole process. But it’s also a consumable item, prone to fouling, cracking, and simply wearing out.

Why it fails:

• Fouling: Excess oil, carbon buildup, or unburnt fuel can coat the spark plug electrode, preventing it from sparking. This is common with older saws or those used with incorrect fuel mixtures.
• Cracking: The ceramic insulator can crack due to age, impact, or extreme temperature changes. A cracked insulator allows the spark to jump to ground instead of across the electrode gap.
• Wear: Over time, the electrode erodes, increasing the gap and weakening the spark.

The Fix:

1. Inspection: Carefully remove the spark plug using the correct wrench. Inspect the electrode for fouling, cracks, or excessive wear. A healthy spark plug should have a light tan color.
2. Cleaning: If the plug is fouled, try cleaning it with a wire brush or spark plug cleaner. Be gentle to avoid damaging the electrode.
3. Gapping: Use a spark plug gapping tool to ensure the electrode gap is within the specified range. For the Stihl MS250, the recommended gap is typically 0.5 mm (0.020 inches). This is a critical measurement! Too large a gap, and the spark will be weak or nonexistent. Too small, and the combustion may be incomplete.
4. Testing: The best way to test a spark plug is with a spark plug tester. These are inexpensive and readily available. Connect the plug to the tester and crank the engine. A strong, consistent blue spark indicates a healthy plug.
5. Replacement: If the plug is cracked, heavily fouled, or fails the spark test, replace it with a new one. Use the correct spark plug for the Stihl MS250. Stihl recommends using a Bosch WSR6F or an NGK BPMR7A. Using the wrong plug can damage your engine.

My Experience: I remember one time, I was cutting firewood with a buddy in the Adirondacks. His MS250 suddenly died. We checked the fuel, the air filter, everything seemed fine. Then I pulled the spark plug, and it was completely black with carbon. He’d been using a cheap two-stroke oil and running the saw at low RPMs for too long. A quick spark plug replacement, and we were back in business. It taught me the importance of using quality oil and running the saw at full throttle periodically to burn off deposits.

2. The Spark Plug Wire and Boot: Connections Matter

The spark plug wire and boot connect the ignition coil to the spark plug. They are responsible for delivering the high-voltage spark to the plug.

Why it fails:

• Corrosion: Corrosion can build up on the terminals, preventing a good electrical connection.
• Damage: The wire can be damaged by heat, abrasion, or rodents. The boot can crack or tear, allowing moisture to enter.
• Loose Connection: The wire or boot may become loose at either the coil or spark plug end.

The Fix:

1. Visual Inspection: Carefully inspect the wire and boot for any signs of damage, corrosion, or looseness. Pay close attention to the connection points at the coil and spark plug.
2. Cleaning: If you find corrosion, clean the terminals with a wire brush or sandpaper. Apply a small amount of dielectric grease to the terminals to prevent future corrosion.
3. Testing for Continuity: Use a multimeter to test the continuity of the spark plug wire. Disconnect the wire from both the coil and the spark plug. Set the multimeter to the continuity setting and connect the probes to each end of the wire. The meter should show a reading close to zero ohms, indicating a good connection. If the meter shows an open circuit, the wire is broken and needs to be replaced.
4. Checking the Boot: Inspect the spark plug boot for cracks or tears. If the boot is damaged, replace it. You can often purchase replacement boots separately.
5. Secure Connections: Ensure that the wire and boot are securely connected to both the coil and the spark plug. A loose connection can cause intermittent spark or no spark at all.

Data Point: In my experience, a common issue is the spark plug boot becoming brittle and cracking, especially on older saws. This allows moisture to enter, leading to corrosion and a weak spark. Replacing the boot is a simple and inexpensive fix.

3. The Ignition Coil: The Spark Generator

The ignition coil is a critical component responsible for generating the high-voltage spark needed to ignite the fuel-air mixture. It transforms the low-voltage current from the engine’s magneto into a high-voltage pulse that travels to the spark plug.

Why it fails:

• Short Circuit: Internal short circuits can develop within the coil due to insulation breakdown or physical damage.
• Open Circuit: The coil windings can break, creating an open circuit and preventing the flow of electricity.
• Heat Damage: Excessive heat can damage the coil’s internal components, leading to failure.
• Air Gap Issues: The air gap between the coil and the flywheel is critical for proper operation. If the air gap is too large or too small, the coil may not generate sufficient voltage.

The Fix:

1. Visual Inspection: Inspect the coil for any signs of physical damage, such as cracks, burns, or melted plastic. Look for loose wires or corroded terminals.
2. Air Gap Adjustment: The air gap between the ignition coil and the flywheel is crucial. Refer to your Stihl MS250 service manual for the correct air gap specification. Typically, it’s around 0.3 mm (0.012 inches). I use a business card folded in half as a makeshift feeler gauge. Loosen the coil mounting screws, insert the feeler gauge between the coil and the flywheel, and tighten the screws.
3. Resistance Testing: Use a multimeter to measure the resistance of the primary and secondary windings of the ignition coil. Refer to your Stihl MS250 service manual for the correct resistance values. A reading outside of the specified range indicates a faulty coil.
4. Spark Test (with Caution): This test is a bit risky and should be performed with extreme caution. Disconnect the spark plug wire from the spark plug. Hold the end of the spark plug wire about 6 mm (1/4 inch) away from a metal part of the engine. Crank the engine. You should see a strong, blue spark jumping from the wire to the engine. If the spark is weak, yellow, or nonexistent, the coil is likely faulty. Be extremely careful not to touch the wire while cranking the engine, as you could receive a severe electric shock.
5. Replacement: If the coil fails any of the above tests, replace it with a new one. Use a genuine Stihl replacement coil or a high-quality aftermarket coil designed for the MS250.

Personal Insight: I once had a Stihl MS250 that would start fine when cold but would die after running for a few minutes. After troubleshooting everything else, I finally suspected the ignition coil. It turned out the coil was developing a short circuit when it got hot. Replacing the coil solved the problem. This taught me that sometimes a coil can be partially functional, making diagnosis more challenging.

4. The Flywheel: The Magneto’s Partner

The flywheel is a heavy rotating disc that is connected to the engine crankshaft. It contains magnets that interact with the ignition coil to generate the electrical current needed for the spark.

Why it fails:

• Magnet Weakness: Over time, the magnets on the flywheel can lose their magnetism, reducing the voltage generated by the coil.
• Damage: The flywheel can be damaged by impacts or corrosion, affecting its ability to generate a spark.
• Keyway Shear: The keyway that connects the flywheel to the crankshaft can shear, causing the flywheel to rotate out of time with the engine.

The Fix:

1. Visual Inspection: Inspect the flywheel for any signs of damage, such as cracks, dents, or corrosion. Check the magnets to ensure they are securely attached and not damaged.
2. Magnet Strength Test: You can test the strength of the magnets by holding a screwdriver near them. The screwdriver should be strongly attracted to the magnets. If the attraction is weak, the magnets may be losing their magnetism. This isn’t a definitive test, but it can give you an indication of the magnet’s condition.
3. Keyway Inspection: Remove the flywheel and inspect the keyway on both the flywheel and the crankshaft. If the keyway is sheared, the flywheel will need to be replaced. A sheared keyway will cause timing issues, preventing the engine from starting.
4. Flywheel Replacement: If the flywheel is damaged or the magnets are weak, replace it with a new one. Ensure that you use the correct flywheel for the Stihl MS250.

Case Study: I once worked on a Stihl MS250 that had a recurring no-spark issue. After replacing the coil, spark plug, and wiring, I finally discovered that the flywheel magnets were weak. The saw would start intermittently, but the spark was never strong enough for reliable operation. Replacing the flywheel solved the problem.

5. The Kill Switch: A Simple Overlook

The kill switch is a simple but crucial component that grounds the ignition system, preventing the engine from starting. A faulty kill switch can cause a no-spark condition even if all other components are working correctly.

Why it fails:

• Short Circuit: The kill switch can develop an internal short circuit, grounding the ignition system even when it is in the “run” position.
• Corrosion: Corrosion can build up on the switch terminals, creating a poor connection and preventing the engine from starting.
• Mechanical Failure: The switch mechanism can break or become stuck, preventing it from moving freely between the “run” and “stop” positions.

The Fix:

1. Visual Inspection: Inspect the kill switch for any signs of damage, corrosion, or loose wires. Ensure that the switch moves freely between the “run” and “stop” positions.
2. Continuity Testing: Use a multimeter to test the continuity of the kill switch. Disconnect the switch from the ignition coil. Set the multimeter to the continuity setting. With the switch in the “run” position, the meter should show an open circuit (no continuity). With the switch in the “stop” position, the meter should show a closed circuit (continuity). If the switch does not function as described, it is faulty and needs to be replaced.
3. Disconnect the Kill Switch (Temporary Fix): As a temporary troubleshooting step, you can disconnect the kill switch wire from the ignition coil. This will bypass the kill switch and allow you to start the engine if the switch is the problem. However, be aware that you will not be able to shut off the engine using the kill switch. You will need to choke the engine or remove the spark plug wire to stop it.
4. Replacement: If the kill switch is faulty, replace it with a new one. Ensure that you use the correct kill switch for the Stihl MS250.

Practical Tip: I’ve found that a common cause of kill switch failure is dirt and debris getting lodged in the switch mechanism. Cleaning the switch with compressed air or a small brush can often restore its functionality.

6. Wiring Issues: The Unseen Culprits

The wiring harness connects all the electrical components of the ignition system. Damaged or corroded wiring can cause a no-spark condition.

Why it fails:

• Damage: Wires can be damaged by heat, abrasion, or rodents.
• Corrosion: Corrosion can build up on the terminals, creating a poor connection.
• Loose Connections: Wires can become loose at the terminals, preventing a good electrical connection.

The Fix:

1. Visual Inspection: Carefully inspect all the wiring in the ignition system for any signs of damage, corrosion, or loose connections. Pay close attention to the wires connecting the ignition coil, kill switch, and spark plug.
2. Continuity Testing: Use a multimeter to test the continuity of each wire in the ignition system. Disconnect the wire from both ends. Set the multimeter to the continuity setting and connect the probes to each end of the wire. The meter should show a reading close to zero ohms, indicating a good connection. If the meter shows an open circuit, the wire is broken and needs to be replaced.
3. Cleaning: If you find corrosion, clean the terminals with a wire brush or sandpaper. Apply a small amount of dielectric grease to the terminals to prevent future corrosion.
4. Secure Connections: Ensure that all wires are securely connected to the terminals. Use crimp connectors or solder to make secure connections.
5. Wiring Harness Replacement: If the wiring harness is severely damaged or corroded, replace it with a new one.

Data Point: When dealing with older saws, the insulation on the wires can become brittle and crack, exposing the bare wire. This can lead to short circuits and a no-spark condition. Replacing the wiring harness is often the best solution in these cases.

7. Low Compression: The Underlying Problem

While not directly related to the ignition system, low compression can sometimes mimic a no-spark condition. If the engine doesn’t have enough compression, it may not be able to generate enough vacuum to draw fuel into the cylinder, resulting in a weak or nonexistent spark.

Why it fails:

• Worn Piston Rings: Worn piston rings allow combustion gases to leak past the piston, reducing compression.
• Damaged Cylinder: Scratches or damage to the cylinder wall can also cause compression loss.
• Leaking Valves: Leaking intake or exhaust valves can prevent the engine from building up sufficient compression.
• Decompression Valve Issues: The Stihl MS250 has a decompression valve to make starting easier. If this valve is stuck open or leaking, it can lower compression.

The Fix:

1. Compression Test: Use a compression tester to measure the engine’s compression. Remove the spark plug and screw the compression tester into the spark plug hole. Pull the starter cord several times while observing the gauge on the tester. A healthy Stihl MS250 should have a compression reading of at least 120 PSI (827 kPa). Anything significantly lower indicates a compression problem.
2. Visual Inspection (Cylinder and Piston): If the compression test reveals low compression, you’ll need to inspect the cylinder and piston for damage. Remove the muffler and spark plug. Shine a flashlight into the cylinder and look for scratches or scoring on the cylinder wall. You can also remove the cylinder head to inspect the piston rings and piston.
3. Valve Inspection: If you suspect leaking valves, you can perform a leak-down test. This involves pressurizing the cylinder with compressed air and listening for air leaks at the intake, exhaust, or crankcase.
4. Decompression Valve Check: Inspect the decompression valve for leaks or damage. You can try cleaning the valve or replacing it if necessary.
5. Engine Rebuild: If the compression loss is due to worn piston rings, a damaged cylinder, or leaking valves, you may need to rebuild the engine. This is a more complex repair that may require specialized tools and knowledge.

My Logging Experience: I’ve seen low compression issues more frequently in older saws that have been heavily used or poorly maintained. One time, I was helping a friend clear some land, and his MS250 had very low compression. We discovered that the piston rings were completely worn out. After replacing the rings, the saw ran like new. This experience highlighted the importance of regular maintenance, including checking the compression and replacing worn parts before they cause major problems.

Wood Processing and Firewood Preparation Specifications

As a bonus, let’s touch upon some key specifications related to wood processing and firewood preparation, as these tasks are often the reason we’re using our Stihl MS250s in the first place.

1. Wood Selection Criteria:

• Hardwoods vs. Softwoods: Hardwoods (e.g., oak, maple, ash) generally have a higher BTU (British Thermal Unit) content per cord than softwoods (e.g., pine, fir, spruce). This means they produce more heat when burned. For example, seasoned oak can yield around 24 million BTU per cord, while seasoned pine might only yield around 15 million BTU per cord.
• Moisture Content: The ideal moisture content for firewood is 20% or less. Wood with a higher moisture content will be difficult to light, produce less heat, and create more smoke. I use a wood moisture meter to check the moisture content of my firewood before burning it.
• Species Density: Denser woods burn longer and produce more heat. For instance, Ironwood (Ostrya virginiana) is known for its exceptional density and heat output, making it a top choice for firewood.
• Rot and Decay: Avoid wood that is rotten or decaying. This wood will not burn well and may contain fungi that can be harmful to your health.

2. Tool Calibration Standards (Chainsaw):

• Chain Tension: The chain tension should be adjusted so that you can pull the chain away from the bar about 3-6 mm (1/8-1/4 inch). Too loose, and the chain can derail. Too tight, and it can cause excessive wear on the bar and chain.
• Carburetor Adjustment: The carburetor should be adjusted to provide the correct fuel-air mixture for optimal performance. The Stihl MS250 typically has three adjustment screws: the high (H), low (L), and idle (LA) screws. Refer to your owner’s manual for the correct adjustment procedure. I typically use a tachometer to set the maximum RPM. For the MS250, it should be around 12,500-13,000 RPM.
• Chain Sharpness: A sharp chain is essential for efficient and safe cutting. Sharpen the chain regularly using a chainsaw file or a chain grinder. I prefer using a file for quick touch-ups in the field.

3. Safety Equipment Requirements:

• Helmet with Face Shield: Always wear a helmet with a face shield to protect your head and face from flying debris.
• Hearing Protection: Chainsaws are loud. Wear earplugs or earmuffs to protect your hearing. The Stihl MS250 typically produces noise levels around 101 dB(A), which can cause hearing damage over time.
• Chainsaw Chaps: Chainsaw chaps are essential for protecting your legs from accidental cuts.
• Gloves: Wear gloves to protect your hands from cuts and abrasions.
• Steel-Toed Boots: Wear steel-toed boots to protect your feet from falling logs and other hazards.

4. Firewood Dimensions and Cord Volumes:

• Firewood Length: The standard length for firewood is 16 inches (40.6 cm), but this can vary depending on the size of your wood stove or fireplace.
• Cord Volume: A standard cord of firewood is a stack that measures 4 feet high, 4 feet wide, and 8 feet long (128 cubic feet or 3.62 cubic meters). A face cord (also known as a rick or stove cord) is typically 4 feet high and 8 feet long, but the width can vary.
• Stacking: Stack firewood neatly to promote air circulation and drying. I prefer to stack my firewood in rows with gaps between the rows to allow for better airflow.

5. Wood Drying Tolerances:

• Air Drying: Air drying is the most common method for drying firewood. It typically takes 6-12 months for firewood to air dry to a moisture content of 20% or less, depending on the species of wood, the climate, and the stacking method.
• Kiln Drying: Kiln drying is a faster method for drying firewood. It typically takes 1-2 weeks to dry firewood to a moisture content of 20% or less in a kiln. Kiln-dried firewood is often more expensive than air-dried firewood.

Industry Standards and Regulations:

• OSHA (Occupational Safety and Health Administration): OSHA has specific regulations regarding logging and tree care operations. These regulations cover topics such as personal protective equipment, machine guarding, and safe work practices.
• ANSI (American National Standards Institute): ANSI develops voluntary consensus standards for a wide range of industries, including forestry. ANSI standards can provide guidance on safe chainsaw operation and other aspects of wood processing.
• Local Forestry Regulations: Be aware of any local forestry regulations regarding tree cutting and firewood collection. These regulations may vary depending on your location.

Final Thoughts

Troubleshooting a no-spark Stihl MS250 can be frustrating, but with a systematic approach and a little patience, you can usually diagnose and fix the problem yourself. Remember to prioritize safety, refer to your owner’s manual, and don’t be afraid to ask for help from experienced chainsaw mechanics or loggers. And always ensure you’re following best practices when processing wood, from selecting the right species to ensuring proper drying and safe handling. By following these guidelines, you can keep your Stihl MS250 running smoothly and efficiently for years to come. Now, get back out there and get those logs cut!

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