Stihl MS 250 Chainsaw Won't Start (Troubleshooting Tips For Arborists)

The Stihl MS 250 Chainsaw: A Love-Hate Relationship

It’s a paradox, isn’t it? The tool designed to conquer wood, rendered helpless, silent, and stubbornly refusing to roar to life. The Stihl MS 250. A workhorse for many, a source of frustration when it decides to become uncooperative. I’ve spent countless hours, literally years, in the woods, relying on chainsaws like the MS 250 to fell trees, limb branches, and prepare firewood. I’ve felt the elation of a perfectly executed cut and the simmering rage of a saw that just won’t start.

The user intent behind “Stihl MS 250 Chainsaw Won’t Start (Troubleshooting Tips for Arborists)” is clear: a frustrated user, likely an arborist, homeowner, or firewood enthusiast, is seeking immediate, practical solutions to get their chainsaw running again. They’re looking for a systematic approach to diagnose the problem and fix it, preferably without needing to take it to a repair shop. They need to get back to work.

This isn’t just about starting a chainsaw; it’s about getting back to the job, earning a living, or simply enjoying the satisfaction of a completed task. It’s about the frustration of a stalled project and the need for a reliable solution. So, let’s dive in. I’ll share my experiences, insights, and a step-by-step guide to get your Stihl MS 250 roaring again.

Diagnosing the Silent Beast: A Step-by-Step Guide to Starting Your Stihl MS 250

Before we even think about tools, let’s understand the core principles. A chainsaw needs three things to run: fuel, spark, and compression. If any of these are missing or insufficient, the engine won’t start. This troubleshooting guide is designed to methodically check each of these components.

1. Safety First: The Golden Rule

Before you touch anything, always ensure the chainsaw is switched off and the chain brake is engaged. Disconnect the spark plug wire to prevent accidental starting. Wear gloves to protect your hands. Remember, safety is paramount. I’ve seen too many accidents in the woods to take it lightly.

2. Fuel System Check: Is the Saw Getting Gas?

The fuel system is the most common culprit for starting problems. Let’s start with the basics.

2.1. Fuel Tank and Fuel Cap:

Check the Fuel Level: Obvious, but often overlooked. Make sure there’s sufficient fuel in the tank. I’ve wasted countless hours troubleshooting only to realize I was simply out of gas.
Fuel Cap Vent: The fuel cap has a vent that allows air to enter the tank as fuel is used. If this vent is clogged, a vacuum can form, preventing fuel from flowing. Loosen the fuel cap slightly. Try starting the saw. If it starts, the vent is likely the problem. You can try cleaning the vent with a small wire or replacing the fuel cap.
Fuel Type and Mix Ratio: Stihl MS 250 chainsaws require a specific fuel mixture: gasoline and 2-cycle engine oil. The recommended ratio is typically 50:1 (50 parts gasoline to 1 part oil). Using the wrong fuel mixture can lead to engine damage and starting problems. Always use high-quality, fresh gasoline with an octane rating of 89 or higher. I once used old, stale gasoline and it completely gummed up the carburetor. It was a costly mistake.

2.2. Fuel Lines and Fuel Filter:

Fuel Line Inspection: Check the fuel lines for cracks, kinks, or leaks. Over time, fuel lines can become brittle and deteriorate, especially with ethanol-blended fuels. If you find any damage, replace the fuel lines.
Fuel Filter: The fuel filter is located inside the fuel tank, attached to the fuel line. It prevents debris from entering the carburetor. A clogged fuel filter restricts fuel flow. To check the fuel filter, carefully remove it from the tank using a hooked wire or pliers. Inspect the filter for dirt and debris. If it’s dirty, replace it. Fuel filters are inexpensive and should be replaced annually as preventative maintenance.
My Experience: I once had a saw that would start, run for a few minutes, and then die. After tearing my hair out, I discovered the fuel filter was partially clogged. It allowed enough fuel for a short period, but then starved the engine.

2.3. Primer Bulb:

Primer Bulb Function: The primer bulb is a small, rubber bulb that you press to draw fuel from the tank to the carburetor, making starting easier, especially when the engine is cold.
Primer Bulb Inspection: Check the primer bulb for cracks or damage. If it’s cracked, it won’t create a vacuum. Press the primer bulb several times. You should see fuel flowing through the fuel lines. If you don’t see fuel, there’s a blockage in the fuel lines or the fuel filter is clogged.
Flooding: Over-priming can flood the engine, making it difficult to start. If you suspect flooding, remove the spark plug (see section 3) and pull the starter cord several times to clear excess fuel from the cylinder. Let it air out for a few minutes before replacing the spark plug.

2.4. Carburetor: The Heart of the Fuel System

The carburetor mixes air and fuel to create a combustible mixture. It’s a complex component, and carburetor problems are a common cause of starting issues.

Carburetor Adjustment: The carburetor has adjustment screws that control the fuel-air mixture. The Stihl MS 250 typically has three adjustment screws:
- LA (Low Adjustment): Controls the idle speed.
- H (High Adjustment): Controls the fuel mixture at high RPMs.
- L (Low Adjustment): Controls the fuel mixture at low RPMs.
- Factory Settings: The factory settings for these screws are a good starting point. Refer to your owner’s manual for the specific settings for your saw.
- Adjustment Procedure:
  1. Locate the Adjustment Screws: They are usually marked with “L,” “H,” and “LA” or similar designations.
  2. Turn Screws Gently: Use a small screwdriver to adjust the screws. Turn them gently, as forcing them can damage the carburetor.
  3. Start with Factory Settings: Begin by turning each screw to the factory setting (refer to your owner’s manual).
  4. Fine-Tune: Start the saw and let it warm up. Adjust the “L” screw to achieve a smooth idle. Then, adjust the “H” screw for optimal performance at high RPMs.
  5. Listen to the Engine: The engine should run smoothly without hesitation or stalling. If it hesitates or stalls, make small adjustments to the carburetor screws.
Carburetor Cleaning: Over time, the carburetor can become clogged with dirt, varnish, and fuel residue. This can restrict fuel flow and prevent the engine from starting.
- Disassembly: Carefully disassemble the carburetor, taking note of the location of each component. I recommend taking pictures as you disassemble it to aid in reassembly.
- Cleaning Solution: Use a carburetor cleaner to thoroughly clean all the parts. Pay special attention to the jets and passages.
- Compressed Air: Use compressed air to blow out any remaining debris.
- Reassembly: Reassemble the carburetor, ensuring all parts are properly seated.
Diaphragm and Gaskets: The carburetor contains diaphragms and gaskets that can become brittle and cracked over time. If these components are damaged, they can cause leaks and prevent the engine from starting. Replace the diaphragms and gaskets as needed. Carburetor rebuild kits are readily available and relatively inexpensive.
A Cautionary Tale: I once spent an entire weekend trying to start a chainsaw. I cleaned the carburetor, replaced the fuel lines, and checked everything else imaginable. Finally, I realized the tiny diaphragm in the carburetor was cracked. Replacing it solved the problem instantly.

3. Ignition System Check: Is There Spark?

If the fuel system is working properly, the next step is to check the ignition system. This system provides the spark that ignites the fuel-air mixture in the cylinder.

3.1. Spark Plug: The Source of the Spark

Spark Plug Inspection: Remove the spark plug and inspect it. Look for cracks, carbon buildup, or fouling. A fouled spark plug is often black and oily.
Spark Plug Cleaning: If the spark plug is fouled, clean it with a wire brush or spark plug cleaner.
Spark Plug Gap: The spark plug gap is the distance between the center electrode and the ground electrode. The correct gap for the Stihl MS 250 is typically 0.020 inches (0.5 mm). Use a spark plug gap tool to adjust the gap if necessary.
Spark Test: To test for spark, reconnect the spark plug wire to the spark plug. Hold the spark plug against the engine block (ground it). Pull the starter cord. You should see a strong, blue spark jumping across the spark plug gap. If there’s no spark, or the spark is weak and yellow, the spark plug is faulty or there’s a problem with the ignition system.
Spark Plug Replacement: Spark plugs are relatively inexpensive and should be replaced annually or more often if needed. Use the correct spark plug for your Stihl MS 250. Check your owner’s manual for the recommended spark plug type.
Personal Experience: I once had a saw that would start intermittently. It turned out the spark plug was cracked, causing it to short out occasionally. A new spark plug solved the problem immediately.

3.2. Ignition Coil: Generating the Spark

Ignition Coil Function: The ignition coil generates the high-voltage electricity needed to create the spark.
Ignition Coil Testing: Testing the ignition coil requires a multimeter. Set the multimeter to measure resistance (ohms). Disconnect the ignition coil from the spark plug wire and the engine ground. Measure the resistance between the spark plug wire terminal and the engine ground. Refer to your Stihl MS 250 service manual for the correct resistance values. If the resistance is outside the specified range, the ignition coil is faulty and needs to be replaced.
Air Gap: The ignition coil has an air gap between the coil and the flywheel. This air gap is critical for proper ignition. The correct air gap for the Stihl MS 250 is typically 0.010-0.014 inches (0.25-0.35 mm). Use a feeler gauge to adjust the air gap if necessary.
A Word of Caution: Working with the ignition system can be dangerous. Always disconnect the spark plug wire before working on the ignition system to prevent accidental shock.

3.3. Flywheel and Key: Timing is Everything

Flywheel Function: The flywheel is a rotating component that generates electricity for the ignition system.
Flywheel Key: The flywheel is attached to the crankshaft by a small key. If this key is sheared, the flywheel will be out of time, and the engine won’t start.
Flywheel Key Inspection: To inspect the flywheel key, remove the flywheel cover. Inspect the key for damage. If it’s sheared, replace it.
Flywheel Magnet: The flywheel has magnets that generate electricity for the ignition system. Check the magnets for damage or corrosion. If they’re damaged, the flywheel needs to be replaced.

4. Compression Check: Is the Engine Sealed?

If the fuel and ignition systems are working properly, the next step is to check the engine compression. Compression is the pressure created in the cylinder when the piston moves up. If the compression is too low, the engine won’t start.

4.1. Compression Tester: The Definitive Test

Compression Tester Use: A compression tester is a gauge that measures the pressure in the cylinder.
Testing Procedure:
1. Remove the spark plug.
2. Screw the compression tester into the spark plug hole.
3. Pull the starter cord several times.
4. Read the compression gauge.
Compression Values: The Stihl MS 250 should have a compression reading of at least 120 PSI (827 kPa). If the compression is below this value, there’s a problem with the cylinder, piston, or rings.
Wet Test: If the compression is low, perform a wet test. Add a small amount of oil to the cylinder through the spark plug hole. Repeat the compression test. If the compression increases significantly, the problem is likely with the piston rings. If the compression doesn’t increase, the problem is likely with the valves or cylinder head.

4.2. Cylinder and Piston: The Heart of the Engine

Cylinder Inspection: Inspect the cylinder for scratches, scoring, or damage. If the cylinder is damaged, it needs to be repaired or replaced.
Piston Inspection: Inspect the piston for wear, cracks, or damage. The piston rings should be free to move in their grooves. If the piston is damaged, it needs to be replaced.
Piston Rings: The piston rings seal the cylinder and prevent combustion gases from escaping. If the piston rings are worn or broken, they won’t seal properly, and the engine will lose compression.
A Costly Lesson: I once ignored a slight loss of compression in my chainsaw, thinking it wasn’t a big deal. Over time, the problem worsened, eventually leading to a completely seized engine. It was a costly repair that could have been avoided if I had addressed the problem earlier.

4.3. Decompression Valve: A Helping Hand

Some Stihl MS 250 models are equipped with a decompression valve. This valve releases some of the compression in the cylinder, making it easier to pull the starter cord.

Decompression Valve Inspection: Check the decompression valve for leaks or damage. If it’s leaking, it needs to be replaced.
Decompression Valve Operation: The decompression valve should be closed during normal operation. It should only open when the starter cord is pulled.

5. Exhaust System Check: Is the Engine Breathing?

A clogged exhaust system can restrict airflow and prevent the engine from starting.

5.1. Spark Arrestor Screen: A Fire Prevention Device

Spark Arrestor Function: The spark arrestor screen is a small screen located in the muffler. It prevents sparks from escaping the exhaust, reducing the risk of fire.
Spark Arrestor Cleaning: The spark arrestor screen can become clogged with carbon buildup, restricting airflow. Clean the screen with a wire brush.
Muffler Inspection: Inspect the muffler for damage or blockage. If the muffler is damaged, it needs to be replaced.

5.2. Exhaust Port: The Engine’s Exit

Exhaust Port Inspection: The exhaust port is the opening in the cylinder where exhaust gases exit the engine. This port can become clogged with carbon buildup.
Exhaust Port Cleaning: Clean the exhaust port with a scraper or wire brush. Be careful not to damage the cylinder.

6. Air Filter Check: Let the Engine Breathe

A clogged air filter restricts airflow to the carburetor, which can prevent the engine from starting.

6.1. Air Filter Inspection: Visual Check

Air Filter Type: The Stihl MS 250 typically uses a foam or paper air filter.
Inspection Procedure: Remove the air filter and inspect it for dirt and debris.
Cleaning:
- Foam Filter: Wash the foam air filter with soap and water. Allow it to dry completely before reinstalling it. Apply a small amount of air filter oil to the filter.
- Paper Filter: Tap the paper air filter to remove loose dirt and debris. Replace the paper air filter if it’s heavily soiled.

7. Other Potential Issues: The Devil is in the Details

7.1. Starter Mechanism: Pulling Power

Starter Cord Inspection: Check the starter cord for damage or fraying. If the cord is damaged, replace it.
Recoil Spring: The recoil spring returns the starter cord to its original position. If the recoil spring is broken, the starter cord won’t retract. Replace the recoil spring if necessary.
Pawls: The pawls engage the flywheel when the starter cord is pulled. If the pawls are worn or damaged, they won’t engage properly. Replace the pawls if necessary.

7.2. Choke Mechanism: Starting Assistance

Choke Function: The choke restricts airflow to the carburetor, creating a richer fuel-air mixture for starting.
Choke Inspection: Check the choke linkage for damage or binding. Make sure the choke plate is closing properly when the choke is engaged.

7.3. Electrical Connections: Ensuring a Circuit

Wiring Inspection: Check all electrical connections for corrosion or damage. Clean or repair any damaged connections.
Ground Connection: Ensure the engine is properly grounded. A poor ground connection can cause ignition problems.

Beyond the Basics: Strategic Insights for Chainsaw Longevity

Troubleshooting a chainsaw that won’t start is just one piece of the puzzle. Proper maintenance and preventative care are crucial for ensuring your Stihl MS 250 runs reliably for years to come.

1. Fuel Management: The Lifeline of Your Saw

Fuel Stabilizer: Add fuel stabilizer to your gasoline, especially if you’re not going to use the saw for an extended period. Fuel stabilizer prevents the gasoline from breaking down and forming varnish, which can clog the carburetor.
Empty the Fuel Tank: If you’re storing the saw for more than a month, empty the fuel tank and run the engine until it stalls to remove any remaining fuel from the carburetor.
Proper Fuel Storage: Store gasoline in a clean, approved container in a cool, dry place.

2. Sharpening Your Chain: Efficiency and Safety

Sharp Chain Benefits: A sharp chain cuts more efficiently, reduces strain on the engine, and improves safety.
Sharpening Frequency: Sharpen your chain regularly, ideally after every few tanks of fuel.
Sharpening Tools: Use a chainsaw file and guide to sharpen your chain. Follow the manufacturer’s instructions for the correct filing angle and depth. I prefer using a 2-in-1 filing guide for maintaining both the cutter and depth gauge at the same time.
Chain Tension: Maintain proper chain tension. A loose chain can derail and cause damage. A tight chain can overheat and wear prematurely.

3. Cleaning and Maintenance: Preventing Problems

Regular Cleaning: Clean your chainsaw regularly with a brush and compressed air. Remove sawdust and debris from the cylinder fins, air filter, and spark arrestor screen.
Lubrication: Lubricate the chain regularly with chain oil. Use a high-quality chain oil that is specifically designed for chainsaws.
Bar Maintenance: Clean the bar groove regularly to remove sawdust and debris. Check the bar for wear and damage. Flip the bar periodically to distribute wear evenly.

4. Understanding Wood: The Key to Efficient Cutting

Green Wood vs. Seasoned Wood: Green wood has a high moisture content, making it heavier and more difficult to cut. Seasoned wood has a lower moisture content and is easier to cut. I typically let hardwoods like oak and maple season for at least a year before burning.
Wood Species: Different wood species have different densities and cutting characteristics. Hardwoods are generally more difficult to cut than softwoods.
Cutting Techniques: Use proper cutting techniques to avoid kickback and other hazards. Always maintain a firm grip on the chainsaw and keep your body out of the cutting path.

5. Safety Gear: Protecting Yourself

Personal Protective Equipment (PPE): Always wear appropriate PPE when operating a chainsaw, including:
- Safety Glasses or Face Shield: Protect your eyes from flying debris.
- Hearing Protection: Chainsaws are loud and can damage your hearing.
- Gloves: Protect your hands from cuts and abrasions.
- Chaps: Protect your legs from chainsaw cuts.
- Steel-Toed Boots: Protect your feet from falling logs and other hazards.
- Helmet: Protect your head from falling branches and other objects.

Case Study: The Resurrected MS 250

I recently had a neighbor who was ready to throw out his Stihl MS 250. It hadn’t started in over a year, and he assumed it was beyond repair. I offered to take a look.

Gather Your Tools: Collect the necessary tools, including screwdrivers, wrenches, pliers, a spark plug wrench, a spark plug gap tool, a compression tester, and a multimeter.
Perform a Thorough Inspection: Inspect your chainsaw for any signs of damage or wear.
Follow the Troubleshooting Steps: Systematically follow the troubleshooting steps outlined in this guide.
Replace Worn Parts: Replace any worn or damaged parts.
Maintain Your Chainsaw: Implement a regular maintenance schedule to prevent future problems.
Practice Safe Operation: Always operate your chainsaw safely and wear appropriate PPE.

Conclusion: The Roar of Success

The Stihl MS 250 is a reliable and powerful chainsaw, but like any machine, it requires proper care and maintenance. By following the troubleshooting steps outlined in this guide, you can diagnose and fix most starting problems and keep your chainsaw running smoothly for years to come. Remember to prioritize safety, perform regular maintenance, and understand the principles of chainsaw operation. With a little patience and effort, you can conquer the silent beast and get back to the satisfaction of working with wood. The roar of a well-maintained chainsaw is a symphony to my ears, a testament to the power of human ingenuity and the enduring allure of working with nature. Now go forth and make some sawdust!