Chainsaw Not Starting Stihl S250 (Troubleshoot Pro Tips)

Let’s bust a myth right off the bat: a chainsaw that won’t start is not always a sign of impending doom or a costly repair. More often than not, it’s a simple fix, a matter of understanding your machine and knowing where to look. I’ve spent years in the woods, from felling timber in the Pacific Northwest to prepping firewood in the crisp Vermont air, and I can tell you, I’ve seen it all – and fixed it all. The Stihl MS 250 is a workhorse, a reliable saw, but even the best machines need a little TLC. This guide is designed to walk you through the troubleshooting process, step by step, so you can get your MS 250 roaring back to life.

Understanding the Importance of a Properly Functioning Chainsaw

In the world of wood processing, logging, and firewood preparation, a reliable chainsaw is paramount. It’s not just about convenience; it’s about safety, efficiency, and productivity. A chainsaw that consistently fails to start wastes valuable time, increases physical exertion, and can create hazardous situations. For example, imagine being halfway through felling a tree and your saw dies – you’re left with an unstable situation requiring immediate attention.

From my experience, a well-maintained chainsaw reduces the risk of kickback, ensures cleaner cuts, and minimizes the strain on your body. Whether you’re a professional logger or a homeowner preparing firewood for the winter, understanding how to troubleshoot your chainsaw is an essential skill.

Key Terms and Concepts

Before diving into the troubleshooting steps, let’s define some key terms that will be used throughout this guide:

• Air Filter: A component that prevents dust and debris from entering the engine.
• Spark Plug: An electrical device that ignites the fuel-air mixture in the engine cylinder.
• Carburetor: A device that mixes air and fuel to create a combustible mixture.
• Fuel Filter: A filter that prevents impurities in the fuel from entering the carburetor.
• Primer Bulb: A small bulb that, when pressed, draws fuel into the carburetor to aid in starting.
• Choke: A valve that restricts airflow to the engine, creating a richer fuel-air mixture for starting.
• Compression: The process of compressing the fuel-air mixture in the cylinder before ignition.
• Green Wood: Freshly cut wood with high moisture content.
• Seasoned Wood: Wood that has been dried to reduce moisture content, making it easier to burn. A good target moisture content for firewood is below 20%.

The Four Basic Requirements for an Engine to Run

To understand why your chainsaw won’t start, it’s crucial to grasp the four fundamental requirements for an engine to run:

1. Fuel: The engine needs a proper mixture of fuel and air.
2. Air: The engine needs clean air to mix with the fuel.
3. Spark: The spark plug must generate a spark to ignite the fuel-air mixture.
4. Compression: The engine needs sufficient compression in the cylinder to create a powerful combustion.

If any of these four elements are missing or inadequate, the engine will fail to start. The troubleshooting steps below are designed to systematically check each of these requirements.

Step-by-Step Troubleshooting Guide for a Non-Starting Stihl MS 250

I’ve organized this guide to follow a logical process, starting with the simplest and most common causes and progressing to more complex issues. Remember to always prioritize safety and disconnect the spark plug wire before performing any maintenance or repairs.

Step 1: Check the Fuel

The most common reason a chainsaw won’t start is a fuel-related issue.

• Is there fuel in the tank? This seems obvious, but it’s easily overlooked. Ensure the fuel tank is adequately filled with the correct fuel mixture. The Stihl MS 250 typically requires a 50:1 mixture of gasoline and two-stroke oil. Using the wrong fuel mixture can cause significant engine damage. I once had a neighbor who consistently used straight gasoline in his two-stroke equipment. The result was seized engines and a lot of frustration.
• Is the fuel fresh? Old or stale fuel can lose its combustibility. Gasoline can degrade over time, especially if stored improperly. If the fuel has been sitting in the tank for more than a month, it’s best to drain it and replace it with fresh fuel. I recommend using a fuel stabilizer, especially if you don’t use your chainsaw frequently. This will help prevent the fuel from degrading.
• Check the fuel filter: The fuel filter is located inside the fuel tank, attached to the end of the fuel line. A clogged fuel filter can restrict fuel flow to the carburetor.
  • Procedure: Use a small hook or bent wire to carefully pull the fuel filter out of the tank. Inspect the filter for dirt and debris. If it’s clogged, replace it with a new one. I usually keep a few spare fuel filters on hand, as they’re inexpensive and easy to replace. The Stihl part number for the fuel filter is typically 0000-989-0516.
  • Pro Tip: When replacing the fuel filter, make sure the fuel line is securely attached to the filter. A loose connection can cause air leaks and prevent the engine from starting.
• Check the fuel line: Inspect the fuel line for cracks, leaks, or kinks. A damaged fuel line can prevent fuel from reaching the carburetor. If you find any damage, replace the fuel line.

Step 2: Check the Spark Plug

A faulty spark plug is another common cause of starting problems.

• Remove the spark plug: Locate the spark plug (usually under a rubber boot) and use a spark plug wrench to remove it.
• Inspect the spark plug: Examine the spark plug for signs of damage, such as cracks, chips, or excessive carbon buildup. A healthy spark plug should have a clean, dry electrode.
• Clean the spark plug: If the spark plug is dirty, clean it with a wire brush or spark plug cleaner. You can also use a small amount of gasoline to clean the electrode.
• Check the spark gap: The spark gap is the distance between the electrode and the ground strap. The correct spark gap for the Stihl MS 250 is typically 0.5 mm (0.020 inches). Use a spark plug gap tool to ensure the gap is correct.
• Test for spark: Reconnect the spark plug to the spark plug wire. Hold the spark plug against the engine block (making sure the metal part of the plug is touching metal on the engine). Pull the starter cord. You should see a bright blue spark jumping across the spark gap. If you don’t see a spark, the spark plug is likely faulty and needs to be replaced. It could also indicate a problem with the ignition coil.
• Replace the spark plug: If the spark plug is damaged or you’re not getting a spark, replace it with a new one. The recommended spark plug for the Stihl MS 250 is a NGK BPMR7A or a Bosch WSR6F.

Step 3: Check the Air Filter

A dirty or clogged air filter can restrict airflow to the engine, preventing it from starting.

• Locate the air filter: The air filter is usually located under a cover on top of the engine.
• Remove the air filter: Remove the air filter cover and take out the air filter.
• Inspect the air filter: Examine the air filter for dirt, dust, and debris. A dirty air filter will restrict airflow to the engine.
• Clean the air filter: If the air filter is dirty, clean it with compressed air or warm, soapy water. Allow the air filter to dry completely before reinstalling it. I prefer to have a spare air filter so I can swap it out while the other one is drying.
• Replace the air filter: If the air filter is heavily soiled or damaged, replace it with a new one. A clean air filter is essential for proper engine performance.

Step 4: Check the Carburetor

The carburetor mixes air and fuel to create the combustible mixture that powers the engine. A malfunctioning carburetor can cause starting problems. This is where things get a bit more complex, so take your time.

• Check the choke: Ensure the choke lever is in the correct position for starting. When the engine is cold, the choke should be fully engaged (closed). Once the engine starts, gradually open the choke to allow more air into the mixture.
• Check the primer bulb: Press the primer bulb several times to draw fuel into the carburetor. You should see fuel flowing through the clear fuel line. If the primer bulb is cracked or damaged, replace it.
• Carburetor adjustment: The carburetor has adjustment screws that control the fuel-air mixture. If the carburetor is not properly adjusted, the engine may not start.
  • Locate the adjustment screws: The adjustment screws are typically labeled “H” (high speed), “L” (low speed), and “LA” (idle speed).
  • Initial settings: As a starting point, turn both the “H” and “L” screws clockwise until they are lightly seated. Then, turn them counterclockwise 1 to 1.5 turns.
  • Starting the engine: Try starting the engine with these initial settings. If it doesn’t start, adjust the “L” screw slightly (1/8 turn) in either direction and try again.
  • Fine-tuning: Once the engine starts, you can fine-tune the carburetor adjustments to achieve optimal performance. This requires some experience and a good ear for engine sound. I recommend consulting a qualified mechanic if you’re not comfortable adjusting the carburetor yourself.
• Carburetor cleaning: If the carburetor is dirty or clogged, it may need to be cleaned.
  • Remove the carburetor: Carefully remove the carburetor from the engine, following the manufacturer’s instructions.
  • Disassemble the carburetor: Disassemble the carburetor, taking note of the location of all the parts.
  • Clean the carburetor: Clean all the carburetor parts with carburetor cleaner. Use compressed air to blow out any remaining debris.
  • Reassemble the carburetor: Reassemble the carburetor, making sure all the parts are in the correct location.
  • Reinstall the carburetor: Reinstall the carburetor on the engine.

Important Note: Carburetor cleaning and adjustment can be complex and require specialized tools and knowledge. If you’re not comfortable performing these tasks, it’s best to take your chainsaw to a qualified mechanic.

Step 5: Check Compression

If you’ve checked the fuel, spark, and air, and the chainsaw still won’t start, the problem may be a lack of compression. Compression is essential for creating the pressure needed for combustion.

• Compression test: A compression test measures the amount of pressure in the engine cylinder. This requires a compression tester, which can be purchased at most auto parts stores.
  • Procedure: Remove the spark plug and screw the compression tester into the spark plug hole. Pull the starter cord several times and observe the reading on the compression tester.
  • Interpretation: A healthy engine should have a compression reading of at least 100 PSI (pounds per square inch). If the compression is below this level, it indicates a problem with the piston rings, cylinder, or valves.
• Possible causes of low compression:
  • Worn piston rings: Piston rings seal the gap between the piston and the cylinder wall. Over time, they can wear down, allowing compression to escape.
  • Damaged cylinder: The cylinder wall can be damaged by scoring or scratching, which can also lead to compression loss.
  • Leaky valves: If the valves are not sealing properly, compression can escape through the valve seats.
• Remedies: Addressing low compression typically requires significant engine repairs, such as replacing the piston rings, cylinder, or valves. These repairs are best left to a qualified mechanic.

Step 6: Check the Ignition Coil

The ignition coil is responsible for generating the high-voltage spark that ignites the fuel-air mixture. A faulty ignition coil can prevent the engine from starting.

• Testing the ignition coil: Testing the ignition coil requires a multimeter.
  • Procedure: Disconnect the spark plug wire from the spark plug. Set the multimeter to measure resistance (ohms). Connect one probe of the multimeter to the spark plug wire terminal on the ignition coil and the other probe to the engine ground.
  • Interpretation: The resistance reading should fall within the manufacturer’s specifications. If the resistance is outside of the specified range, the ignition coil is likely faulty and needs to be replaced. Consult the Stihl MS 250 service manual for the correct resistance values.
• Replacing the ignition coil: Replacing the ignition coil involves removing the old coil and installing a new one. Make sure to properly gap the new ignition coil to the flywheel. The correct air gap is typically around 0.3 mm (0.012 inches).

Step 7: Check the Flywheel Key

The flywheel key is a small metal piece that connects the flywheel to the crankshaft. If the flywheel key is sheared or damaged, it can cause the ignition timing to be off, preventing the engine from starting.

• Locating the flywheel key: The flywheel key is located under the flywheel.
• Removing the flywheel: Removing the flywheel requires a special flywheel puller tool.
• Inspecting the flywheel key: Once the flywheel is removed, inspect the flywheel key for damage. If it’s sheared or broken, replace it with a new one.
• Reinstalling the flywheel: Reinstall the flywheel, making sure the new flywheel key is properly seated.

Step 8: Check for a Blocked Exhaust

A blocked exhaust can prevent the engine from starting by restricting the flow of exhaust gases. This is less common, but still worth checking.

• Remove the spark arrestor screen: The spark arrestor screen is a small screen located in the exhaust outlet. It’s designed to prevent sparks from escaping the exhaust.
• Inspect the spark arrestor screen: Inspect the spark arrestor screen for carbon buildup or damage.
• Clean the spark arrestor screen: Clean the spark arrestor screen with a wire brush or carburetor cleaner.
• Check the exhaust port: Look into the exhaust port on the cylinder. Is it blocked by carbon build up? Remove carbon gently.
• Reinstall the spark arrestor screen: Reinstall the spark arrestor screen.

Preventative Maintenance: Keeping Your Stihl MS 250 Running Smoothly

Prevention is always better than cure. Regular maintenance can significantly reduce the likelihood of starting problems and extend the life of your chainsaw. Here are some essential maintenance tips:

• Use fresh fuel: Always use fresh fuel and a fuel stabilizer.
• Clean the air filter regularly: Clean the air filter after each use or at least every 5 hours of operation.
• Inspect the spark plug regularly: Inspect the spark plug every 25 hours of operation and replace it as needed.
• Sharpen the chain regularly: A sharp chain reduces strain on the engine and makes cutting easier.
• Lubricate the chain regularly: Use chain oil to keep the chain lubricated and prevent wear.
• Store the chainsaw properly: Store the chainsaw in a dry place, away from direct sunlight and extreme temperatures. Drain the fuel tank before storing the chainsaw for extended periods.

Case Study: Reviving a Neglected Stihl MS 250

I once acquired a used Stihl MS 250 that had been sitting in a shed for several years. The owner said it hadn’t started in ages and was ready to give up on it. I took it on as a challenge.

Diagnosis:

• The fuel tank was full of old, gummy fuel.
• The fuel filter was completely clogged.
• The spark plug was fouled with carbon buildup.
• The air filter was packed with dust and debris.

Solution:

1. I drained the old fuel and replaced it with fresh fuel and a fuel stabilizer.
2. I replaced the fuel filter with a new one.
3. I cleaned the spark plug with a wire brush and adjusted the spark gap.
4. I cleaned the air filter with compressed air.
5. I primed the carburetor and pulled the starter cord.

To my surprise, the chainsaw sputtered to life on the second pull! After a few minutes of fine-tuning the carburetor, it was running smoothly and powerfully. This experience reinforced the importance of basic maintenance and the fact that even a neglected chainsaw can often be revived with a little effort.

Strategic Advantages of Proper Chainsaw Maintenance

Beyond simply preventing starting problems, proper chainsaw maintenance offers several strategic advantages:

Here are some other essential considerations:

• Wood selection: Choose the right type of wood for your needs. Hardwoods like oak and maple are ideal for firewood, while softwoods like pine and fir are better suited for construction.
• Felling techniques: Learn safe and efficient felling techniques to minimize the risk of accidents.
• Debarking logs: Debarking logs can help speed up the drying process and reduce the risk of insect infestation. I often use a drawknife for debarking smaller logs.
• Splitting firewood: Splitting firewood can be done manually with an axe or mechanically with a log splitter. Hydraulic log splitters are much faster and easier to use, especially for large quantities of wood. I use a 27-ton hydraulic splitter for most of my firewood processing.
• Stacking firewood: Stack firewood in a way that promotes air circulation and allows it to dry quickly. I prefer to stack firewood in rows, with the bark side facing up.
• Drying methods: Allow firewood to dry for at least six months before burning it. The ideal moisture content for firewood is below 20%. You can use a moisture meter to check the moisture content of your firewood.
• Safety gear: Always wear appropriate safety gear when working with chainsaws and other wood processing equipment. This includes safety glasses, hearing protection, gloves, and steel-toed boots.

Costs, Material Specs, Timing Estimates, and Skill Levels

Here’s a breakdown of some relevant technical details:

If you encounter any difficulties or are unsure about a particular step, don’t hesitate to consult a qualified mechanic. Remember to always prioritize safety and wear appropriate safety gear when working with chainsaws and other wood processing equipment.

With a little patience and effort, you can keep your Stihl MS 250 running smoothly and efficiently for years to come. And that, my friend, is a skill worth having when the woodpile is calling your name.

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