Dealing with a Stihl MS250 that refuses to start is a common frustration for both professionals and DIY enthusiasts. This reliable chainsaw, when functioning correctly, is a powerhouse tool for cutting firewood, felling trees, or managing land. However, when it fails to roar to life, the problem can stem from a variety of issues ranging from simple user error to complex mechanical failure. Understanding the systematic steps to diagnose the issue is the first critical move in restoring your tool to working order.
Initial Checks and Fuel Verification
Before diving into complex repairs, it is essential to verify the basics. Many starting issues are the direct result of empty tanks or stale fuel mixtures. A chainsaw requires a precise blend of gasoline and two-stroke oil to lubricate the internal components, and getting this ratio wrong can lead to poor performance or complete failure.
Examine the fuel level in the tank, ensuring there is enough to reach the fuel line.
Check the freshness of the fuel; old gasoline can develop a varnish that clogs the carburetor.
Verify the fuel mix ratio is correct, typically 40:1 or 50:1 depending on the model year.
Ensure the fuel tap is turned to the "ON" position and the filter is not submerged in sediment.
Investigating the Spark Plug
The spark plug is the ignition source, and if it is dirty or damaged, the engine will not fire. This component is often the first suspect when troubleshooting a no-start condition. A fouled plug can result from using the wrong oil, dirty air filters, or simply carbon buildup over time.
To inspect the plug, remove the spark plug cap and pull the plug out using a socket wrench. Examine the electrode for heavy carbon deposits, oil fouling, or cracks in the ceramic insulator. If the plug appears dirty, clean it carefully with a wire brush or replace it entirely. While the plug is out, you can check the gap with a feeler gauge to ensure it matches the specifications outlined in your user manual, as an incorrect gap will prevent a strong spark.
Air Filter and Intake System
Clogged Air Filter
A dirty air filter restricts the airflow necessary for combustion. Without the right mixture of air and fuel, the engine will sputter and fail to start. This is one of the easiest issues to address and should be checked regularly as part of routine maintenance.
Locate the air filter cover, usually on the side of the saw, and remove the filter. If it is coated in dust or oily residue, it needs cleaning. Paper filters should be replaced, while foam filters can be washed with soap and water, dried thoroughly, and then lightly oiled before reinstallation.
Fuel Line Blockage
Fuel travels from the tank to the carburetor through a small line. Over time, this line can become brittle, crack, or get clogged with debris from old gasoline. If the line is pinched or broken, fuel cannot reach the engine, resulting in a dry start.
Disconnect the line from the carburetor and blow through it to check for blockages. If the line is stiff or cracked, replace it immediately to ensure a consistent fuel supply.
Carburetor and Fuel System
The carburetor is the heart of the fuel delivery system, mixing air and fuel in the correct ratio for combustion. If the Stihl MS250 has been sitting unused for a while, the fuel may have evaporated, leaving behind thick residue that clogs the jets.
Attempting to start the saw with the choke engaged indefinitely can flood the engine. If you smell gasoline when pulling the starter, the saw is flooded. In this state, you should lay the saw on its side without the air filter cover and pull the starter cord several times to expel the excess fuel. Reset the choke to the "Run" position before trying again.
