my stihl ms 250 chainsaw won't start when it's cold, what am i doing wrong?

Diagnosis

The primary cause of a cold-start failure on the STIHL MS 250 is typically "flooding" the engine or failing to properly engage the choke sequence. When the engine is cold, the fuel does not vaporize easily; the choke restricts airflow to create a rich fuel-to-air ratio. If you pull the starter cord too many times with the choke engaged, excess liquid gasoline pools in the combustion chamber, saturating the spark plug and preventing ignition. Another common culprit is a loss of prime in the fuel line or a clogged fuel filter. If the fuel isn't reaching the carburetor during the initial pulls, the engine will never fire regardless of the choke position. You can confirm a flooded engine if you smell strong gasoline around the cylinder head or if the spark plug is wet and dark when removed. If the engine is "dry" (no smell of gas), the issue is likely a fuel delivery blockage or a fouled plug.

How to Fix It

Follow this precise sequence to ensure a successful cold start on your MS 250:

1. Set the Master Control Lever (the choke lever) to the "Cold Start" position (full choke). This is the bottom-most position of the lever.
2. Ensure the chain brake is disengaged (pulled back toward the handle) to reduce drag on the engine.
3. Pull the starter rope firmly and quickly. Continue pulling until you hear the engine "burp" or attempt to fire for the first time. This usually takes 2 to 5 pulls. Stop immediately once you hear this change in sound; do not keep pulling in full choke.
4. Move the Master Control Lever up one notch to the "Half-Choke" position.
5. Pull the starter rope again until the engine starts and runs consistently.
6. Once the engine is running, move the lever to the "Run" position (top notch). This opens the choke fully.
7. Allow the saw to warm up for 30-60 seconds before applying the chain to the wood.

If That Didn't Work

• Clear a Flooded Engine: If you have pulled the cord 10+ times on choke and it won't fire, the engine is flooded. Move the Master Control Lever to the "Run" position (completely off choke). Hold the throttle trigger wide open and pull the starter rope 5 to 10 times. This forces air through the cylinder to clear out the excess fuel. Once it fires, let it idle before hitting full throttle.
• Inspect and Gap the Spark Plug: Remove the spark plug using a 7/8" socket. If it is black and carbon-fouled, replace it with a fresh NGK or Bosch plug. Ensure the electrode gap is set precisely to 0.5mm (0.020 inches). A gap that is too wide will prevent the spark from jumping in cold, dense air.
• Check the Fuel Quality: If the saw has sat for more than 30 days with ethanol-blended gas, the fuel may have phased or gummed up the carburetor jets. Drain the tank and refill with a high-quality 50:1 pre-mix fuel containing a fuel stabilizer.
• Verify the Spark Arrestor: Remove the small metal screen (spark arrestor) from the muffler exhaust. If it is clogged with carbon deposits, the engine cannot breathe, causing it to stall immediately after a cold start. Clean it with a torch or a small wire brush.

Heads Up

• Avoid using "cheap" pump gas; always use a dedicated 50:1 mix. Ethanol attracts moisture, which causes internal corrosion in the carburetor and makes cold starts nearly impossible in winter.
• Never start the saw while it is lying flat on its side, as this can cause oil to leak into the air filter or flood the cylinder.
• Keep your air filter clean. A clogged filter restricts airflow, making the engine run too rich and increasing the likelihood of flooding during the choke process.
• If you are starting the saw in temperatures below 32°F, consider using a winter-grade fuel mix or allowing the saw to sit in a garage for 15 minutes to take the edge off the freeze.