stihl ms 250 runs fine then suddenly bogs down and dies in cold weather, like below freezing.

This is carburetor icing. Moisture in the intake air freezes on the carburetor throttle plate, blocking fuel flow.

The MS 250 has a winter operation mode to prevent this. For temperatures below +10°C (50°F), you need to open the intake air shutter.

1. Remove the carburetor box cover (the plastic shroud near the spark plug).
2. Lift out the shutter vertically. It's a small metal or plastic plate in front of the spark plug.
3. Turn the shutter 180 degrees.
4. Refit the shutter and reinstall the carburetor box cover, securing it with the knob.

This allows the engine to draw in heated air from around the cylinder to warm the carburetor.

If that didn't work: Ensure you are using fresh fuel mix. Old gas absorbs water, which exacerbates icing. Also, check that the air filter is clean and dry. A saturated filter can introduce more moisture.

Heads up: This setting is for cold weather only. At temperatures above +20°C (68°F), you must close the shutter again. Running with the winter shutter open in warm weather causes engine running problems and overheating.