my stihl ms 250 chainsaw won't start, pulled the cord like 20 times and nothing. what's wrong?

The engine is flooded or the ignition system is blocked. Check three things, in this order.

1. Set the master control lever to the STOP position. Pull the starter rope 5-6 times with the throttle trigger locked. This clears a flooded engine.
2. Move the master control lever to the cold start position (choke). Engage the chain brake for safety. Pull the starter rope until the engine fires, then immediately move the master control lever to the warm start/half-throttle position.
3. Pull again to start. If it still won't start, the spark plug is likely fouled. Remove it, clean the electrode, check the gap, and try again.

If that didn't work, the fuel in the tank is old or the fuel filter in the tank is clogged. Drain and replace with fresh, properly mixed fuel. A clogged air filter will also prevent starting—clean it.

Heads up: Always use the chain brake when starting. Never drop-start the saw. If you've gone through these steps and it still won't fire, the carburetor may need service.