There's been quite a few complaints regarding this saw not starting.  My father has the same saw and we've found ourselves pulling so much at times we can't pull anymore.  From what I've read the main thing with this saw is to make sure when you have it on full choke (starting cold) to never pull more than two or three times otherwise you'll flood it and it will never start.  It should "pop" or "fire" on the second or third pull on full choke.  If it doesn't after two or three pulls on "full choke", move the selector to the "fast idle" position and try to start it.  It should fire right up after a few pulls and from what I've read there should be no chance of it flooding in the "fast idle" position.  Since I learned the above I've had no problems starting my dad's MS250.  Good luck!