I have a new star 250 it runs and idles for about 20 minutes then loses spark what could the problem be??

 

First, are you absolutely sure you have lost the spark? If so, it is most likely a spark plug or stator problem, as they get hot quickest. Try a new plug. If that doesn't cure it, do the tests in the "help diagnose stator" sticky in the engine problems section of this site.