after rebuilding top end on my yz250 it was idling and just died and now it wont start any idea's

Little more help here. Idling right after you rebuilt it, ran it for a awhile before it died?

Standard question ------ Did you clean your carb (main, pilot, etc jets) after the rebuild? Fresh gas (mixed)?
 
Tell me if the plug looks wet or dry to begin with. Along the lines of what CD says, how long did it sit? Sounds very much like a fouled plug, happens frequently when someone rebuilds, probably puts way more oil on everything than they should, and lets it idle.
 
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