I bought a used 2006 yz250f in great condition this past winter.  it has an aftermarket air/fuel mixture screw. the bike started to run funny when it started to warm up outside and so i adjusted the screw and now the bike will not start AT ALL.  I do not know the factory settings and am not very experienced with working on bikes.  any suggestions?

  Thanks, Drew

Gently close the screw. Thern open it 1.75 turns.

You may have crap in the carb. Remove the cap on the bottom and see if there is any garbage sitting in the bottom of it. If so, the carb needs to be cleaned.