Motorcycle Repair: vibration, honda cr250, wheels tires

Question
my mate owns a honda cr250 motocross(2002 model)...he,s getting severe vibration through the handlebars...any ideas what could cause this.

Answer
take off the chain, and run it see if it vibrates from engine alone or if putting it in gears causes it. there may be a counter balance assy out of time, or bad bearings in the power train. if its smooth in all ranges with the chain off look into the wheels, tires.