do you use the choke?
when its cold (i.e. you havent tried kick starting it) try taking the air cleaner off, and squirting a little gas into the carb, then try kick starting it. if it starts and dies about 30 seconds later, you have a fuel delivery problem, (like it wont prime) if this is the case, but after it runs for a while i would imagine that it is part of the fuel petcock . you might need to take it off (plug the fuel lines so they dont run all your gas out of the tank) and take the petcock apart and clean it really well. while you are at it, take the carb off and clean it real well....its too simple of a thing not to do.
hope that helps