How do you fix a 4 stroke dirt bike run on 2 stroke fuel?

My neighbor has a 4 stroke Yamaha dirt bike. They didnt have any regular fuel, so they ran 2 stroke fuel with oil mixed in it. Now that they want to put regular fuel in, its tought to start and will run for a few seconds and then cuts off. I have taken the carb apart and cleaned it completely out. The only way to get it started is if I cover the carb from getting any air. Does anyone know other things I need to check or clean to get this dirt bike running right again?