Either the float is not set to the right level or the seat could be worn out. I just had the same issue with my 250. The needle had some small garbage stuck to it and it kept the needle from fully seating. Pull the needle out and look at it in some good light and make sure it's clean and the tip is not cut up looking and it should still have the nice taper to it. If the needle is fine then the seat has something on the walls. Some seats are removable and can be easily cleaned out. If not then use a small plastic bristle brush or q-tip and clean the seat really good. Then with good light see if it is clean. The way to test it before putting the carb back in is to leave the bowl off the carb, hook the fuel line to the bike and over a bucket test it out by turning on the fuel and raise the float and see if it shuts off the flow of fuel. Keep working on it till your satisified with the results. You could even remove all the jet's, power jet, choke and what ever else you can remove and let the carb body soak in a can of carb cleaner over night and then blow it out with compressed air in the morning.