I would say that you should replace the float needle sets,which include the float needle,seat,etc. Mikesx has a good kit which includes all you should need except elbow grease,cleaner,etc. I went through the same thing with both bikes I have,leaking,cleaned seat with Q-tips and acetone,and replaced,it ran well for a while,then started flowing again.Put in new kit,and no more probs with overflow. Don't be frazzled when you read the part about fits 650,they use the same carbs as 400s. If you have an air compressor,blow all the passages until they are clear. lha