I bought a 10k miles 640E with a BST 40 carb that ran fine. I took the carb apart and found the needle was almost worn through so I replaced it with new plastic and metal washers. Bike runs fine except at idle will stall and occasionally die (when stopped at lioght, etc). I messed with the fuel screw with little change. I did notice when I turned the screw all the way in the bike didn't die like I thought it should. Any Ideas? Want to get it ready to sell.