Please help. 01 Yamaha Beartracker starts and runs but rough idle and no steady rpm's while running. Set valves, no air leaks that I can find. Replaced carb with known good carb from another machine. It will start but when you give it gas with new carb it backfires thru carb. does not do this with orrig. carb. What in the world could be the prob? The machine i took the carb from runs like its brand new.