my son has an 06 vulcan clssic 900. got in december 06 and weve put about 5000 miles on it. the backfiring problem led us to check the air filter, and when we removed the cover we poured out about two tablespoons of oil. kawasaki told us that the oil level window on the side of the engine is in the wrong position (manufacturing defect) so when filled to the level of the window the engine is overfilled and pukes the excess oil into the breather. we were told to fill the engine withe the amount specified in the owners manuel and ignore the site window. backfiring stopped. we love the bike, but i am glad we bought the extended warrenty, cause if i am gonna run the oil level low like they told me to, we got 4 years for it to blow up and they replace it