Bought from a local resident that said the bike was in great mechanical condition. After getting it home and using it the first day the problems started. The plug needs to be changed out every 2 to 3 hours. It is black and sooty but not oily "rich fuel mixture ???". Also using a quart of oil every 3 to 4 hours of use. Very little visible smoke out of the exhaust. It runs really well and engine is really quiet. No knocks or any odd noises. Please help with any and all suggestions. Thanks