there've been some good plug threads in the past if you can retrieve them with the general concensus that quality of most of em has deteriorated (like most else). As said I'd figure out if it is gas or oil fouling & if gas then check the choke and or the AF ratio. Timing in the ballpark. For oil, new valve stem seals (if that is it) or maybe a rebuild (read "breakin secrets" at www.mototuneusa.com