Am stripping down my new 98 WR400F to clean her up etc.. I know the jetting isnt right just yet, but when I removed the spark plug I expected light brown, lean running signs as the bike pops and splutters on full throttle (the T/Stop pin is the right length!) But I found the plug to be black and very slightly oily !! I guess this means a head rebuild (piston/rings) or could this be just down to valve clearences ? or perhaps I'm completely wrong and it's down to jetting ?