We have been there and done that...
Good spark plug pics to compare with here...
http://www.peachparts.com/shopforum/showthread.php?t=168126&highlight=spark+plug+horror
We did the valve stem seals and it cured this problem instantly.... smog is another issue, but definatly start with these oil seals..
tech article here...
http://www.peachparts.com/Wikka/M103ValveSeals