The same thing happened to my dads 1979 Stihl 028WB. As Dave Fix stated, the Nylon gears behind the chain drive clutch under the plastic cover have probably stripped. I couldn't find a replacement for this about 10 years ago, however Oregon made an upgrade/conversion kit. This replaced the oiler, oiler worm gear drive star and chain drive star. Gave it the full floating chain drive.
Luggnutt
February 2012
inside the oil tank there is a oil line with a pick up screen on the end could be blocked,if not under the sprocket drum and clutch( left hand thread )you will have to take cover off first plus need circlip pliers is two nylon gears that wear out might have trouble getting spares
Dave Fix
May 2011
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