Rather than guess,I'd get a service manual & adjust things to spec.That is After you get a replacement drive (rubber lined) disk.Also that age--I'd say it's the old style clutch.Either way-you never put oil of any kind on said wheel,causes it to really slip-think about it.And yes it's possible your eng.is running at a lower rpm,but you should be able to hear that from how it ran before.If not,,get a "tiny-tact"--easiest way to set rpm.BTW,eng.is normally set @ 3200-3600 rpm at wot.