KZ 700 Motor Mounts?

I am getting alot of vibration running through my 84' kz700 above 4500 rpm (particularly with a passenger). The vbration is not unsual, it harmonic. I suspect the motor mounts may be "hard" given the age of the bike and may need to be replaced. Is this a valid assumption? Anyone have similar experience?

thanks

It's a good hypothesis. The rubber will harden up leaving you with "random" rpm zones of vibration - two identical bikes will buzz at different ranges. I have a GPz that's buzzy at 1500-3500 rpm, then smooths out nice at 4k +......never had my rubber engine mount plugs replaced, but I'm thinkin' it's time!