Originally Posted by 2door
There's your problem. If you are using any kind of rubber or resilient material between the engine and the frame you are increasing vibrations, not decreasing them. This explains why you can't hold onto your handlebars and probably why you keep breaking fuel tanks.
It has been proven time and again that you can't mount the engine in rubber and expect not to get excessive vibration. Get rid of your "dampeners" and mount the engine as solid to the bike frame as possible. I won't get any argument from any experienced builder here.
Read this. It is the answer.