Astro boy.

Its not totally a standard 1200.
Its a 1200GX.Standard GX engine.
They can rev to 6700 but max power is at 6400.
And yes, I can get 6000rpm in 1st,second and third gear.
Third gear just takes a while, but it pulls all the way to 6000 never stopping or showing that it cannot go anymore.
It is just this vibration that I need to sort out. My max power of 83hp is at 6400rpm.
I must get 6400rpm if it can go to 6400.
My flywheel was torqued by the guy.
If the flywheel is'nt loose, can it be that it is out of balance?

I thought of something else.
That sound I talked about at 5500 to 5700rpm.
Can it be the timing chain being worn and rattling at that high revs?
Thanks for all the replies.