Dunno if this is any help?http://www.therevcounter.co.uk/threads/21097-Engine-won-t-rev-over-6000-rpm-First-thoughts
Over fueling...Choke or float heights?
What exhaust do you have on your bike? Is it clear, check the baffle has not collapsed or something like that. Seems you have checked the inlet side of things, so it could be being restricted on the outlet side of things.