How big of a motor do you have, with what carbs? What fuel pump/s? Sounds like you are starving yourself for fuel on the top end...
I would recommend adding another fuel pump to your system, either mechanical or an electric depending on what you currently have. You will need to know if you are running a low or high press system (do you have a regulator) so you can match to what you have. Basicly, you are wanting to keep the same pressure but add more flow.
Give us a call and we would be happy to explain how to plumb in another to your system..