Ok. Not really an into as such :-).
You say fully rebuilt. By who, when ? Are the important questions.
Follow that with whats your skill level with the spanners. And is this your first bike.
Gut feeling is carb. If that pv is pegged wide open it should be horrible to ride. But if someone has it backwards or its still on the stock 200main it will run out of puff at around 7k.
More than one small issue spoiling a good bike i think. Be interesting to know what main jet is in there and what pipe it has.
Pics would be even better....
Oh and welcome...