Before you disassemble the carburetors, verify that the air filter is not dirty and has good air flow.
It happened to me once, I oil the filter too much. I could accelerate the bike through all the gears being in the central support until 9000 rpm, but under load, riding, in 5th and 6th gears could not go beyond 5000 rpm.
I had to use a compressed air gun to remove excess oil and the change was drastic.
Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -Albert Einstein.
2007 Ninja 500R