nrg power revs keep dropping 2 7000rpm and wont move properly?

i have a 2005 nrg power dd top speed down hill is 63mph tested against a gsxr 600 and have a laser x pro exhaust , 4.5 grm rollers mallossi kevlar belt and it wont pull away and when i am moving as soon as i let off the throttle and get back on it revs drop from 9500rpm to 7000rpm and the bike slows to about 28mph from 55mph and refuses to rev untill i go down hills and also i can do 60mph onto a hill by the time i get to the top im doing 13mph and again revs drop to 7000rpm ive taken it to a garage and they tried 3grm rollers and made no diffrence and opened main jet from 60 to 63 and said it was plenty and went up hills ok but wouldnt pull away and when get off and on throttle revs drop stiill but after a 2 days it stopped working on hills again and its really annoying me they dont know what else to try and also is their a restricter in the carb as a mate of mine said theirs a metal plate with holes init is that true and is it the cause of all this YOUR HELP WILL BE APRECIATED THANKS