59 plate YBR 125cc

Hi everyone im stil new to this site and i duno if ive posted this in the right place (:

Right i have a YBR125cc 59 plate havent realy done much to it apart from rim tape and stickers i was going to leave it as that but i wanted it to sound a little bit deeper with a nice tone i custom made and exhaust for mine mainly for sound but now im looking at the power side of things the exhaust is pritty much hollow so its letting out enough air and i took the air filter and the cover of the air box for a while to drive about and see if that helps and it didnt realy just badly smelt of petrol (rich) so ive stuck it back on but i have brought a fancy air filter and removed the air box competly and just got the filter poking out under the side fairing ive been told i will need to Jett the carb because i will have black spots on the acceleration side of things and i think i have in low revs trying to pul away it does seem like a slug sometimes but on the high rev's it seems fine, but like i said i do want t get a bit more out of the bike HERE'S THE PROBLEM ive heard that the 59 plates are injected would that make a diffence at all jetting the carb's?? also would i just need to do the main jet?? or is there any others i would need to do ?? also do i go up one size or 2/3 ?? (I am getting badly confused by all this ) and i am having troble finding stuff out on the web SO PLEASSEE HELP AND ADVICE WOULD BE LAVLY