surging

I own an apollo 125cc RFZ dirtbike and have had it for a few years. As with all chinese made quads, threadlocker is forbidden, and every bolt comes loose........ the throttle assembly on top came loose, the one with the throttle cable, huge spring, plunger, little clip and big needle. i got it...

Revs sort just drop off then pick back up and drop off again when I give it more than 3/4 throttle, doseit in gear and nutrle, engines a lifan 125 with a mikuni copie on it.. Plug is black ( again ) replacing fule filter and switch got it running good again but now after running fine, it's...