Runner 180 bad acceleration and top speed

My gilera runner used to work fine (good accelearton and 130kmh top speed) but then it started backfiring and i couldnt find the problem. Today i changed the sparkplug cap and spark plug and its ok now. But the top speed is around 100kmh and the accelaration is the same as my friends runner 70cc. It used to wheeli with no problem but now it doesnt. Everything is stock except s6 vforce 3 reed valve, rms 23mm intake manifold ( i think ita for 50c but its bigger than original 50cc), runner 50cc triangle airbox without foam and with 2 10mm holes in it, 13.5g rollers, 21mm ysn phbg (85 main jet). What could be the probelm. It used to go much better with same parts.