How much does your boy weigh? Stock for the KX65 is 13-46 or 47. I went up to a 49 rear this year because my kid weighs in at 95lbs. I also put in a V-Force reed valve. This pulls him out of the hole quicker!! He might have lost some top end but the tracks we race at no one rides in 6th. gear pinned, so top end means nothing.