A worn needle and seat could cause it to not idle, also if his idle screw isn't set high enough. with a high compression piston, I'm guessing it'll take a little more gas to idle than it would've with the old worn out top end
that's my two cents. carb kits are only about 25 dollars, and they come with pretty much everything but the slide.
Caution
Do not "POP WHEELIES" and run for any distance with the ATC. The oil capacity is relatively small and the oil will drain out of the oil pump area thus causing loss of oil pressure, resulting in costly engine damage -CLYMER