Gary1968Chevelle

My friend has an LS 6 in his 68 Chevelle, 454 Cubic Inch. It has a solid roller cam, and it suddenly started making a popping sound from the carb when you rev it. But seems to idle ok. Could this be a broken valve spring? Burnt valve? Also need the valve lash clearance on this motor. We were using the TDC method, to adj the valves, which I found out was wrong. So going to use the I/O , E/O method next time. Im thinking .018 int, and .022 ex for valve clearance. I have some exp. with the 396, but not with a 454 LS 6 engine. Any help is totally appreciated. Thanks, Gary.