when they smoke due to oil in pcv pipe and inlet its normally down to piston ring blow past , also on over run as crankcase pressure gets to high that is one level, over level is when do valve stem seals must lag them in oil when doing them, also when push valves through the oil seals you are meant to use protector over the end of valve stem to avoid hurting the rubber lips on stem seals,, that said i get around that by slowly and gentle rotate the valve as it goes past the stem seals to avoid damage the seals , i also use engine builders grease to avoid such issues,