What you are probably seeing is the blow-by vapors that are making their way back into the valve cover from the combustion chamber and crankcase. that is one of the functions of the pcv system - to ventilate the crankcase and re-burn that "smoke" that you are seeing. It is pretty normal for a slightly worn engine. Adjusting the valves is good for an older engine, but it won't help you if you've got worn piston rings. Hope that helps