Installed the intake manifold back to the block (with new gaskets) after have it cleaned and painted. Some minutes after fired it up, the engine was bad idling (erratic) and shaking/vibrating a little. I need to keep high RPM (about 800/900) to improve idle. After a while, it started to make a rhythmic and loud noise like something (metallic) loose inside. It seemed came from the top end passenger side and intermittent. I've made the valve lash adjustment as much I know and learned online but the noise remained. After sometime when idling, it disappeared but the erratic/poor idle is still there as for the low power. Took the car out today for a road test and it really lacks power specially if you drive it in a small ascent street. It has a small leak where the exhaust manifold joins the head near to the #5 cylinder but I don't believe it's the cause for this low power and bad idle. Everything was fine just before restoring the intake. Engine is from an '67 Impala stock sbc 283 2bbl with power pack heads on it. Video post here: https://www.youtube.com/watch?v=w1g0jn07Ljw. Can it be some vacuum leak from intake/heads or something else? Any suggestions are welcome!