Sr20det Low vacuum at idle..pondering

[SoguRacing]

ok guys so I had my engine rebuilt a few months back due to 2 spun rod bearings during tuning. Anyways long story short, My engine was rebuilt where it was tuned and I recently did a vacuum test and the reading was 12inHg. I know that normal reading range between 17-21inHg. So, My question is..What do I do or go about this problem? A reading that LOW would indicate either an intake manifold leak or late valve timing. So again..How would I go about the process or figuring out how to resolve this situation? Keep in mind that I haven't modified anything since the rebuild. Stock rebuild with hks step 1 256 cams...very mild and shouldn't cause low vacuum. There isn't enough valve overlap at idle. Any Ideas? Should I take off the valve cover and see if the timing marks line up???