A question about a manifold vacuum leak?

on the manifold theres a series of fittings, as seen here:


and here’s the diagram:


i understand that the first fitting isnt used but should there be a cap/plug that should be on there?

It should have a rubber cap on it. You are running a little lean without it.

Plug that open port. It will certainly upset the idle…But if you were smart enough to post the picture and the diagram, you are smart enough to know that…