Good news Jason, your correection has given us some positive results and some new questions.You enrichened the idle mixture and helped.Does that mean now we can compensate for a small air leak?
Manifold vacuum readings are a measure of the engines ability to take air in and the throttle plate trys to prevent that intake so a vacuum is created.If there is air coming in other than at the throttle the reading will be lower than normal, such as valve guides, gaskets, flaws in castings, valve leakage, etc.Since you have eliminated most of these now what?
I had a tractor engine once that operated a lot in an industrial situation--low load, much idling,low operating temperatures that developed serious idling problems.The usual tune up procedures and external parts replacement, compression check etc revealed nothing so a head removal was in progress not knowing what else to do.
When the inlet manifold was removed it revealed a great buildup of carbon in the area under the inlet valves.The passage was choked down to about a 3/8 inch diameter passage.Opening this area up brought back decent performance.
I don't remember a vacuum test but a restriction like this would have resulted in a lowered manifold vacuum reading with other conditions being normal.
Also a lot of cam wear would result in a lowered reading but it would have to be fairly uniform to not have fluction observed in the guage reading.
Cam wear can get well advanced and still allow good low speed performance.
Observe the valve action with the engine idling and see if valve lift appears uniform.
Another thought to remember.We are used to sometimes using vacuum reading to set spark timing-BUT, spark timing only changes vacuum readings because it changes idle speed at a given throttle opening.
