Technical Ford FE 390 Vacuum not dropping

So after many weeks I finally got a new Vac Can and a Vac Gauge, only now, if I hook the gauge up to the full manifold on the Carb, I'm getting about 18"hg at idle but when I rev it, it doesn't drop off at all, sometimes it even goes up, so when I connect my vac advance, it doesn't drop back to initial timing when I rev it, it stays at whatever I've got my advance set to

I'm ***uming this is most likely a vacuum leak?

 

If it's a no-load condition, like revving in neutral, the vacuum won't change much. The engine can freely increase revs to use all the air coming in. Try it under load, like climbing a steep hill. Should see the vacuum drop down.

 

I had a plugged exhaust system once that caused that same type of symptom


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No load is the answer , try driving it


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That's not correct. Revving in neutral, healthy engine, should show a very quick drop to around 4" of vacuum and then rebound to 25"+; this is the diagnostic characteristic of healthy rings and cylinder compression.

 

Sounds like you need to ditch the can. Try it without it JMO Lippy