Normal Engine: On a normal engine, accelerate to around 2000 rpm and then quickly release the throttle. The engine should snap right back to a steady 17- 21 "hg vacuum. Steady low between 5-10 "hg vacuum: This indicates that the engine has a leak in the intake manifold or the intake gasket. This leak should be easy to find because it would be making a loud hissing noise.
Steady low between 10-15 "hg vacuum: This reading indicates late valve timing. There's a good chance the vehicle has jumped timing. Check the timing belt or chain depending on the application.
Steady low between 15-18 "hg vacuum: This just low reading indicates retarded ignition timing. Advance the timing on the distributor to correct this problem.
Fluctuating Needle: A fluctuating needle indicates there's a problem with a valve or a there's an engine misfire.
Needle drops during acceleration: If the needle drops steady during acceleration there's a restriction in the exhaust or intake. This is typically due to a clogged CAT or muffler.
one clue is " seller told me the engine was rebuilt".......when? by whom? any receipts?........something could have been put together wrong or the intake gasket is screwed up
