When you "rev it up", the vacuum can drops out and you are then seeing the mechanical advance in the dizzy.
Well sort of ..no
When u intially rev, the vac drops , VA cuts out...as the engine speed catches up with the throttle it pulls a vac and the VA actuates...a good rev up is is too short a time to establish any VA in or out or reliable advance numbers.
Follow toms post above...except..to clairfy
if you disconnect and plug the vacuum line, thats your initial advance.
then hook up the vacuum advance, the difference, at idle, no other changes is how much timing the vacuum can is pulling in.
When the VA is connected it advances, the advance increases the rpms, IF it has a reasonbly quick curve the cent advance could also be advancing....it is important that both measurements are taken at the same idle rpms
It does sound very much another case of an old school engine designed for manifold vaccuum being run on a dizzy with specs designed for later model (about post 1974) ported vac dizzy specs.....which are very different...as discussed many times over and over.....they run, the often run "well" but are still way off any potentual of power or economy.
There is a narrow range between about 15 and 18 degs after TDC that the max explosion must reach its peak.....about +- 2 to 3 degs. at any given rpm.,, and varies as to how much load the engine is under (and a few other things)
you have a low intial...longer battery life, less loading of started stops the dreaded so called heat sink....dry, melted solder joints on armitures
Engine fires, engine Vac established in the manifold...the VA is sucked in and activated
This is the idial advance...determined by cam, fuel type, dynamic compression etc, usually 12 to 18 degs.
The engine accelerates the rpms increase which requires more lead time....this is usually a curve untill around 3000 rpms is hit ..there is no need for more timing after this point because the milli secseconds required to time are far to small to at these rpms and therefore too small to make any significant change (over simplified)
More load requires less advance...changes in compression ..lot of stuff....less load more advance...can run leaner mixtures, less compression, the VA kicks in to provide this.