Interesting. Wouldn't there be misfire without the vacuum advance due to no dwell time? I'll try it sometime, just to see what happens. If that's the case, it could be that the old VA has not been working for some time. I know that it has never idled as smooth as it does now.
When the throttle is opened vacuum drops in the manifold so the source of vacuum to operate the advance unit is gone and so the timing is retarded to base timing.
The car accelerates faster with base timing. Base timing/retarded timing yields power.
Advance timing fuel economy low emissions.
Dwell is the amount of time the points are closed.
Vacuum advance changes when the points open in relationship to where the piston is reaching the end of the compression stroke.
As timing is retarded the spark happens with the piston closer to the end of the compression stroke/TDC.
As the timing is advance the spark takes place with the piston farther away from reaching the end of the compression stroke/TDC.
Whether there is spark advance or no spark advance the points are still going to open and fire the plugs.
When a vacuum advance unit does not work it is usually due to the vacuum advance unit having a vacuum leak. Engine idle quality will always be effected by a vacuum leak. Jeffrey
