I have a 1955 Thunderbird with a stock Holly 4000 carburetor and tach drive Loadomatic distributor.  I fount that I could set the timing with the vacuum line removed with no problem.  The timing did not change once the line was reconnected however if I rev the engine the vacuum will advance but will not retard when the throttle is returned to idle.  I tinkered and found that if I loosened the spark valve 1/4 turn on the carburetor, it seems to work OK,  I am not sure why the system holds the vacuum when the valve is fully seated.  Should I just add another gasket to the spark valve or, by loosening the valve, am I simply allowing the vacuum to equalize because the loosened valve is not making a seal.  Thank you!

This may be related to the distributor check ball in the carburetor; if it’s there, it could be stuck and/or not moving freely.  Those are plastic and if soaking the carb body in a good solvent or carb cleaner, that ball can be damaged.  To gain access to those balls, the distributor check ball retaining screw located at the bottom of the carb must be removed.
 
As a footnote to this, I’m told that only the standard shift Teapot carbs had the plastic check ball while the automatic transmission equipped Teapots did not.  For most Teapot carb rebuilds, I do not reinstall those plastic balls and just leave the passageway clear.

Lorena, Texas (South of Waco)

thank you sir, I will check that out,  it is an automatic transmission.  

I checked with my son who did the rebuild.  He said that he removed the screw in the bottom of the carburetor and the ball was there.  he said it looked good and he put it back in.  Next time we remove the carburetor, I will take the ball out and see if that makes a difference.  Thanks