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!