FWIW,
A quick and dirty way to check a vacuum advance unit is to crank the
engine,with strobe attached to #1 plug. Un plug the vacuum advance unit and
cap the tube. Observe the position of the timing mark(s) with the strobe.
Re-attach the vacuum advance unit. If it is working at all, the timing should
advance. This will only tell you if the unit is working at all, whether or
not it is functioning "normally" takes the services of a distributor machine.
When the timing mark advances as the engine is revved, this is an
indication the mechanical advance is functioning. It has nothing to do with
the vacuum unit.
Rick "Just my tuppence worth" Morrison
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