Yes, that is the good way to check. To be safe, I would disconnect the vacum to the distributor altogether and just have mechanical being the ONLY advance it sees. Of course, plug up the hose end, otherwise you've got a small leak.
If this takes care of the problem than you know your vacum is not pulling off when at WOT and your distributor continues to see additional advance beyond just the mechanical.
If that is that case, what is your source for the vacum advance?
oh ya, ive disconnected the vacuum advance and its still advancing too far.
