Just because everything was good 2 years ago doesn't mean anything. Valves stick and so do rings from sitting.
Any vacuum source that originates below the throttle plate is the correct vacuum for the vacuum advance.
The check ball doesn't have anything to do with the low speed circuit. However, depending on which position it is stuck could cause different issues. If it's stuck in the closed position you won't get a squirt when you open the throttle . If it's stuck open you will probably get an overly rich condition since the plunger will force fuel backwards.
A cool tool to find vacuum leaks is was of those cheap party fog machines. I bought one last winter and attached a hose to it to connect to the intake on my BMW. I had smoke coming out in several places I was convinced weren't leaking. Car runs like a dream now.
