[TR] Vacuum advance/retard

[Nolan]

> At the other end, if the port is on the manifold or the base of the
> carb its vacuum will go down progressively as the throttle is opened,
> so it must be meant for VA.  If the port is right at the butterfly
> its vacuum goes away completely as the throttle is opened even a
> little, so it must be meant for VR.

That's a dangerous assumption to make.  Many a vacuum advance is connected 
to port vacuum, not manifold vacuum.  The same for vacuum retards.  That's 
why you've really got to know the parts to be sure which way things go. 
I've held what appeared to be identical distributors with identical looking 
vacuum advance cans, yet by the numbers one was meant to be connected to 
manifold vacuum while the other to port vacuum.

This, in no small measure, is why many older cars with replaced distributors 
have some very odd performance characteristics.  The ignition advance is 
working incorrectly because it's receiving the wrong vacuum signal.  As well 
incorrect mechanical advances for the application.