Honestly I didn't realize that there were variants of the modern "replacement" vacuum pots that aren't adjustable. It just seemed like every replacement dizzy we found, had instructions about how to make your "new" advance behave like your "old" one (a limiter). Ted is right - cheap may be the ruling factor for some of the rebuilts. Just looking at the picture below (and lacking the proper part) - I think that I would engineer a threaded rod or long screw - tapped through the side of the dizzy to stop the pivoting point plate where I wanted it to stop - and locked with a nut.
The advance scheme that I have been running (and I believe Mark too) is for engines that are pretty much stock (low to moderate compression / factory style cam characteristics):
10°-12° Initial
13° in the Dizzy (26° at the crank) for a total of 36°-38° degrees of combined advance (vacuum pot installed but not connected).
Spring set in the centrifugal replaced with lighter action - to get full mechanical at 2500 -3000 rev's (2800 target).
We've then got the vacuum advance pot limited to an additional 8° (crank). Both Mark and I are running live vacuum advance at idle, so after using the initial to start - they are idling at 18°-20°. It keeps them cool and runs up the manifold vacuum.
I use a dampening orfice in my vacuum line from the carb to the dizzy. Be aware that on some enignes (high compression most likely) - it doesn't work. The dampener is simply an orfice that prevents the vacuum pot from slamming around too quickly. For mine, I inserted an electrical "butt splice" and crimped until it worked to suit me.
Steve Metzger Tucson, Arizona