The fully vacuum advance VW distributors took a signal off a port on the carb between the venturi and the throttle plate; as airflow through the carb increased, more advance was added. It worked similar to the earlier mechanical advance distributors, until you changed something, like install a big-bore kit; the increased flow would increase the vacuum signal, advancing the timing at a lower RPM. Standard practice was to switch to a mechanical advance distributor like the 009. If the carb was changed to a Holley bug spray, the mechanical distributor was mandatory, as there was no port on the carb that would mimic the Solex.
Most older cars used a combination of mechanical advance and vacuum advance, but while the mechanical advance was RPM based, the manifold vacuum referenced advance was based on engine load; at light load/high vacuum, advance was added on top of the mechanical. At high load, full throttle type situations, the manifold vacuum drops, and the timing decreases. If the GL vacuum advance is referenced to the manifold, as in under the throttle plates, the normal Solex distributor port won't work properly.
