Attached is a link to a dual diaphragm schematic and text explaining the operation.
I don't understand how the timing is retarding at idle from the carb front vacuum line.
The only time it's supposed to retard the timing is on sudden acceleration just momentarily to avoid pinging (explained in the text). The inner diaphragm which is connected to the carb on the front of the carb only pulls in the advance direction.
The outer diaphragm which is connected to manifold vacuum is only for the momentary retarding of the spark on acceleration.
Sal
https://r.search.yahoo.com/_ylt=AwrJ...ZYEQ_dd0wsWrA-