I would first eliminate a few "variables" when using idle vacuum to adjust the ignition timing (and the carburetor mixture):
- disconnect the wiper "vacuum motor" and use that port on the intake manifold to measure vacuum
- disconnect (and plug seal the line) to any other device that uses vacuum (like the brake system hydrovac unit/line)
- disconnect (and plug line) to any other device that uses vacuum (2-speed rear end and speedmeter selector, if you have these)
- there is no need to disconnect the distributor vacuum advance
This way you are eliminating other possible "sources" of vacuum leaks.
