<VV> Carb problem - any ideas?

[Kinzelman, Andy]

Guys,
Helping out an older gentleman with a 64 Monza vert - tweaking the carbs.  When I blocked off the driver's side carb a very noticeable vacuum leak occurs.  I couldn't isolate it's location; so took off the carb to check it out.  The base gaskets and insulator looked OK.
There appeared to be remnants of an old gasket between the top & bottom of the carb, so I disassembled, cleaned the gasket surfaces, and inspected the parts.  The top of the carb seemed straight; so I reassembled with a new gasket and re-installed it.
There was no obvious change to the leak.
The vacuum balance tube hose looks OK.
The carb shaft doesn't seem worn.

I called a friend of mine; he said he'd seen the plug opposite the vacuum balance port fall out.

The next time I go out I plan to bring a stethoscope (or hose) to try to locate the problem.  I'll also bring a spare carb.

Anybody have any other ideas to check?
Thanks,
Andy K.
Wisconsin