I'm trying to track down the cause of a rough idle and occasional stalling that has started happening on my carbed 350 mated to a T-5.

I checked vacuum, jumps from 12-13" at idle. Then, I attached my vacuum pump to the vacuum advance and could not get it to hold a vacuum. If I pumped it vigorously I could get it up to about 5", but that would drop down to 0" about 2 seconds after I stopped pumping.

I know that the vacuum advance is advancing the spark somewhat, because the engine will stall at idle as soon as I unhook the vacuum line from the carb (manifold port).

Am I in the market for a new vacuum can?

 

You're on to something. It shouldn't leak down like that. I don't know how big your cam is, but you should probably have higher manifold vacuum than you do. Advancing the timing (as long as it's not detonating) when using a bigger than stock cam can help increase manifold vacuum at idle. Replace the vacuum advance mechanism, they're pretty cheap. AFTERWARDS, take another look at your base timing.