yes that does sound like a bad vacuum can, but only one side. The inside only bumps the timing up about 6* and that is only when it is cold. The outer one bumps 12* whenever it receives ported vacuum. When I had this problem I manually advanced my timing by approximately 5* and left the vacuum hose to the inside nipple plugged. This worked fine with some minor driveability issues arising. Drove this way for a couple of years till I happend to find a vacuume can at pull a parts.