[olddog]

Possible vacuum leak between the carb and the manifold. Two gaskets to leak with the spacer, plus the spacer itself could be cracked.

Pull it and inspect it. If there is no leak evident, reassemble with a different, known to be good, carb and see what happens. Any carb off of a similar cid engine that is tuned well to that engine should be close enough for all but WOT. If you have a wide band just make sure it doesn't go too lean. Fat is OK for a while.