I had a guy brought me a 69 Camaro once, ran fine under power but missed at idle. He had thrown everything but the kitchen sink at it. I told him it was a vacuum leak, but he argued that he had replaced the carb base gasket and the intake gasket. Its still a vacuum leak I told him. Pulled the intake back off, looked inside it, and found the leak. Someone had used a bolt that was too long to hold the coil bracket on the intake, and made a nice hole under the bolt. After putting some sealer on the bolt and putting it all back together, it ran good. (As good as a 69 Camaro will run, anyways)