I recently purchased a 66 convertible with a 390. It starts ok and idles well at cold idle but when at temperature, the idle is erratic and sometimes, the engine will barely keep running. I found and fixed several vacuum leaks but still cannot get the vacuum above 14 inches. Adjusting the idle screws on the carb seems to make no difference in vacuum or idle quality. The carb is clean and shiny which makes me think that it has recently been rebuilt or replaced. Any ideas other than replacing the carb? I have also noticed that when I step on the brakes the idle speeds up slightly however, blocking off the vacuum line to the brakes did not improve my low vacuum situation.