Vacuum leak is the first suspect. After that, at around 3500 - 4000 is where the slide opens enough that you are running on the main jet. Below that, you are running on the idle and transition jets which are fed by the pilot jets. If it idles well with the idle screws around 2 1/2 to 3 1/2 turns out, the idle jets are probably OK. Take a look at the transition jets, 2 (maybe 3) holes in the bottom of the carb throat just before the throttle plates. Also make sure the pilot jets and related passages are clear and properly seated.
