If you can run up to 2000 rpm & maintain it smoothly yet when you goose it it goes flat for a moment then it's likely in your acc pump system. If you just have a stumble in that area that doesn't go away even when maintaining that rpm you likely have a lean condition & it may be that you need a lower powervalve to fill in in your low vacuum condition. Or it could be something else completely like clogged passages or failing gaskets. I helped a friend rebuild a carburetor yesterday that had holes in the acc pump diaphram. It had a huge flat spot on acceleration, can't imagine why...
