I put a new carb and intake on this year and could not get the pump shot to come in off idle, and it was causing a stumble. I checked everything, then I took a look at the arm that pushes down the acc pump lever. The arm appeared to be bent a little sloppy, not nice 90*bends. I took it off, tweaked it a little and replaced it. The carb works great now. My point is maybe it is not always something internal, check the most basic parts first. Carb was a Holley 4150 1k annular.