From what you describe, it sounds too lean at 2 1/2 turns. The barking sound is most likely the engine going lean and causing pre-ignition. This can also cause the engine to backfire. Both of these conditions could be the barking sound, both are caused by the engine being too lean and both can cause damage to the engine. The gradual slowing down at 4 turns sounds like the engine is too rich and is slowly loading up with too much fuel. Have you tried running it at about 3 turns or so? After you make a change to the high speed needle, run the engine up to full throttle for several seconds to see how the change affects the engine. Always make the changes in small increments, maybe 1/8 of a turn at a time until you get close, then just a click or two at time.
Paul