Leaking petcock: if the petcock is leaking while the bike is just sitting, it should fill the bowls until the needle valve is seated which then shuts off the flow of fuel. While a leaking petcock is never a good thing, the needle valves should prevent an overflow while at rest. Of course, that's all theoretically. If there's crap in the mix, the needle won't properly seat. If the needle taper is worn, it won't properly seat. If the float level is too high, it will run out the overflow before it ever properly seats. And if your overflow tube has a tiny crack in it....and so on and so on. First, fix the leaky petcock, but also check the overflow tubes and float levels and drain the bowls looking for any signs of crud.
Crappy running: Idles good, so it's probably not the air screws & slow jets - they impact 1/4~1/8 throttle on down. Needle clip position impacts operation from 1/8~3/4 throttle, so I'd consider there, however this would be kinda an odd fix in that this condition suddenly cropped up. Needle taper impacts 1/4 on up, and main jets 3/4 on up, maybe something here, but again an odd fix for a sudden problem. I'd start looking for crud in your bowls and maybe float height adjustment. If the problem occurred directly after a rebuild, then maybe some of the mechanical/physical stuff like clip position and needle/jets/etc. But a sudden problem smacks of crud someplace. And it ties in with your leaking petcock thing.
