You know what, I had almost this same exact thing happen to me a couple of weeks ago. I took it in to the shop, and it turned out the diaphragm in the petcock was toast, causing gas to flood into the rear cylinder.
You might want to see about rebuilding the petcock to see if that cures the problem before digging into the carbs. Good luck : )
7800 orig mi when purchased Jan 2011
Turned 11,000 mi Nov 2011
Taillight relocation kit
Chad's highway peg mounts
Otherwise, all stock
"But it's a dry heat - like the inside of an oven"