Sorry for my slow response. Anyway, have you pulled the nozzle and red PTFE tube out lately? What happens is that burned ABS leaves a hard substance that can block the nozzle from the inside. Believe it or not, but dust too from the filament can do the same thing as it doesn't pass through the nozzle. One way to try and clean it it to loosen the filament tension screw, get some fresh good ABS filament in a couple of 12 inch pieces cut square on the ends via a sharp blade, and heat the head to 220 C (No hotter!) then hand feed filament enough until it comes out, them pull out fast to suck any contaminates back out stuck to the hot melted filament, It might take once or twice.
Another common issue was that improper cutting of the red tube when assembling. If you cut it off flush with the end of the stainless tube or since you ran the head hotter than 230C melted, it pinches off the end or leaves a gap near the nozzle. The gap condition leaves a large room in the area above the nozzle in which the ABS doesn't flow through against the walls and burns, blocking the nozzle, or the melted PTFE could be blocking the nozzle. I see from the photo your metal plate in the middle is stressed meaning you might have jammed the PTFE into the nozzle and it's closed off from running too hot.
See this thread which is related and has some great pictures of the problems
http://wiki.makerbot.com/forum/t-374079/stepstruder-mk6-issues