I just recently cleaned mine out, the carbon was atleast 1/8th inch thick or more in spots, bike was run rich for awhile so it built up in there for some time.
i tried a few diffrent methods, but the one that i found to work the best (but discolored my chrome to purple/blue) is to use a blowtorch and a large air compressor with a blowing nossle attachment.
start at the headpipe where it meets the head, heat the carbon up redhot till its ready to burn, then start blowing some compressed air into the pipe (to blow the flame and heat through the pipe)
you have to make sure you dont use too much air as itll blow the blowtorch out.. and then its really hard to reignite the carbon.
its a slow process to burn it all out, but you have to do a steady burn, you can see a red ring going through the pipe, thats the carbon burning out..
after its all finished you can tap on it with a peice of wood to get any loose carbon to fall out,
it did turn my pipe blue/purple but it was free and the pipe is clean and ready to roost like new