You know, there are some engine cleaners that may work better for exhaust than home-brewed stuff. I'm going to try some ford carb/engine cleaner. It's in a blue can, and it looks like a spray, but youre supposed to just pop off the top, and pour it in the engine.
I'll go to the Ford house and get a part number.
It will TOTALLY disolve any carbon in an engine, and probably work great on exhaust parts. I don't think it costs more than $3-$4.
I'll try it tonight and see what it does.