I frequently have the same problem!! I start by putting the pipe in a stand, stem turned up & filling the bowl with 190 proof grain alcohol. After an overnight soak, dump it and let it dry a few minutes. After having tried every reamer over the years. I just start with a strip of sand paper a little wider than my index finger. Leaving a flap to cover my finger tip, I just go into the bowl and start to rotate. For thick, hard cakes I start with 80 grit. For finishing or more fine cakes, I’ll use 150-220 grit. Some of these cakes take some work, but serial sanding is the way to go rather than flaking or chipping a thick cake with tools.
The stummel vent gets brushed with grain and I’ll also use ‘prickly’ pipe cleaners until it’s clean and draws well. You can rinse with warm/hot water, just let it dry before the next round. Chances are that sourness will require you to take that cake down to the bowl briar.
Once I’m happy with the cake and I may do 2-3 consecutive nights with grain alcohol soaks on those really tough cakes, I will do the same with inverted stem and soak overnight with a high proof bourbon 114-125). I’ve never had a stinky pipe survive my liquor soaks! A wee dram of the bourbon on your lip buys a little a little extra as well (
Hope this helps, Mike