I just whent through this exact thing.....
I tried looking for the easy way out by calling a transmission shop ( they have steam tanks) they told me it would be a waste of time because its impossible for the water jets to get in there. I also called rad shops and that was a no go also.
The conclusion I came to was that there is no miracle tool to go in there and clean it. You cant even see in the middle.
Heres what I did.
-tape off the lower air inlet and fill it with boiling HOT water.... Let that soak for 10-15min. Then go back for another round and shake it like a maniac. I did this 3-4 times
I had a really nasty sludge in mine kinda a mix of dust and oil , this nasty stuff was baked on and almost impossible to get off.
I even made a few trips under the cover of darkness to the local car wash
to blast them with hot water.
Just to be sure there was nothing left in there I loaded them with VERY small stones ( recomended by a gas tank shop) to again shake like a maniac.
Mine are looking new
but it was alot of work.
Good luck