How to clean nozzle and coupling with acetone?

Soaking works even on burnt ABS, but only as a softener.  I use a small paperclip to scrape the remainder as it fits in the threads, and through the nozzle hole.

I don't do any cleanout for simple nozzle changes, but sometimes it can be hard to reassemble after a teardown when there is plastic in the threads.  It may have been easy to push through the plastic when melted during the teardown, but hardened plastic can get left behind where it is in the way for reassembly.   I've often had to use the torch to get everything melted enough to take apart if heating with the band isn't an option.  Usually that comes after a disaster of some kind.

I've torn mine down and soaked the nipple and coupler in a tuperware with acetone to loosen and clean out ABS.  With out tearing it all apart, I've soaked paper towels in acetone and placed them on or in the threads of the coupler and then wrapped  it in tin foil to prevent evaporation.  I have the two piece  vertical mount that I take off and just put wrap the port with tin foil to prevent evaporation out that way.  Helped to soften the ABS to wipe the coupler threads.  A dental pick helps a bit to force the acetone soaked rag into the the threads to clean them.

Just how I do it.

There are solvents for PLA, but you don't need to use them. You can just run ABS through it at normal temperatures, and it'll purge the PLA in 15 minutes or so.