As some of you may remember, a while ago I was trying to figure out a way to clean Zippo pipe interiors, which tend to build up carbon more quickly because of the angle it is held that. Well I finally came across something that works, partially at least. Put a pair of Alka Selter (or Polydent Tablets) at the bottom of a small disk, then the insert, then enough water to cover the insert. The bubbling action of the tablets will loosen the carbon enough in a few minutes to remove the insert and clean with a small wire brush or old toothbrush.
Hope this works for you!
Hope this works for you!