You dry burn by removing the filler and sticking the carto onto your batt and hopefully if you have a manual battery, you can just heat the carto until the gunk is burned off. You can only do this with the horizontal cartos and in my opinion, is more trouble than it is worth.
I suppose you could try hydrogen peroxide, but I would imagine that if they are dried out really well that it would leave a nasty taste to your
juice.
Best bet is to do a hot water (not BOILING mind, just hot from the tap) soak and let them air dry for a couple of days or bake them on the lowest setting possible for about fifteen-thirty minutes. If they still aren't working well after that, toss them and buy new ones. Five cartos still cost less than 3 packs of smokes and lasts a lot longer as well, especially if you keep them dedicated to one type of
juice.
I buy different colored Cartos just for that reason. I can usually get about 4-6 months out of each carto by not cleaning them and keeping the juices seperated by carto color.