Hi, I have used these a few times but have not experienced anything like a burn taste from them, i have used them on a ego type battery as well as a VMod V2 in unregulated mode, what voltages or watts are you putting though them,
Carto`s and indeed tanks can taste horrible if you don`t let the old flavour finish, i personally would use a new Carto for a new flavour, especially if a tainter juice has been put in it (Menthol, some chocolate, chilli chicken and Diablo Loco to name a few) .
For Carto`s best practice is keep topping up every 10+ tokes, depending on the size of the carto, that way its always topped off, although one that takes as many as 40 drops should last a fair few tokes before becoming "dry", it all depends on your "Vaping" style and how often you take a toke, not very helpful but everyone has a different method of enjoying the act .
you will be able to tell when the carto is dead..it will not work..lol, but usually you will notice a "Drop Off" in performance well before the actual carto goes dead on you, although not always.