Yeah, cartos and carts are two totally different things. A two piece KR808 uses cartos, which you don't take the filler out of usually. If it starts to taste burnt, then it probably got too dry and the filler is starting to burn. Not good. lol
Cartos are meant to be disposable, and you can usually get a few days to a week out of them, topping off with juice often to keep them wet. But you'll notice the air flow starting to get harder or the vapor production will start to lower - I think because the old juice starts to gunk up the atty inside. They are washable though, so long as you stop using them before that nasty burnt taste.
You can find blank KR808 cartos and fill them with whatever juice you like, or buy prefilled and use them for a day and toss them or refill with a similar juice.
I find that with the way I vape, I put 10 drops in to top off a carto fairly often, maybe once an hour? Never really paid attention. Keep a paperclip handy to pop the top off and look at the filler, if it looks dry on top, put a few drop in until you notice the filler not sucking any more in. You can also use a drip tip to make it easy to check inside, they pop off and on without needing a paperclip. Keep a paper towel handy in case you get a little too much, or drop any down that center tube - it will leak out the bottom. Cartos are fairly easy to work with, but it takes some practice to get a groove down to where you know when to top off and when to move to a new one.