Once you get the burnt taste, the carto is probably shot and needs to be thrown away. If you like you can try to clean it and get some more life out of it, but since they're like $2 each anyway, you might as well just toss it. As for filling it, here's what I do: I will drop 5 to 10 drops in at a time, and wait a little bit for it to soak in. While doing this I'll be holding a napkin or paper towel below the carto to catch any juice that falls out the bottom. I fill, wait, fill, wait, and repeat, all the while checking that napkin. As soon as a drop falls out the bottom, I stop, wait a little bit more for everything to soak in/drip out, and then I'll blow through the carto to make sure the center airway is clear. This whole process takes about 30 seconds to a minute depending on the size of the carto (I'm using Smok Mega dual coil cartos at the moment which advertise a 5ml capacity, though it's really closer to 3 or 4). Once you use a particular carto for a while, you will get a sense for how many drops it can hold, how often it needs to be refilled, etc. To prevent the burnt taste, make sure you're using an appropriate voltage/wattage for the given resistance of the carto. Also, waiting too long to refill it can burn the polyfill. Often you can just tell by the taste and the amount of vapor coming out when you need to refill it, but I'll still take my drip tip off and look inside sometimes. If the polyfill looks white and dry everywhere but the center most area of the polyfill, it's time to refill. Hope this helps.