Few people actually use the cartridges. They are an OK way to start vaping. That's about it. If you want to use them occasionally I recommend replacing the used stuffing with the precut blue foam for 510 carts from strictlyejuice.com (search there for foam - under $1 for 10 pieces). On a full fill, fill the juice cup 1/2 way, insert the tapered end of the foam, push the foam down into the juice to wet it (an unbent smooth paperclip helps), reposition the foam at the top edge, and top off with 2-4 drops or whatever it needs to look damp on top but not a puddle. As you vape, top off as needed. When topping off does not help, do another full fill. The blue foam holds its shape and consistency and is easy to take out, rinse, pat dry, put back in.
What people prefer to carts -
510 drip tip on the atomizer to test juices of vape occasional juices or ones that are too thick for cartomizers (and I recommend a plastic or delrin drip tip because you can also use it on some cartomizers as a mouthpiece)
Cartomizers - most people prefer cartomizers for most of their vaping of their favorite juices. A cartomizer is a disposable 1-piece cart+atomizer. I can get a week or more use out of each by using thin clear juices (20% or less VG, and I choose juices that don't seem to leave much flavoring sediment in the stuffing).
Boge cartomizers are the standard stuffing type of cartomizer, no syringe needed, the most popular cartomizers on 510-threaded devices. I use the standard resistance because I share them with a slim manual 510 (slim ecigs should only use standard resistance). I recommend you learn on standard resistance because they are more forgiving and won't singe as easily as you learn to sense when the carto needs juice. And I love the vape of the standard res ones on the Riva 510 from Liberty-Flights because the Riva is 3.7v and gets a warmer bigger vape out of them. Once you learn to use them, you may want to switch to the hotter 2.0 ohm LR ohms (also use battery charge faster). Many people like the 2.0 ones.
There are also more exotic cartos like the dual coils and the clear plastic cartos (the old ones crack but Gotvapes has an early release of the new CE3 ones which they are calling Fluxomizer Deluxe - supposed to have a flexible tube that won't crack or leak). These DO need a syringe so if you buy any there also select your choice of filling device (on the same page) to be added to your order. These are new - no real reviews yet on the new ones.
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How I fill a cartomizer (Vapor4Life for KR8 or Boge 510 for 510 which are all soft-cap w/ no inner seals and defined central air channel)
I use thin (20% or less VG) liquid or Vapor4Life WOW liquids, preferably clear liquids w/ no sediment
Filling a blank carto - I pop off the soft cap (unbent paperclip carefully in the mouth hole, not catching any wires or stuffing!). I hold some tissue against the threaded end hole. I tilt the carto and slowly drip 18 drops onto the inner wall of the carto so it slides down into the stuffing while I rotate the carto. Then I wait 15 seconds or so then I drip however many more drops the stuffing wants to soak up. Then I do clean-up. If it is leaking, I clean that up. Then I tilt the open end down into the tissue and puff hard a few times into the threaded end to clear any juice out of the central air channel (I want to see light through there). If it looks too wet above the stuffing, I gently touch a bit of tissue to the stuffing (otherwise that liquid will be gurgling or spitting in my mouth). Then since this was a fresh fill of a blank, I now let it settle. I lay the carto on different sides for 15-20 minutes. Then it should be ready to vape. If you use thicker juices than me, then you may need to let it settle longer.
As I am vaping - if the vapor gets a bit light or a bit dry or tastes bit off or the carto is getting unusually warm, I will take the carto off the battery, open it, add 6-8 drops or whatever it seems to want (I will either hold a bit of tissue against the threaded end hole just in case, or skimp on how many drops I add to make sure I avoid drips if I have no tissue). I check to make sure the threaded end is dry, give it a couple of puffs into the threaded end to make sure the air channel is clear, and continue to vape.
Note - it is easy for me to "open" my main cartos for adding juice because I replace the white end cap with a plastic or delrin drip tip (I get them at LiteCigUSA.net). So I can easily pull that out with my fingers when I add juice and it keeps my lips cool if the carto is hot.