VG is the thick one. PG is the thin one and lets through the most TH. but thin juices work best/easiest in most cartomizers and tanks.
What are you vaping now? What PG/VG?
You can lower TH with
less nicotine
less heat (i.e. don't use low resistance)
more VG
For a nice vape without too much TH, try Boge 510 3 ohm (standard resistance) cartomizers with 80PG/20VG juice and see how that is. In general these should give less TH than a 510 atomizer, especially less than a 510 LR atomizer. The carto smooths out the vape. I use the 3 ohm ones. And if it is still too much TH then consider trying 1 level lower nicotine. You won't have a nic fit. Once you are away from the additional addictive chemicals in cigs, nicotine alone is not as addictive. The important thing is to have a TH that is comfortable to you and a vaping method that works well and consistently - if it feels good you can take bigger drags - you will get enough nic.
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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 (per ml of carto capacity and these are 1ml cartos) onto the inner wall of the carto so it slides down into the stuffing while I rotate the carto. Note - the first few drops may take longer to wet the stuffing and soak in - wait for those to soak in before putting in the rest of the 18). Then after the 18 I wait 15-30 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 a couple of 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.