Blow out the atomizer into a tissue if it is gurgling and make sure the threaded connection (both sides) is dry. If it is crackling that is good - leave it alone.
There are few prefilled cartos for 510 because it is born as a 3-piece but 510 threading has the largest assortment of blank cartos. And the most popular blanks are Boge. Slim batteries can only use the standard resistance ones, 2.5 ohms or higher, typically closer to 2.8-3.2 ohms. The hotter LR low resistance ones are only meant for batteries 450mah or larger (like Ego, Riva 510, and mods)
I use Boge standard resistance on my slim 510 manuals and fat-battery 750mah Riva 510 (Riva 510 from Liberty-Flights.com since they have the latest model which is higher voltage - and fyi under $30 for a kit of 2 with their 20% off)
For your KR808D-1 you can get cartos at Vapor4Life, Smokeless Image (Volt), Bloogplanet (Maxxfusion). Halocigs (G6) and elsewhere, again anything down to 2.5 ohm (Vapor4Life WOW carts are 2.5 ohm)
I vape both manual 510s (w/ Boge cartos) and manual KR808D-1 - I love both - have both sitting on my desk here with contrasting flavors.
And just FYI - here is how I fill blanks and top off prefills (same way I add juice when vaping)
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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.