I've tried them all and here is what works best for me. I smoke a cart until I figure it's about half done, then drop a few drops of
juice into the
battery end. (Take a tiny wad of paper towelling, stuff down into threads to soak up any extra). This extends the life of the cart. Do this every so often, the cart lasts an extra day or so. When a cart is empty, I throw it in a pile for later use. Carts from this pile,I test for burnt taste, and if it is good I put 8 drops in the battery end and then put 8 to 10 in the mouthpiece end. This seems to fix air issues, eliminates a ton of ancillary hardware (syringe, eyedropper, spinning paraphenalia, etc...) and provides a pretty good smoke.
The key to refilling, no matter what method you use is:
Don't smoke the cart dry!! If you do, it will taste burnt. Took me a long time and a lot of wasted carts to figure that one out.
When I get some blue foam (for my 801 and kr808), I'm going to try a VK cart mod using it.