I believe the tanks are supposed to be filled with a syringe through the hole the atty pokes in them. That is the best way to avoid leaks.
Boge cartos (and BTW on a slim 510 you can only use the standard resistance ones - LR suck power too fast and can damage slim batetries. Those need 450mah+ batteries)
How I fill cartomizers...
I fill cartos as intended, by popping out the soft cap and adding 18+ drops of juice into the stuffing (for a standard 1ml-ish) carto. If your carto has an inner donut-shape seal you need to remove that too (Boges do not have that annoying seal). I hold a tissue against the threaded end hole as I drip juice into the stuffing. Some people instead of a tissue use the bottom condom and the little plug for the bottom hole in the carto - V4L cartos come with the little plug. When I am done I clean it up - clean/dry the threads, blow into the threaded end to clear the air channel (I only buy cartos with a central air channel like V4L and Boge), sop up any loose juice above the stuffing. Then I cap and lay it on different sides for 15-20 minutes to make sure every bit of stuffing is damp with juice so I don't singe the stuffing when i vape it. My juice is fairly thin (70pg/30vg and 80pg/20vg) so if your juice has more VG you may need to let the carto settle a lot longer. As I vape a carto during the day, if the vapor seems a bit light or a bit dry or the carto is getting extra warm, I add more juice. That way I don't singe them and get great reuse out of them. I also find that for me, thinner juices seem to help the carto vape well longer.
Note - if you have auto batteries then never add juice to a carto while it is on the battery and make extra extra sure the carto is not going to leak before putting it back on your battery. If a carto leaks then it will leak right over where an auto battery can be damaged.
Re flavor - some juices just aren't well made, have too little or too much flavoring. If a juice has a reasonable amount of flavoring and smells good when you smell it in the bottle, then it should be a pretty good vape. But you have to smell the vapor!! The human tongue can only taste sweet, salty, bitter, sour and umami (MSG). Everything else for vapor is aroma because unlike food, vapor has no visual or texture clues. Remember to let your nose have some vapor.
Re the actual flavor part (the sweet, sour, etc) - some things can dull that... VG, vaping the same juice continuously, eating flavorful foods or drinking hot or alcohol liquids. I find vaping a contrasting flavor or drinking some cold water help.
