The stuffing in carts gets tired after a while - looses its fluffiness, stops wicking as well, etc. Once white stuffing does that I replace with blue foam stuffing. You can actually
buy precut pieces cheaply for standard 510/901 carts at
StrictlyEjuice.com, Flavor Shop! - give those a try and if you like blue foam, you can buy the uncut stuff as Marineland Rite-size U Foam Sleeve in the aquarium dept at Petsmart (when they have it in stock - runs out frequently at ours - I wonder why haha)
I love the blue foam. I started with the precut pieces and cut them down for my smaller M401 carts. It rarely gets a burny taste when too dry, it holds its shape, it lasts and lasts, wicks at least as well as brand new white stuffing, and easy to wash if you want to (just hold in a stream of hot tap water then pat dry on a paper towel)
Re getting juice in your mouth - the blue foam should be better at holding the juice in than than old white stuffing. And I do not have a Riva, but when I set down my cart models I set them so they are angled slightly down toward the mouthpiece (to hopefully prevent leaks) then when I pick it up to vape I do that with mouthpiece higher than battery so hopefully if any DID leak it will go in the atty and not my mouth.
I also don't overfill my carts and don't fill alot from the top. With blue foam it is easy to pop out the stuffing (I use an unbent very thin paper clip). Then I fill the cart 1/2 way with juice, put the blue foam back, push it into the liquid, reposition at the top where it should be (using paper clip) then if needed I add a drop or two to the top. I only do that a couple of times per day. The rest of the time I just top it off with 1-3 drops.