Some VG juice vendors do sell unthinned juices. I know vaporgalaxy.com does. Being a VG juice user, and DIYer, myself, this does become an issue with carts and cartos.
I assume you mean carts, as in cartidges to be used on an atty, and NOT cartos, as in cartomizers. If it is carts, then you can remove the filler, soak it in vodka or even just warm water, paper-towel dry it, and put it back in that cart. If the juice looks too thick, try adding about 5% water to it. So if it is 10 mL, add 0.5 mL of water. That may well be enough, but up to 10% is not unreasonable. Flavoring will generally thin a juice itself, and my rule of thumb is %flavor + %water = 22%. You don't know your %flavor, so start at 5% water and maybe increase it if needed.
I would also try dripping directly ~4 drops of the unthinned juice onto the atty bridge, and using either an empty cart or drip tip. You may find that the unthinned is excellent for dripping, as is my experience. You may like it better than a filled cart. But the dripping will also tell you if the atty and batt are working properly.
Original carts are not much favored by the more experienced vaping community. They leak, or do not wick well (you problem, I believe since the juice is too thick), have to be topped off all the time, a general pain. When I use an atty now, it is to drip. 99% of the time I am using a cartomizer with a drip tip. I like Boges. You did not say the model of the PV you are using, but if it is an original 510, then 2.2 - 3.0 ohm cartomizers should be fine. With larger batteries, you can go to lower resistance, for a hotter hit, without risking batt damage.