If your carto's have a removable plug at the mouthpiece you can get most of the juice out of them quite easily using the "Reverse Taryn Spin". It's also fun.
I use one of those pencil grips that the kids have ( the soft slightly gooey feeling one at the moment) and a lanyard for my spinner. I stick the lanyard's clip through one end about a 1/4" from the end.
* Pull the white plug out of the mouthpiece end using a paperclip or dentist's pick (or equivalent). (I use the 510 cartos)
* Take a long plastic end-cover (condom) and put it on the mouthpiece end about half way
* Put your spinner on the threaded end. No condom here. I push it down about a 1/2"
* Spin your lanyard round and round quite briskly for about 10 to 20 seconds and check out the condom. It should now have about half filled your condom with juice. Pour it into your mixing bottle
* Put the condom back on and start spinning it again for 10 to 15 seconds. It should now have more juice in there, but not quite as much as the first time as it has to pull it up from the battery end of the carto. Empty it into your mixing bottle
* Do it again if you want, but you might have most of it out by now. Give it 20 to 30 seconds this time
* Your carto's filler material should look pretty clean by now.
* Mix your new juice to the strength you want and refill your carto.
**** I use the "Regular Taryn Spin" when filling my carto's. On my regular 510 cartos I get around 40 drops of PG and only 30 of VG in each one when new.
Hope this helps.