First off, Want to quit, if you run them under the kitchen sprinkler hose, very easy water pressure at the hottest temp your faucet puts out you can see the water and residual juice running out the hole in the battery end. I usually run them for about ten minutes and I keep them elevated so the water can run freely out the hole in the batt end. Then I give them a good shake and suck on the battery end to see if I taste any of the old juice flavor. If I do, most likely the menthols, I continue the process. Then I let the cartos drain with the lip side down on a towel for about a half hour. Then I put them battery side down on a piece of tinfoil on a cookie sheet in the oven and bake at 300 degrees for 1 hour. I turn the oven off after an hour, leave the cartos in there to cool and go to bed. Next morning I fill the ones I want with juice and have no problems. some of the cartos are on their second washing.
Here is a few things I've noticed. With a carto that has been washed there is some compression of the filler. What this means is that the carto will take less juice than a fresh, new carto. The juice will take longer to reach the bottom where the coil is as the filler is compressed. It's best to fill a washed carto and let it set for a couple hours and top it off again so it fills completely and when you add the juice the filler will re-expand back to it's origional volume. Some have filled washed cartos till the drips come out the bottom and then talk about a burned taste. this is because of the compressed filler and the additional time for the juice to settle and the filler to re-expand.
Hope this helps.