If you are building with cotton and you taste cotton (or an "off" taste) your wick is too thick. Try a less dense wick.
I highly HIGHLY recommend Koh Gen Do Japanese organic cotton. It is the easiest I've used to make the perfect wick. You simply snip off a piece, just twist up the tip (leave the rest alone) and pull it thru your coil.
If it binds at all when your pulling it thru it's too thick.
After using the KGD cotton I wouldn't use the CVS again unless there was an emergency and I couldn't get any more KGD.
Here's a vid I made on it
MICRO COILS 101 - PART 3 WICKING USING KOH GEN DO JAPANESE ORGANIC COTTON - YouTube
I also use rayon and mix it up between KGD and rayon.
Again, if you can taste anything but your juice the cotton is too thick