It's happening with 2 different flavors neither are dark colored. One is completely clear. They are sweet flavors though. I'm not so much worried about juice coming up the mouth piece as I am about the blank gunk coming up
Well, I did say "or a sweet juice." That's the nature of heating up juices that are sweetened. They caramelize. Think of taking a bowl of sugar water and boiling off the water to make candy. If you go to long it gets burnt and "gunky". It's the same process going on inside any e-cig equipment but on a mini scale inside.
Again, change out the coil more frequently, or change flavors. That's about your only recourse.
I've had this with Kanger, Aspire, and RDA's (with cotton wicks), with various PG/VG ratios (higher VG will do it by itself because it is somewhat sweetened), with and without nicotine. So it's not the wicking material.
About the only way to stop it is use straight (100%) unflavored PG. Remember this things we use are just miniature fog machines. Real fog machines use PG, but have a temperature sensor to turn itself off when the heating element gets to hot and prevents the PG from burning.