Hey, hey ... for those following my sub-tank blog. I've come up w/ the "Toothpick Mod' and the tank and I are now getting along.
The sub-tank head has 4 LARGE air passages. For me trying to throttle down the air flow - this was the root cause.
So I took a toothpick and ran it through 2 opposing holes. Caution: Look for 2 holes that are not obstructed with any of the coil legs that might get messed up when inserting the toothpick.
Then cut the toothpick off flush with the bottom of the head on both sides. I mean flush, as close as you can, so it does not interfere when inserting the head into the bottom section.
So here's a shot of taking an easy single mouth to lung draw and exhaling. The air ring is set to the 1 hole exposed position. I can live with this. An easy double pull is even better.
This is with a DIY 1.8Ω coil with 30g Kanthal (2.5mm id) in a standard head w/ KGD wick. Vaping on an iStick at 3.7 volts (and we all know the iStick runs hotter than set - so probably closer to 4 volts. I actually like it. May have to get a Kanger Nano sub tank now which would be a better fit size wise on the small iStick.