there cant be any restricted space where the juice is in the tank and where the wick go into the cup, that needs to be exposed but it doesnt need to be as big as a bottle, the juice just needs enough space there so the wick s or ptb can do its job.
the e2 r4 unmodded at the proper ohm can work ok with some thicker juices and not others but performs pretty well with thinner juices but it has the issue of the new sleeve, that new sleev makes it even tighter and for some reason restricts proper juice flow, it can be a combination of wick issue and that its to small. the mods seem to prove this. but the r4 with cut wick and ptb also works well for me but i like the syringe mod better, its slightly wider and its see through and works better becuase its a little wider, the bigger tank mods work very well and let the wick do its job. I dont see why microcig doesnt just make plastic cases that are wider and be done with it, whats the hold up.