Cable wonkiness

[lph]

(The slack is between the plastic thing under the bottom bracket that holds the cables and the shift lever, somewhere.

The spring that pulls the cable taut(ish) after you've shifted gears is in the derailleur. If the cable is tight between the derailleur and the bottom bracket, then the der is working and as you said, the slack is elsewhere.

Check one more time that the cable moves freely and smoothly under the bottom bracket. I found out after a ton of trouble that my cable had dug a v-shaped gouge into the plastic bottom bracket thingy, and was sticking there. I fixed it by taking the cable out and reshaping the gouge into a u-shape carefully with a thin drill bit.

Otherwise I'd suspect a fraying cable too. When that happens a cable can grow more and more slack as it gradually unravels. Usually happens inside the shifter lever, but I don't know your type of levers well.

PS. Barrel adjusters work by lengthening the housing, i.e. giving the cable a longer way to travel, ie. tightening the cable, when you screw them out. So start by screwing the barrel adjusters all the way in, clockwise* (you can see them growing longer or shorter after a few turns, you want them as short as possible) before you start tightening the cable by hand. If you've screwed them a bit too far out in advance they de-attach and can be a little fiddly to re-attach. Check by wiggling it sideways.

(*I get the feeling I may be confusing barrel adjusters on brake and gear wires here. Please correct me if I'm remembering wrong, fellow mechanics )