Right- If I solve it you can slip me half the price of a new TB...
Is the cable wound around the TB ring the same way as in the photo? I assume it is, I don't think it can be put on backwards. make sure everything else in the photo looks just like that.
Undo nut A right to the bottom of the thread. You can slip the rubber cover off an take the nut right off the thread, but don't run the engine with it lying loose on the cable, it may prevent the throttle closing!
Then wind nut B down as far as it will go. Watch the TB ring as you do this, and if the ring starts to rotate STOP and take B back up a bit. If the ring hasn't moved when nut B gets right down, fire it up and check what the revs do.
That should make sure that the cable opens the butterfly as soon as you push the pedal. If the ring is already moving when you touch the pedal then the problem is inside the TB. In that case you'll have to pull it all apart and post photos up here!
The return spring, if not broken, must be assembled incorrectly. Not wound around enough or mounted in the wrong hole or something simple to fix but hard to find.