Hopefully a return spring like rarefish said, is all you need.

or

The problem with the cable kits is they assume everyone has the same throttle lever on their transmission. You may need more reaction on the transmission end which would be a shorter lever.

Presently you are only making a linear adjustments. You can adjust it forward or backward and find it is always a compromise on the other end.

The solution is a ratio change. The lever needs to be longer or shorter. Short is easy because you can just drill a new hole inboard, closer to the pivot.