Work your twist grip, it will pull the cable back into the sheeth, that is acceleration, and if you let go of the grip and pull on the cable that is return to idle, that is what the spring will do, return to idle, the sheeth should be fastened so it is held still, but not smashed into the cable, that would cause drag, you don't wt that, the cable must move freely back, and forth. The lever that you anchor the cable to ,when you pull it, it makes the motor go faster, the cable does the same thing, and the spring will pull the lever back to the idle position, sometimes you have to be creative and make a spot to hook the other end of the spring, I usually purchase the springs I use for return to idle springs from my local hardware store, but in a pinch I found that those springs on those wired desk lamps work really good, remember it is very important to have you idle speed set to kill, if a child is riding the bike, that way they can let go of the twist grip and the motor will stall.