Something else to look at, the throttle cable itself. Unhook the cable from the linkage and work the the linkage manually. If it's working correctly you may have a binding throttle cable/twist grip. With the cable unhooked,, twist the throttle, and see how much force it takes to pull the cable back to return the throttle to it's idle position. Remembering all you have is that little return spring to do the same for you.

On mine, I didn't really trust that little spring, so I just went ahead and added a second return spring.