Yea a lot of guys have the problem of the throttle not returning to zero, I added, and most add, an extra spring to the throttle. I show a picture of it, but you drill a small hole at the top of the throttle plate there, attach one spring end there and the other back to a hole on the lever. You don't need a super strong spring, just enough to pull the throttle back, not stiff enough to make it harder to twist.
Good luck!:thumbsup: