I searched the net for the best value twist grip throttle for my project bikes.. spent a lot of time on EBay but the best deal I found was this throttle on Dennis Kirk! I bought two and I am very satisfied with the quality and operation of these throttles.
Will not work on Yamaha xs650 without heavy modification. Without modification there isn't enough throw for your throttle...you won't even be able to hook it up. I cut about 3/8 out of the stop (all that I thought was safe) and I was still short by an inch or more from being able to hook up to my carb.
While it looks really cool its is a very cheaply made part.