Sounds to me like an issue with the throttle assembly, somewhere between the handle bar throttle and the slide. I would check that the throttle cables are well lubed and functioning properly. Disconnect the throttle cables from the carburetor, and observe the speed of return, and ease of return, of the throttle assembly when you open and release it. This doesn't strike me as a jetting or diaphragm issue. Without actually hearing, seeing and feeling what is going on at your end, most anything from the forum will be a WAG. [Wild a_ _ guess]
My loudest suggestion would be DON"T MODIFY ANYTHING ELSE UNTIL YOU GET THIS ISSUE RESOLVED.
This does not sound like a fuel delivery issue. It sounds like the RPM's rise when the load from the engine is removed, as it should. The problem, if I understand correctly, is that the RPM's don't drop when you release the throttle. If this is accurate, I would focus on the throttle components, which runs from the handle bar, to the needle jet.
“many a trip continues long after movement in time and space have ceased”- Steinbeck, [I]"Travels with Charlie"
[FONT="Century Gothic"][I]Sometimes your only available transportation is a leap of faith[/I] [/FONT]