are you saying as you Lower the Stick from Full Throttle to Idle the RPMs come down but stay high when it should be Idling, then the RPM slowly come down to a lower Idle,, ??
if you say yes the Idle could be to lean or it has an Air Leak somewhere, try tightening all bolts, on the Head and Backplate, make sure the Carburetor is not leaking

Jim