This 90 idles great hit the throttle and it runs great, but does not idle down when you let go of the throttle. The cable is working properly, the thumb throttle not sticking,just wont slow at all without hitting the brake. Any help would be great, my son wants to ride but to dangerous til its fixed. Carb and needle have also been cleaned completely.

 

Seems to be an air leak.
Check around the carb and such.
I suspect crank seals.