my 90cc back wheels keep spining when I release the throttle. what can I do to fix this used, but nice machine, before I give to my 4 yr old for christmas.

 

IMO (and others),,,Your 90cc is way too big for a 4 year old.
Sorry, no help available.

 

I agree with LT80 you want to start him/her off on a little 49 or 50cc machine with a remote or manual (lanyard) safety shut off and of course make sure he/she has a helmet & gear! A 90cc can blast me around at a good speed and i'm 180lbs! I'd say sell or trade it for a nice smaller quad.

About your problem, I'm far from an expert but I'd consider turning out the idle screw on the carb untill the wheels stop spinning. Sounds to me like the idle is just too high. but again I'm not an expert just a consideration. Good luck.