put the gearbox back together and reshim carefully the gears! it should work i had one of my aks running with the same chipped tooth on the spur for like 1000 rounds ... was waiting for a replacement one to arrive
here ya go i found you a great shimming guide!
http://www.858airsoft.com/upgrades/shim.html i know it's boring and time consuming but it's a VITAL thing to do to your aeg! hell i properly reshimmed 2 cyma mechboxes and runned them on a 11.1v lipo without a problem!