I have a sachs commuter....If the cable slipped out the return spring would stop the slide from reving at all. ..I bet the cable is frayed , grip is hanging up , cable came out of the hole on the grip mount or the return spring is broke..
REmove left side cover...there should be 2 screws in the top of
the carb where the thottle cable goes in .....loosen these screws slowly and equally until the top and the slide assem. come out.
you should be able to see the problem if it is in the carb..be careful. there are clips and washers to keep track of.
this site www.themopedjunkyard.com/ has some pics under sachs/carb or bing carb