I have a 1978 Columbia Commuter with a Sachs engine and it runs like the throttle is wide open. Speed can only be controlled by the brakes (after it is run up to speed).
I have replaced the throttle cable with a cable from mopedwarehouse but the problem continues. The throttle grip does move about 1/4 turn, and that corresponds with how far the carburetor jet travels. I'm guessing the the fault is not the cable, but an air leak that keeps the motor humming at full speed once it gets there.
It will idle at a low rpm when I first start it--but after I've accelerated, it stays at higher rpms. Nothing seems to be stuck in the carburetor.
Are these your symptoms?