Remove the air cleaner housing and check the throttle linkage. If your 420 is like my 380, it is likely that one or both of the nylon balls that the fore and aft shaft of the linkage rides in have disintegrated causing the shaft to no longer be captive in its slot. Thus the car starts and idles fine but will not accelerate. Cheap parts to replace. When it happened one night on my father in law's car I made a temporary fix out of coat hanger to keep the shaft captive. Mark