If I understand you correctly, the lathe runs OK in back gear, i.e. the lowest speeds, but there's no drive to the spindle in a high speed setup.
I've just found a copy of the manual, there should be an Allen bolt or similar to key the two headstock gears together. It's described in the manual as "a sliding key, locked in position by means of a cap head screw."
The bullwheel is the largest gear in the headstock. It needs engaging for direct drive. I don't have an ML7 but I just thought I'd reply so you weren't feeling left out.
Welcome to a friendly and helpful corner of the internet.
Bill
Edited By peak4 on 02/09/2019 23:07:29
Edited By peak4 on 02/09/2019 23:22:03