Throttle adjusting screw not working


i have a 1984 charade turbo with the CB60 carby motor. There are two screws at the back of the carby to adjust idle and the air/fuel micture. The idle adjusting screw (small one) works fine but the throttle adjusting screw (big one) doesn't seem to be doing anything. If anyone knows what i'm talking about or is familiar with the engine, can u please tell me what the problem could be? Why doesn't the screw do anything?
And also where are the timing marks on this thing?