Watching the throttle butterfly move is an important clue when adjusting the mixture. When the mixture is right, the butterfly doesn't move much at no load. If you can't make it rich by turning the mixture screw anticlockwise, you have a fuel feed problem (dirt in the system or a bad fuel pump diaphram). You can tell when it is rich by the exhaust smell, and it slows down a bit and opens the throttle slightly. When you turn it clockwise it should run better and faster, and the throttle should be open slightly less. Then if you keep turning it clockwise the engine will slow slightly, and the throttle will open a bit then start to work back and forth quite noticeably. At that stage you might find the big speed variations you mentioned (hunting).
