Hi adeas, that carburetor is supposed to have adjustable idle mixture, but it seems the previous tenant has thrown away the spring and screwed it in hard to try to suppress the symptoms of a malfunction which was making it run extremely rich. Because it is intended to adjust only the idle mixture, not the main mixture, his efforts were presumably fruitless - though he may have prevented it from idling, and also damaged the adjustment needle and its seat.

Here is the operator's manual for the Rover 45 engine:
http://bsintek.basco.com/BriggsDocumentDisplay/flmvtLX-nfBhU7y.pdf

Here is the Illustrated Parts List for the same engine:
http://bsintek.basco.com/BriggsDocumentDisplay/18jsBLXBnfBhU7y.pdf

The carburetor is a Pulsa-Jet - you can see an exploded view of it on Page 5 of the Illustrated Parts List.

The usual cause of extremely rich mixture on old Briggs engines, is a blocked air filter element. I suggest you run a brief test: remove the element from the air filter, and inspect it for dirt build-up. Run the engine briefly without the element fitted, and see if the exhaust smoke is eliminated. Do not continue to run it without the filter: this would allow dirty air to enter the engine, and wear it out. If the underlying problem is just a dirty filter you will need to buy a new one and fit it. We can then talk about how to deal with the butchered idle mixture adjustment. However if the black smoke still occurs with the filter element removed, we will have to discuss a carburetor overhaul.