That is not the mixture screw, its the float adjustment screw or needle valve screw, if its leaking then you have to remove the carburettor top hat, punch out the pin holding the float assembly and screw the needle valve inwards till it removes itself, change the O ring and put it back, the little glass you see on the side of the carburettor is the sight glass to adjust the fuel level from that screw, Once its set you never have to change setting.
The jets on this carburettor cannot be unscrewed, they are punched into the body, and they do not enlarge themselves overtime as petrol is not abrasive, the steel plate with two screws you see below the sight glass is there to cover the cavity that aim straight to the main jets, thats how the factory was able to punch them in - at best you can aim carburettor cleaner and blow them out. The slow/idle jet is in the centre of the carburettor and can be unscrewed after removing the top hat.
The usual and most problematic culprit that causes fuel usage in this engine is clogged air bleeds/emulsion circuits in the carby top hat and messed up fuel float level, excessive float will cause it to suck fuel like its free of charge - clogged air bleeds will cause raw fuel to be sucked into the throats making it even worse - Also make sure the secondary throttle is opening up when signalled, if its jammed or not responding then your engine will bog down at high throttle application. In correct tune it should pull very quickly and smoothly at maximum accelerator.
btw - the idle mixture screw is the tiny screw which is located in the base of the carburettor and faces the rear of the car.
IMO - its best to remove the carby from the engine (easy 4 nut removal job) and completely clean it out and check adjustments properly. - You will need to find a mechanic who understands carburettors, Most mechanics today are just parts fitters who follow "ye sensor woh sensor"
