Unfortunately the amount of info I have on this particular machine is fairly limited, but as far as I can ascertain the horizontal screw is the throttle stop, the lower screw is for the idle mixture and even the owner's (Triumph Tigress version) handbook gives no information on settings for the (I believe?) 175 Zenith 18 MXZ carb.
The *175* carb should have a tickler button to flood the carb for starting, and I think there should be a slide choke?
I would suggest that you get copies of the workshop manual, parts book and owner's handbook from somebody like Bruce Main-Smith (they are listed) if you do not already have them.
http://www.brucemainsmith.com/If this scooter hasn't been used for a while then I think it would be a good idea to completely strip and clean the carburettor parts, fit a new spark plug, also check the ignition timing.
