MF, thats the auxiliary top speed mix screw - you probably need to richen the idle and variable jet side up - if you look at the top of the carby, on the rotating part there will be a brass centre. deep down the hole is a little mixture screw. given the age of the machine it should be a plain flat head. if its a mini pacman head, you will need a tool. then they came out with mini single and double D heads and now they are using a-circle mini heads.
sometimes there is a rubber bung filling the hole - flick it out. sometime there is a plastic bung pressed deep down it - head up the tip of a jewellers screwdriver, plunge it into the plastic then let it cool and pull the bung out.
Other times its glued in with epoxy - remove the whole brass jet block (larger flat head around the small hole) soak it in petrol for a few days to melt the epoxy filling the screw head.
Sometimes the factory coats the threads in threadlocker as well so the fuel doesn't work, hold it in vicegrips and heat with a butane torch. with the epoxy liquid, unscrew the mix screw completely try to clean up the threads with carby cleaner and a brass brush.
the correct setting is around 12-14 turns from lightly seated
