ok i was able to open the buggered screw with a good tip from neighbour , grinded the edges of a cheapy socket.
sensor works but doesn't seem very stable. for instance, in continuity test, i read 30-90 ohms while i still have the beep, pulling the throttle wire slowly ohmage increases gradually but even when the beep stops ohmage continues to rise to around 240-300 ohms before reading turns to OL . near the beep limit if i pull the wire very slowly at some stage beep stops and starts again, it almost feels like the switch was stuck on borderline limit and got corroded over time. idling is better now after adjustment. i didn't have any feeler gauge etc.. but gave it a bit headroom and adjusted it while engine was idling, found the sweet spot where engine reacts to throttle and set it there.

i ll see how it goes.. not a cheap part for how small the bugger is. might get one if acting up continues. thanks for the help guys