@calum the more i think about it the more it makes sense to me, foubd couple of pictures online and all of them have brass ball fitted, im just not sure if its fixed or its with spring, also found that the part is ventilation channel to the main nozzle/mixing tube, closed by a pressed-in ball, on here:
https://dtpedia.de/mediawiki/index.php/Vergaser:_Mikuni_TM28SS
No wonder it would bog and not idle, air going strait to main nozzle and airscrew having little to no effect..