hi stephane, best way forward is to take off the carb for this job & get the use of a vice to hold the carb steady. if you have enough of the idle screw sticking out you may be able to cut a slot in it with a junior hacksaw blade, should be easy as the screw is made of soft metal. you can then unscrew it with a straight edged screwdriver. if not then use a small hammer & a sharp dot punch & tap the screw loose anti clockwise. be careful not to damage the carb body & treads. if that does'nt work you will have to drill it out will a small bit & use a tool called an easyout to get the rest of the screw out of the carb. hope this makes sense.......happy motoring.
steo
May 2009