[Sirc]

although sacrificial, there is one approach which, apart from very carefully machining out the seized idle mixture screw** and then hooking out the remaining brass thread shreds*, is the best method for me after all these years, and that is to remove the top part of the tubular extension cast into the carb body leaving the top part of the idle mixture screw/jet exposed. Only now can you finally get hold of the screw and start to carefully free it off. Best method for me was to file to opposing flats so that a tight-fitting open ended spanner could be used. How much of the **you remove is a careful decision as you do have to encroach on the threaded part of the idle mixture screw in order to expose enough to get a grip on.

Heat can help sometimes with lightly seized screws but rarely had success myself, plus risk of distortion and permanent damage.

* a time-consuming horrible task, and the last part isn't always successful - reluctantly I would then use a tap but unless you have cleared the majority of shreds there is no guarantee that the tap will pick up the thread correctly in such soft material.