Easy to explain that one :-)
As the throttle has gradulay carboned up and the rpm dropped over the years someone has compensated on the adjuster air by-pass screw , this may have happened three or four times since the throttle was last bright and shiny .. now you have restored the air flow through the throttle at idle there is too much with the additional air coming through the adjuster , so you have to turn it down back to where it should be ..
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