A part of a carb service on a 2005 GS500e I am intending to do the 'recommended' pilot jet upjet from 17.25 to 20, but will I also need to make any adjustment to the idle mixture ? Cheers
Will you have to? No. But you might find that the idle mixture is not optimal even with the rejet and you may eventually decide you want to adjust the idle.
But if the bike runs and idles with 17.5 jets it will run and idle with 20s with no problems that must be remedied.
Now if it were me and I was taking the carbs apart that far, I'd replace all the o-rings including the idle mixture needle o-rings, in which case you would definitely have to adjust the idle mixture to get the bike to run close to right since you'd have had those needles out.