Good that you have solved it !
It sounds like you need to make sure the cable is oiled and has a nice smooth run.
The advance lever itself also can use a spot of lubrication.
And the plate in the magneto can have a trace of lube too, so it can advance and retard nice and smooth.
I wouldn't think 11.1 mm makes much difference to 12mm, there seems to be some variation in the recommended number.
You can experiment of course, but if it doesn't make much difference....
Have fun !