Dennis. You're confusing a weak mainspring that causes a clock to not run, at all, with an undersized suspension spring that's causing it to run slow.
I don't have any KoMa midgets that have lead in the weights. they're all solid brass, IIRC.
You should just get a new suspension spring that's slightly thicker.
And... Be absolutely sure you're turning the adjuster the right direction. These are backwards from most.