I think I remember a discussion about that now. No harm in trying the hot water trick before messing with the actual jet. 'Course you'll need a kit either way. I like my cheapie USC for stubborn carbs. Paid for itself with the first carb it revived.
Could stiff diaphragms be a culprit too?
What sort of USC you got Poge? Wouldn't mind 1 someday when funds allow. Would be handy for push bike parts too