I'd want to take a look at the parts involved. I'd remove the dome, piston and needle. Are their shiny streaks on the needle, or is it worn flat due to misalignment with the jet? Look straight down at the top of the jet. If its worn it'll look oval. Is the needle secure in the piston? Just to get a starting point you'll need to do some investigating. Thinking a problem through is good up to a point - I'm guilty of going past that point many times myself - but the SU is so simple and easily disassembled I'd get right to it --- after water spritzing just to be sure there are no vacuum leaks. - Dave