I second the vote for needle/nozzle adjustment issue. Since you've already loosened the needles, suggest you CAREFULLY remove one and see how the piston action is. This should narrow things down quickly.
Also, another plug here for the late lamented Dime Quarterly, the last "newsletter" before everything went totally internet. Tons of good technical info, and the editor did several articles specific to SUs. All issues are available as pdf. If you can't access the site, let me know.
https://drive.google.com/file/d/0ByCvxnHNk90SZTVlNTU1N2YtZmQ2OS00ZjQwLWI1YTItMzlkZmIwNmZhZjJk/view?usp=sharing
https://drive.google.com/file/d/0ByCvxnHNk90SYWExMmM2YjMtMDNkMS00MDM0LTljZDQtNDU4ZGFlYjc5YjI2/view?usp=sharing
ISSUE INDEX -- https://drive.google.com/drive/u/0/folders/0ByCvxnHNk90SYzc4N2E1MWEtMzg0MC00YTE4LTkxZGQtM2RjODA5ODA1YjU1?hl=en
TOPIC INDEX -- https://drive.google.com/file/d/0ByCvxnHNk90SREtNSjQ2Wm9INzg/view?usp=sharing