the spring was the experimental exhaust brake. i wanted to see if it would help warm-up. it doesn't. but about an hour after i put the cap on, i welded a bolt with a piece of rubber glued to the head on the backside of the flapper strap. that works great (no more clanking). and it's held up a lot longer than i expected (it's been on for almost a year).
edit: stomp, are you still running 4"?