Is that a cafe4euro collector? Did it fit pretty snug for you? Mine doesn’t seem to fit snugly and leaks pretty bad.
You need to coat the inside of the collector with exhaust putty, coat the pipes up too then I mated up the surfaces, tightened the exhaust to its final position, wiped away any excess then initially heated it dry with the heat gun before running the bike to make sure I didn’t blow any out on start which would remove the seal - works perfect.