A lot of mirrors have some bolts on the back side to tighten the actual mirror to the pivot point on the stem but it doesn't look like you have those bolts. They are probably on the inside behind the mirrior. If you can get the mirror off the housing you would have access to them and you could tighten them because I think it's the pivot point that's giving you trouble. I'm not sure how to get the mirror off though so maybe someone else can help you with that.
That's a beautiful color by the way. How do you like your Triumph?
That's a beautiful color by the way. How do you like your Triumph?