It looks like you have the same carb as me, just different intake and air cleaner.
The rubber I was refering to, I think is peened into the end of the brass slug although I could be thinking of a different carb. I don't recall any means of accessing the choke from the bottom other than maybe driving out a plug of some sort. I would think that if you can see through this area from top to bottom, you really have a problem.
Hopefuly, someone with a carb in hand will try to help, if not, I will pull one of mine and try to describe the mechanism in detail.