It is part #11. They are large rubber things under the top cover of the carb. If you haven't had the top covers off yet be careful with it. Use a good proper fitting screwdriver. Anti-seize the screws when you put them back together. You may consider new screws if the ones you have are beginning to show their age. It is best to have the carbs off to take the tops off.
Once you get the slides out hold them up to a strong light and stretch them. You are looking for holes in the rubber.
I'm sorry but I don't think poor running after twenty mins is a symptom of diaphragim problems. Oh, and I don't know how well you will do from other sources but I found one that was almost $40 each.