How can Change a Diaphragm on a Briggs & Startton lawn mower engine?

I have been told that because the mower starts then runs fast and the slow and drinks all the fuel that the diaphragm on Champion R484Sp Factory Ref R484TR needs to be replaced but can't find any where how to do it so I am hopeing some good person will tell me. I was told by the Technical Assistance or the Customer care desk. That I need to remove the Air Filter, don't toch the 2 springs. there is 2 bolts holding on 1 A/F 1/2 and the other 1 A/F 7/16 then remove the Rod on Carburetor there's about 6/7 screws on tank and carefully lift off. All I want to know is where is the Diaphragm, where I can buy one and how to replace it as I can't find any hand manules on my mower. When mower is going it drinks so much fuel and runs fast and slow and because of this that is why I was told it id the diaphragm that needa replacing so please can some one help us.
Thanks Vera