Bing diaphragm replacement question

[rolyak]

'77 R100S 90K- Noticed fuel? stain at the left carb top and diaphragm interface, removed the diaphragm but found no tears however it was swollen and could not be reinstalled. Picked up a new one from Tony at RePsycle BMW, installed it and noticed it seems to run smoother (imagination?) although I had no complaints before?
Recently pulled 50.9 mpg.

I'm guessing the P/O had new diaphragms installed at the 10K service (about 3K ago) so wondering if swelling is normal due to exposure to crankcase vapors and fuel. My question is; would the swelling alter the vacuum in any way? BTW I plan do to replace right side also.
Tom

 

[20774]

Tom -

At a minimum, the swelling will change the way the diaphragm responds to the forces of the vacuum...it could be stiffer or looser. At any rate, they shouldn't swell, even with the new gasoline blends. So, maybe the previous diaphagms are of a non-standard variety?

 

[lkchris]

Have never encountered diaphragm swelling so would suspect aftermarket part.