This is a quick video showing how to replace the slide diaphragm on a Mikuni BST40 flat slide carburetor as found on the Yamaha Virago XV750 and XV1100 from 1988 up. This part (slide and diaphragm) is available from Yamaha for around $189 - seeing as the bike requires two, this is an expensive proposition.
I purchased a 'replacement' slide and diaphragm from Amazon which turned out to be nowhere near close (I show that part in this video for your reference).
The part that fits came from JBM Industries out of Ohio (great to see American made parts for a change). They fit well, and seem to be functioning properly. I have yet to reinstall on the bike, but I see no reason they will not behave properly, however I will update these comments when I get the carbs back on and synched up.
I am in no way associated with JBM, I have not been compensated in any way, I found the parts on line and I purchased them - I have made this video to help others who may be going through the same search that I have just gone through and can maybe save you some time and money.
UPDATE 2/28/22:
I have started up the bike and the diaphragms are working fine. I watched both slides actuate in a steady and deliberate manner. Will probably work even better once the temperatures get back above freezing.