Air cut off valve leaking

[Bombasstic]

I have been hunting a cause of my GL 1100 #4 cylinder spark plug burning sooty black after a relatively short ride . . . it starts mis-firing.
As per my reading of the plug the mix is too rich.
I have found the air cut off diaphragm to be perished and with a small tear.
I have been told that this could indeed create a rich mix ?
If anyone has knowledge of this and could confirm it, I will be most grateful.
Thank you

[JoBoy68]

OK the air cut off valve works on vacume and its job is to stop backfiring so if there is a rip you have a vacume leak and they can cause many weird problems