Hi Carl. That is the gasket I showed the image of in my post above. My opinion is it makes the warping worse in that it allows the corners of the bonnet to be drawn down further. It doesn't take much effort to warp that potmetal. The gasket was created as a "shade tree fix" of the problem I mentioned. Jon H doesn't include it in the kits he makes for the YF models and while I could make them, I don't include it in the gaskets I provide for the YF either. But Jon can tell you about that gasket and who created it.
The only YFs I've taken apart which had that gasket were those which were "rebuilt" by a commercial rebuilder. It is pretty easy to correct the warping problem using a piece of wood and some sandpaper, but I doubt a commercial rebuilder would ever take the time to do it. My experience has been once that bonnet has been warped the best fix is to make it level again and be done with it. Usually about .005" to .010" is all you need to remove, but any size opening will let gas be sucked in there.
Possibly a Viton gasket made in that shape would help, but it would need to be very thin. Even raising the height of the bonnet by 1/32" is going to increase the tension of the lower diaphragm spring and that means unless the vacuum present beneath the diaphragm increases, the mixture might be richer.
