One thing I'll add...some of the YF models I've seen have suffered warped diaphragm bonnets. What happened here was some person over-tightened the 4 screws too tight and that caused the 4 ends to compress. This leaves the center of the bonnet higher...but it gets worse. In some kits you'll find a fiber gasket made to correct this situation. It actually allows the ends to compress more and increases the warping. If the center section of the bonnet does not seal against the float bowl recess at all points the ever present vacuum in there will suck raw gas right into the manifold.

You can see here: https://tinyurl.com/2p8ff69b

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end