Unless the diaphragm retainer is causing an issue itself, I tend to keep them in, or more often, replace them with a small flat clip (Like the one used in the G4 carby)
By memory, A is not the normal setting, I believe your mower should be "C" for the poppet valve.
An air leak on either of the intake seals, or the starter seal, will case the fuel mix to lean out, and this will in turn lead to severe over revving. If you dry out your carburettor when turning the mower off (By turning the fuel tap off and letting the mower "run-Dry" you would recognise the quick rev increase just as it dies, this is the mix leaning out, a leaky seal would mean severe excessive revs, and little control on the throttle, as well as often being harder to stop.
