I bow to your vast experience and include a pic of my disassembled pump, in case you can see something I don't. For a minute I thought I had erroneously used the air cut off valve spring in the accelerator pump, which is shorter, but no such luck. It is slightly narrower so would not fit properly. And in any case, the maximum travel of the rod is set by its diaphragm end, 'bottoming' internally against the pump case (pushing on the brass valve bottom) ... it is in no way limited by the spring.
Puzzling ...
