The high speed spring come into play only at 1/400. I can't clearly recall the configuration, but if incorrectly installed the "peg" that catches the leg of that springs will pass by it and not catch-and-engage. If your shutter is a Wollensak-style rim set, then filing of the step cam is a remedy. But to me, it is a false remedy IMO. That is to say if you have to resort to that, then the shutter parts are obviously worn enough that "lateral" shift in the motion of parts is additively causing slow movement. In other words, there is only so much you can do. I'm convinced that an iris shutter is one of the poorer inventions of man, but they exist only because there IS nothing else. I think they wore out after the first 5 rolls of film, and have been inaccurate ever since. Or something. I think an iris shutter would be a watchmaker's nightmare. You can spend weeks in experimenting, bending new springs, re-fabricating parts worn by the measurement of a germ's hair, and end up in an asylum before making one behave the way it's supposed to. Sometimes you have to say "it is what it is". Try not to get too annoyed when you have to open it up again for the 40th time.
Remember, I'm no shutter expert by a long shot. I'm just a fellow who refuses to be just a regular boob.