Just had a quick read so forgive me if ive missed something.
Two things, i doubt the spring in the starter is broken. If its been wound the wrong way the spring can come off and will not hook back on to 'wind up'. Only way to tell is to pull it apart. I think ive had one broken spring, plenty of jumpped off one.
Second thing is, did you put a new O-ring seal on the bottom of the replacement starter before you fitted it??? If this is a bad seal it will over rev.
Will also do the same if the bottom crank seal (its part of the bearing) is no good. Sloppy bearing is usualy the give away.
The other thing would be to check over the carb for cracks or splits or wear in the diaphragm cover or body.
