Legarm,
With you timegrapher set to auto, if the beat rate comes up as 28800 then your escapement is running too fast. As others have said, this is unlikely to be a mainspring problem unless it is making nasty noises.
Check the hairspring. Are any of the coils touching each other? If so, it could be dirty or magnetized (hence the question above about rheumatism bracelets). Also, make sure that the second coil of the hairspring is not touching the regulator boot.
If the hairspring is good, i would suggest you check the train count to make sure you have all the right wheels. There is a calculator at the bottom of this page http://www.vintagewatchstraps.com/watchmovement.php. You know the answer should be 21600, so this should help you diagnose which is wrong.
S