you can replace only "HANDLE PIVOT SPRING ASSEMBLY " but not only the spring. THi site does not allow me to put a link so follow those steps
Go to goodvac.com
then Kirby Vacuum
then FAQ
then Kirby FAQ
then How to replace Kirby fan blade.
First 5 pictures will tell you how to remove Kirby shell, then under the shell you will find that "HANDLE PIVOT SPRING ASSEMBLY " that is attached with 2 screws. When installing the new one you may want to plug in and try using Kirby without the shell in case if you will need to reposition "HANDLE PIVOT SPRING ASSEMBLY ". It has to be positioned so when handle is released it comes to a neutral position and Kirby do not just move by itself. If it is a bit off then you wil have Kirby shooting forward or reverse when you let go of the handle.
Good luck
Rocky
Rocky
December 2008