John
I never succeeded in getting the spring back on. The only way would be to remove the butterfly first by Dremeling off the split screw at the bottom, then shove the axle out, but generally this is not recommended as getting the valve back in with a new screw would be difficult to secure.
Loosing that screw by vibration would mean an instantaneous demolishing of Yr engine... a new carb would be cheaper, alas.....