I hate electric chokes. I have had nothing but problems with them. Go manual, or a water choke.

Depending on your application a manual choke might not be what you want to do. No where to mount the pull knob. And the water one may be a real plumbing pain the arse to install. Given that and the fact you may want to keep your electric choke, try this:

Take the coke housing off with those three little screws. try running a hot lead to it, and ground the case of the choke. See if the spring starts to move when it heats up. If it doesn't its probably defective. If it does check to see if your choke linkage moves freely.

Chime in on this someone please as I don't know if my testing method will damage the heat coil in the choke. As I am unsure if it takes 12V line voltage or if there is a resistor inline some where.

EDIT:
See what you have for voltage at the vehicles lead for the choke. Try running a hot wire to your choke with out pulling it off first and see if it opens. You may not have power on your choke wire.