So I just did a full tear down and rebuild on my carbs and I was wondering why my throttle cable is stiff and won't spring back. This is my first bike and first rebuild so I know it could be something dumb....but I'd still like to know what I did wrong

 

there is a big chance that the cables going from the handle bars to the throttlehouse is laying in a wrong turn, or is a bit squeezed between the valvecover and the airbox.
Happens to me all the time.
Unless you forgot to mount the spring on the throttlehouse
Without the cables mounted the throttlebodies closed firmly by themselves?