The slide isn't installed correctly. Unscrew the top of the carb, and make sure the slide drops all the way down. Make sure it's installed with the cutaway towards the rear (airbox). Once it's installed correctly, you'll have no slack in the cable and you'll be able to hear the slide snap close when you snap the throttle shut.
The XR80 wheel is a direct bolt-on to the XR100, so the XR100 wheel will be a direct bolt-on to the XR80. Only factor might be clearance at the fender -- I don't know if the XR80 forks are shorter than the XR100 forks? That should be pretty easy to check, though. Let me know if you're looking to sell your XR80 front wheel.