Has the carby been taken apartprior to the throttle not working properly anymore?

The only thin I can think of to stop the throttle working properly like that is either one of 3 things.

Throttle cable is kinked (unlikely)
The 3 pronged bit that has "this side down" written on it is not in the grooves in the carb body
Or the diaphragm is on backwards.

Thanks Joe.