I had to re-think this problem, you said it would not slow down.. From that I assume the twist grip was able to pull up the slide and the cable is still connected in the slide, the slide is the round cylinder that lives in the carb.
Mayby the cable/shield need lubbing or the slide is sticking? I thought to pass this on and avoid for you having to open the upper carb section.
You should hear a click when slide returns to home position on mine I hear a nice snap from the slide and twist grip after letting the twist grip go. Motor off of course.
It would be highly unlikely that the slide's massive return spring could break. Keep us posted