Kohler 16 HP butterfly stuck

[d12bn]

My Dixon Kohler 16 HP mower idles well but full throttle it runs rough and has black smoke. Took the plug out and it was black. Changed it same problem. Pulled the air filter and the carburetor butterfly is almost closed even at full throttle. I can open it with a screwdriver and it runs good at idle and full throttle. The choke closes all the way when choked but won't open but about 1/4 at full throttle. Any ideas?

I know nothing about this small engines but since I'm out of work I need to try to figure this out.

Thanks for any suggestions