Why does my Generac generator engine sputter and die when I turn off the choke?

I start it with full choke, run it at half choke and it runs well. It carries an electrical load well. I can run it like this for hours and it doesn't skip a beat. As soon as I try to move the choke from half to run (no choke) position the engine sputters and dies with and without a load on the engine.