It could be that you are running too lean when the choke is open. Meaning that when you pull the knob out for the choke you are actually closing the choke plate creating a richer mixture for start up. As the engine warms and the choke opens it creates a leaner mixture as more air is allowed in. So when you state that it will only run on the choke, it sounds like the engine needs that richer fuel mixture. Have you adjusted the mixture screw? I do not know a lot about the carbs on these atv's, but I think a carb is a carb no matter the application.