you might have an air leak where the carb goes onto the intake manifold.
normally, you only need the choke for cold starting. you should never have to close the choke after the engine is running.
in fact, i never need to use the choke to get mine started unless it's like 50 degrees out.
get some starting fluid and spray a little around the area where the carb goes onto the intake manifold with the engine running. just spray a little bit of starting fluid at the joint.
if the engine races up, you have an air leak there.