Even though you mention a carb rebuild, it sounds like a carburator problem. The need for partial choke is a tip off. One subtle item with these carbs are some very small drilled passages which, if plugged, result in just the symptoms you mention. Only after I disassembled and cleaned my carb the second time, and ran some fine wire through those passages, did it run right. Suggest you try going over the carb again and look for the holes.

John