the next step would be to remove the carb, and inspect it and clean it. if there is a fuel blockage in the carb, from debris, the bike will run much better with choke on, because the choke adds extra fuel to teh carb. once you open the throttle, or sometimes turn the choke off, there is not nearly enough fuel to throttle up the engine, so it stalls.
you will need to remove your carb and clean it. make sure you remove all the jets inside the float bowl, and clean them very well. when the jet is clean, you should be able to see a complete circle through the jet. if the jet is clogged, you will not seee through it, or see some debris inside it.