I have to agree with the above advice. The Express carb is one of the most tempermental I have ever worked on.
Be sure to clean Everything very well, and follow the proper method for setting the screws.
The manuals are a little unclear on the subject, and the instructions sound like the opposite of what you think you need to do, but if you follow them they do in fact work.
When adjusting, you usually need to turn the throttle stop screw outwards until the bike is just bairly still running, running at such a low idle that you're amazed it doesnt die, and making that click click click sound the spring starter makes when the engine's on the brink of stopping.
Then you can do the final adjustment on the idle mixture screw, and turn the throttle stop screw back to where it belongs.