It may be something simple like a dirty carburetor.. or something about the carb needs adjusting. It often takes 2 or 3 tries to get a carb really clean. Or it might be a few loose nuts or bolts.. Or the previous owner may have done one or another things or forgotten to do something.
There are a couple Express manuals on THIS page Study them.
On top of that page is Fred's Guide.. take a look at that.. and "Search" this forum for Express-related threads as far as specific questions..
Get really familiar with the bike ..check everything from one end to the other.. learn how to tune it up.. And build yourself a dependable, good running bike. It takes time and effort but it's worth it.