get some carb cleaner and spray it all around the intake and where the carb mounts. you might have a bad casting in the manifold with a pinhole leak. if you have to keep it throttled to run, keep it at the minimum, like where it should idle. if there's a leak, it'll sputter and die when the carb cleaner hits it.

also, this may sound totally stupid, but make sure your choke isn't stuck on.

the factory setting on the idle screw is 3-4 turns out. try loosening it a little more.