If all the jets and ports are clear and things like your float is adjustment and spark gap are right then you have to look elsewhere and a vacuum leak is a good place to start. I'm also thinking that if you're not getting enough voltage to or from your electronic ignition or your spark gap is to wide for the voltage to jump to ground at low rpm then it might lead to an engine shutting down. You might want to check your ignition grounds and spark gap. At high rpm's you would have a stronger spark and the bad ground issue might not show up until the engine slowed down.