tryin to break in new engine, ruff idle

Have you checked float levels?? if one or both are too high, it could be flooding, and that will cause a hiccup at fast idle, as you described, and will likely die at idle.. just a thought.. if you have a full face shield,, look down the carb while it's running, to see if fuel is dripping down the throats, any dripping will be your hiccup... WARNING.. that hiccup is a backfire, and will burn your eye-brows off, without the face shield..

Definition of experience,.... the ability to recognize when you're about to make a mistake.................... AGAIN...