Here's stab in the dark....when you throttle off and come to a stop, something is completely shutting off your fuel supply...could there be a blockage in your carb/carbs? I did this once to a mower engine, and it worked....try lightly tapping your carb body with a small hammer. The carb float was stuck, and the tapping loosened it. Maybe try this... Empty most of your fuel from your tank, then add some fuel treatment like STP, or some carb clean-out. Start up and rev the stuff through the carbs.
