Those air bubbles going for the regulator to the carb are normal. The YS goes from injecting to drawing fuel at idle (below 2400RPM). I wouldn't use a different plug than an OS F or YS 4 as I learned that one the hard way but I am wondering why you are on your third plug? Are they fouling?

Before we dig into what the issue might be can you give us some info on your set up? Fuel? Prop? Idle RPM before it dies and what the transition is like? WHat did yo clean and soak the engine in?