Sounds like that's what you need to do though.(adj. the idle mix.) If it sputters, and spits fuel out the exh. then stops, then it's too rich. If it just kinda sags smoothly, and/ or shuts off suddenly, it's too lean. If you haven't touched the idle mixture though, chances are good that it's too rich. Most needles are set rich at the factory, and you should try turning it in, clockwise, an eighth of a turn at a time, until it smoothens out. Keep doing that until you get a good idle and transition. If the engine is new, it will take some running-in before you can really get a good, reliable idle. Also, make sure you have an O.S. type F plug in it.