But, you also must keep in mind that later E.P.A. mandated carbs like these don't show much change when adjusting their mix screws, not like older less pollution controlled carbs. On those older carbs, you go a little past the best running "sweet spot" in either direction and the idle speed drops off and the cylinder starts stumbling or running rough. That usually doesn't happen on later pollution controlled carbs. You may not see any change until the mix screw is nearly closed or way far open (like 4 or 5 turns). So, for tuning these later carbs by ear, I mostly go by plug color and use small throttle blips to test my setting. Get the setting too lean and the throttle blips will produce a hanging idle or lots of popping. Too rich will blacken the plugs.