Of the few 55AX engines I have delt with they all seem to be sluggish around mid throttle until well broken in. Seems I read this here on RCUniverse also. Mine come around after a gallon or so of fuel. I don't understand why it was only sluggish at mid throttle but it was. It acted as though it was rich except at WOT or idling and no amount of tuning the low or high side altogether fixed the problem until after a gallon or so of fuel