Have a Briggs 16 hp L Head in a Bolens G16XT. Cab has been thoroughly cleaned, new kit, new fuel filter and lines and fresh fuel in the tank. It starts right up every time but when we get the carb adjusted for slightly fast idle, it won't rev up. When it is adjusted for full throttle it wont idle worth a darn. Real rough in both situations. It acts like carb adjustment situation. Do these engines normally run rough at idle ? ? My first experience with a Briggs twin. I welcome any and all suggestions.