I have a 60" eXmark TTHP with a Kohler 23 hp engine on it. The engine specs are: Kohler 23hp CV23S Spec: 75559 S.N.: 3225503391. The symptoms are every time hot or cold, when I go to full throttle it runs rough like it is missing. I have noticed a little black smoke on occasion while it is doing this. One plug I forget which side is blacker than the other. I can just a consistently clear it up by throttling down and then quickly back up. After this it runs smooth until I slow it down or restart it. It also, most of the time, back fires when I shut it off. It will do this even when I let it sit at idle for ten or more seconds although it is worse when I throttle down quickly and shut it off. Once in a great while it starts hard needing choking and this is usually when it is warm. I have had the carb all the way apart and cleaned it thoroughly not to long ago. Also, I replaced the carb diaphragm for a different problem last year. Any thoughts will be appreciated.