I just bought a 2008 toro z5000 with a Koehler courage 21 engine. It ran a little rough and wanted to die when I bought it. I cleaned the carb, put a new plug, and changed the oil. Now it will mow, and does great until it hits even a slightly thick area. Also when I really bog it down, or when I just throttle it down as I do every time before I kill it, it will blow smoke. It appears to be blue, and I can smell it burning oil occasionally. It did leak oil before I made a new valve gasket, so it could be residual on the smell.