I would check carb adjustment and spark plugs/wires. Todays spark plugs are not the quality that they once were(Chinese). The Z and R series are known for fowling plugs too. Seems like I end up replacing plugs in my R's and Z's every two or three years.
By the way, I have not had too good of luck with Champion plugs lately. I just pulled a set of nearly new ones out of my 670 Super today and replaced them with Autolites. I hope they work longer than the Champions!