I have the honda GXV390, on a Snapper. Has ran grreat year after year. This year it began to smoke, on ruff terrain or turning. It does not constantly smoke. Recently it slowed to a idle while mowing. I switched it off and finished with another mower. Later, I pulled the valve cover and found a broke rocker arm. I replaced both rocker arms, and reset the valves at .006 intake, and .008 exhaust. I also replaced the valve seats and valve seals. Again running perfect, except smoking more regular. What is causing the smoke?