Craftsman ZTS 7500 w Briggs 31M977 motor 20hp single cylinder ohv

mower was running perfect then as I went down an incline it started smoking worse than a fog machine, as soon as I leveled out it ran fine.

then as I go on any grade, worse down or with the left side lower it smokes, as soon as it is level it quits. I have changed the gasket at the oil filler, head gasket intake valve guide seal,reed valve in the intake.

Any help or suggestions are appreciated.

Thanks, Neal