nummy, Back for a minute to the intake . If your tractor was running when you suspected the intake was sucking leaked air, did you look at the plug coloring? I would expect a lighter , even whitish color and that the engine ran faster and lacked power. The o ring v groove sounds weird , usually the machined recesses for o rings are U shaped or square recesses. Only a little o ring protrusion above the metal is normal. Did you check for flatness with a straight edge and not overtorque?