John Deere LX 280 spark plug fouling out on one of the cylinders

Every time I change spark plugs which is often one of the spark plugs is fouled out on the left cylinder, it ran great until I inadvertently put in old gas have since put on a ne carb, new fuel pump, an gas additive because according to the local John Deer gas with alcohol clabbers up the carb. It also blows out a grey smoke,

John Deere LX 280 spark plug fouling out on one of the cylinders