Though I had cleaned the tank up to the conditions shown in the first foto, the gasoline had the color of the second photo. In this case we had no sludge, but a thin powder that was circulating in the whole circuit. The greatest amount was collected in the bottom of the cup of the fuel filter. The thing finished when we changed the two metal hoses (to and from the engine) and extracted the fuel from the tank several times using the fuel pump itself, filtering the fuel and obtaining a liquid each time more clear. At last the engine was perfect.