Ok getting back to this one, thinking about it I'm beginning to think it is a design problem with these in the fact that the crank breather just discharges into the filter chamber and as Tyler suggests maybe an oil mist gets on the inside of the filter. Most other mowers that vent into the filter area have a shield that contains the vent vapours and this is directed straight into the carby throat. This is not the case with these later Briggs motors. As a trial when I get the new filters I will remove the vent from the filter housing, plug it and run the vent tube away from the motor and see if this solves the problem. As a long term fix I think the crank vent would need to be fitted directly into the filter housing/carby throat. I will have to investigate how I can do this.
