Peter, I have also encountered problems with welding BMW motorbike castings, notably, I suppose they would be called rocker covers off a 1950's era bike.
The castings felt heavier than normal aluminium castings so that gave me some doubts from the start and I was to be proved correct. No matter what method I tried (pulse/no pulse, high AC cleaning/low AC cleaning, hybrid wave forms etc) the result finally beat me and I had to give in.
Does the casting you're welding feel noticably heavier than it should for an aluminium casting? If so, perhaps it's made of the same material.
Regards Andrew. (also from down under (Melbourne))