Had both heads off recently on my X1 for various things , one reason being duff stem seals . After scraping carbon from the combustion chambers and removing the valves, mild pitting was evident on each heads' inlet seats but not the valves, but on the exhaust seats there was none, but the valves had it .The pitting was easily removed by facing the valves on a stone and by lapping in the seats , just wondered if the inlet valves and seats were different material to the exhaust, both valves appear to be stainless steel .(stock and original) . Modern fuels have been blamed for this , but i used to run a 1200 shovel when 4 star was still available and having rebuilt that a time or two can remember the same phenomenon. It's not a major problem but thought it may affect air cooled engines in general, especially hot running ones .

Hi mmmmm could be air cleaner suking in rain water,dont think its any thing to worry about if its removed by a bit of laping in. sounds like you are a bit like me ......BUT I NEED TO KNOW ! I would have thought the exhaust valves would pit because of the heat and Crap fuel we all have to use, well thats my theory