There is an alternative way to stop fuel getting into the sump via the air box . Pop out the air filter and shine a torch inside and it will become clear what the problem is . There is a step inside the air box that means that any fuel will go straight down the crankcase breather instead of out of the other drain in the rear left corner. The fix is simple , extend the crankcase breather a few inches up inside the air box and job done . You'll have to find a suitable bit of tube from the scrap bin . This won't cure the source of the problem but it will save buying new oil when it happens. Talking of which, it happened to me yesterday, my bike starts spluttering and dying a hundred yards up the road so I went back and sorted it out . Before I take the carbs of I always try this first . Make sure the long drain tubes are on the float bowls , undo the drain screws and let both bowls completely empty. They should both stop flowing eventually, if not then the tap is knackered . Then while still open , switch to Prime and let more fuel flow through to wash out the debris . Close the tap to Run or Res , close the drain screws, set tap to Prime and as the bowls fill up again repeatedly whack the carbs with a rubber mallet . Works for me most of the time . Have another look in the air box to see what's going on. Good luck.