I have had that same problem with my bike - 2002 883 xl... before the 1200 conversion. Dont have enough miles on the conversion to see if it still does it or not. Hoping that the increased vacuum will help some. I have an S&S Style teardrop air cleaner on my bike, so it blew all over the side of the bike after it filled up the filter... Fire Hazard. At least with the HD "Ham Can" it doesnt spray all over the side of your bike
I have heard that a thunderslide kit may help, but I havent tried it personally. Also, I have read on Sportster.org that there is a vacuum vent hole you can drill out to a specific size to cause the Slide to react faster. Read a littke in this month's American Iron about that too. The vacuum operated slide sometimes doesnt react fast enough. and the gas doesnt get sucked in, causing gas in the filter... If that makes any sense - its a little late at night/early in the morning for me here...