I have been tod that a large percentage of dirty belly problems are traced to over filling of the crankcase (on VW conversions anyway) often attributed to the dipstick not calibrated correctly, which causes excess oil to be blown out the vent tube, and that once the oil level is adjusted to the correct level the dirty belly problems go away.
I haven't tried to see if my own crankcase is overfilled yet, so I can't confirm this rumor yet. Todd A few years ago I was having problems with a dirty belly on my kr. I got an ld oil separator from a wet to dry vacuum pump conversion I did on another ircraft. I installed the separator and ran the return line to the intake ox of the carb. The waste was recycled through the engine and voila - no ore problems. This is a very simple fix for a very annoying problem.
