This is an explanation of what wet sumping is on a Harley-Davidson motorcycle, and what to do to keep your Harley Davidson motorcycle from getting wet sumped. If your Harley engine hasn't been started in a few weeks, and your oil pump is old, oil can slowly fill up the sump at the bottom of the engine. If too much oil is in the sump, the Harley's engine can't pump it all back into the oil tank, so the oil goes out through the crankcase breather and out onto the motorcycle. On a stock Harley-Davidson motorcycle, the oil will come out of the air cleaner. To fix the wet sumping problem on your bike, replace the check valve in the oil pump, or replace the whole oil pump. If you start your engine every week, your Harley won't wet sump unless the oil pump check valve is bad. If a lot of oil comes out of your air cleaner, your engine is wet sumped.