Date registered: Aug 2005
Vehicle: 1972 250C, Ducati ST
Location: Norfolk, Virginia
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RE: air filter
I have the same engine, and you're correct - it's not designed to have an oil-soaked filter. Is this a recent development?
What's happening is that your PCV (Positive Crankcase Ventilation) system is failing. It's designed to feed fresh filtered air into the crankcase, and then air from the crankcase that has been contaminated with combustion blow-by gets sucked into the air filter and re-burnt.
Excess crankcase pressure can cause oil to get forced back through the fresh-air intake tube into the air filter. The two ways this can happen are either the PCV valve has failed and clogged shut, or there's excess combustion blow-by from worn piston rings.
If this came on "all of a sudden", it's probably the PCV valve. If replacing the valve doesn't solve the problem, I'd suggest having a compression test done to see if one or more of the pistons have excessive ring wear.