Certified CarbonCubs do also have oil breather tube residue buildup problems.
http://www.google.ca/url?sa=t&rct=j&...eA4ILEPM_d4uOA
And they don't look to have any valve on the breather tube.
Why put a valve if they don't??
The way the breather tube is oriented in the exhaust pipe should only create vacuum.
On a single exhaust pipe, why not put a 90 degree bend in to the flow?