Certified CarbonCubs do also have oil breather tube residue buildup problems. http://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB8QFjAA&url=http%3A%2F%2Fwww.cubcrafters.com%2Fpdfs%2Fservice-instructions%2Fsi0023.pdf&ei=nbb8VL_mOZCYyASRi4LACQ&usg=AFQjCNG-auHGEJ1P8RWMG7u73h3aFm7ecg&sig2=ToF55JmeeA4ILEPM_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?