[Jim Smith]

The fuel vents are clear. I have been flying with the valve selectors in their respective normal positions without changing them for quite some time, so there should be no reason for the valves themselves to be out of correct position. From what I understand about their design, they can't accidently change positions unless manually operated and then a cable breaks. We are not going to change any position of the valves until we do a one hour test flight and top it back off to see if the condition persists. Also, from what I understand a blocked fuel vent or fuel cap would just cause starvation to the engine and not create a forced crossfeed situation. We have not checked for a complete blockage from that tank as of yet, but I don't know that if there was a complete blockage if it could cause a forcible crossfeed situation. It would seem that if this is possible, it would have been noted from past occurrences somewhere or somebody would be aware the possibility of unknowingly sucking one tank with both engines exists. I would imagine over a span of 50 years there would be numerous crashes due to this or some type of AD out on this. Understanding the plane and it's fuel system this is just quite baffling.