OK yup, I was probably splitting hairs with the pump comment. Now that I read your explanation it could make sense when the vent is exposed to a sudden impulse in the opposite direction to flow, provided there was some liquid fuel in the line from a previous impulse.. The pump would only serve to clear the line in a near steady state condition.

As for vapor trails, I don't think any one of us can definitively state which side wins this one. I don't know if Tom's fuel system would allow for it in the Firebird, but someone should try installing a check valve to allow flow in only, with a ball valve in parallel to allow air flow out for fueling. The aircraft could theoretically be flown with the ball valve open or closed to simulate both scenarios. If the vapor trails exist in one scenario, but not in the other (in roughly the same atmospheric conditions), I suppose we would have a clearer understanding of the situation.

Personally, I hope someone tries it and gets some pretty serious vapor with the on-way valve functioning...

I know its an entirely different situation, but I've flown a jet that sprung a fire hose leak of kerosene in flight. The fuel was invisible until landing, where it was gushing out of every orifice and the only indicator was the lack in top en power as the leak got worse and worse.

Either way, the JMP Firebird is one fine jet.[8D]
Kelly