Here is an off-the-wall theory from the RV spy....

I am suspicious of those cute little NACA vents you guys use for tank vents. I am thinking they may not be as effective as they need to be at higher altitudes (lower indicated airspeeds). I think maybe NACA vents work better when there is airflow through them, rather than being used to generate a static pressure.

I am thinking a good positive pressure inside the tank would reduce these problems.

My RV8 has streamlined tank vents that stick out about 1 1/2 inches into the air stream to get out of the boundary layer and generate a positive pressure inside the tank. I never turn the boost pump on except as a precaution on take-off and landing, and I have flown it for 3 hours at a time at 17,500 msl. I never turn the pump on switching tanks.

My IO-550 powered C185 has big ugly tank vent tubes hanging below the wing (hiding behind the wing struts to keep from icing up). I have flown for hours at 19,000 IFR and never touched the boost pump. I have taken off from Porterville, CA at 105 degrees and climbed to 17,500 eastbound and never touched the boost pump.

Just a thought,

John Huft (RV spy)