Today I was flying side by side with a friend's Super Cub and he called me on the radio and said "every now and then I see a white vapor streaming back from your right wing." I asked him to move in closer to my SS7 Kitfox and see if he could tell exactly where it was coming from. He did that and said "every so often I see a spurt of fuel shoot forward out of the fuel cap vent tube on the right wing; then it vaporizes and streams back". I immediately checked my fuel sight gauges and saw the right tank showed full and the left tank about 2/3 full. Needless to say I was concerned and immediately turned for home and landed without incident. By the way, the air was somewhat bumpy today, but not what I consider very bad at all. Both tank fuel caps were on tight and proper with fairly new good (the blue BMW or Mercedes) gaskets. Both vent tubes were clear and open.

I had taken off for that flight with both tanks full, but not overly full. I have noticed since day 1 (175 hrs ago) that the right tank always seems to stay much fuller than the left during flight. Eventually the left tank shows empty, but the plane keeps on flying just fine as the right tank draws down. I had quit being concerned about it until today. The actual amount of fuel venting out of the forward facing tube must be very small because I have never noticed any more than a normal fuel consumption for any flight.

In thinking about this, I am sure it is impossible for the forward facing vent tubes to pressurize each tank exactly equally with the corkscrewing airstream coming off the propeller along with other small differences on each side. So a somewhat unequal draindown of each tank is to be expected. But actually losing fuel out the vent really bothers me. I suspect that after both tanks are down to the equivalent of about 1/2 each this burping of fuel out the vent probably stops. I also know that during an aggressive slip it is normal to vent some fuel from the high side wing tank.

My question to you smart people out there is: Have any of you noticed this during straight & level flight maybe on a somewhat bumpy day? Is there anything that can be done about it other than installing fuel shutoff valves in each tank line? I hate to go the valve route because then I will have to actively manage my fuel system.