I've been using a Hayes 3 oz tank on my Flite Streak. At first I could not get a good run - the engine leaned out to a runaway after a lap or so no matter how rich I set it on the ground. The tank was vented to atmosphere and the engine was an FP25. The tank was mounted with the "foot" against the fuselage. To keep the engine run time down to about 7 minutes, I was short tanking - only about 2 oz.
After much trouble shooting with no improvement, I realized that with the short tank, on the ground the vent port was uncovered so it acted like a conventionally vented tank. In flight, the fuel was thrown outboard, covering the vent, thus becoming a uniflow. I tested this theory by filling the tank so that the vent was covered on the ground as well as in flight. Nice consistant runs of almost 9 minutes.
I now have the tank mounted with the foot down and muffler pressure to the top port on the tank. Engine runs are consistent with only a little leaning out in flight. Tank is centered on the venturi.
Steve