I think rlipsett is on the right track/asking the right questions. I in no way can imagine that even with an 8oz tank, It schilling could be only getting 5 - 6 minute flights that are running the tank completely empty. It would have to be running at a sloppy rich 4-cycle that I would question if it could get off the ground without a real fight. The plane would be a gooey mess to boot. If fuel is leaking out of the tank, either it is leaking outside the plane out of a vent or it would be in the plane somewhere and you wouldn't miss that. Are you positive that you are actually filling the tank completely.

I've got a H9 Toledo Special with a stock 11oz tank running a .46AX and I can fly it for 20 minutes using normal varied power levels. I almost never fly it that long just because I'm about tired out after 12 - 13 minutes.

Lars