I'm running an OS 61VF, 12x8 apc prop, Hatori 605 pipe and a 10 ounce tank in an MK Silent 90. I can get about 8 minutes maximum flight time, but that is flying a turnaround schedule and not WOT. More fuel does not always equal more power. My YS 60FRs would use slightly (~1 oz) more fuel than my OS 61VFs with the stock (with insert) carb (same prop and fuel) yet the YS would SIGNIFICANTLY out pull the OS. I think you're probably wasting fuel - is the engine piped or muffled? How much exhaust residue is on the airplane after a flight?