More nitro will result in shorter flight time. With more nitro, the engine needs a richer mixture to run "right". It will sound the same, but fuel consumption will be a little higher (and it will make a little more power). My two Irvine 53s used quite a lot of fuel when new, but during the first gallon of fuel I could gradually lean the needle valve. My Four-Star 40/Irvine 53 could stay in the air for more than 20 minutes on an 8 oz tank. I agree with oldvet70; your engine might be running too rich. Most of my engines have required that both the high speed needle and the low speed needle be leaned by quite a bit during the first one or two gallons of fuel. When they get properly run-in, they run on a much leaner carb setting than when new.

Good luck, and let us know how it works out.