Are you able to visually see that the tank is empty? I don't think that the mixture being a little rich would cause a drastic change in flight times. My guess would be the clunk fell off and it isn't drawing all the fuel (unless you can see that the tank is dry). I've got a .46 that I always run rich and can easily get 18 minutes out of a 10 oz tank. If it was that rich it wouldn't be running remotely right.