If when flying your airplane, the carb draws all of the fuel out of the tank, there is no need to invert your aircraft to defuel. Just plug you fuel pump into the T that you have coming off of your carb line and suck away. You will draw every drop out of your tank from the clunk that serves as your pick up line going to the carb. If you can't drain your tank this way, then your clunk is not far enough to the rear of your fuel tank. There is never a need to invert your aircraft and drain from the vent line. You will not suck air from the carb. I have always used the two line system on my gas engines, with a T coming off of the pick up line for fueling and defueling. Never have I given it a second thought because it has always worked flawlessly.