I haven't met a gasoline engine we use for our hobby yet that uses tank pressure, the vent line on the tank is vented to the atmosphere, perhaps you are talking about the pulse line that the carb would use for case vacuum pressure to pump fuel? Get the plumbing straightened out first, and take your time setting the carb for your location, temp, altitude...etc. no factory can preset a carb and have it work fine everywere. the engine might start at those settings, but that might be about all you could expect. If you are talking about the air pressure inlet, it is also vented to the atmoshphere. The fitting is there in case airflow around the engine causes a change in atmospheric pressure, and you have a need to route this vent, say into the fuse of the plane, where the pressure is more constant. Definitely don't route this vent to your tank