harphunt, you connect the vent line to the muffler to pressurize the tank. That gives you a more even run from full to empty tank. w/o pressure, you end up running too rich @ the beginning, to keep from going too lean @ the end of the flight. Some engines won't run w/o pressure, or they'll run un-reliably. You can get pressure fittings @ a hobby shop, or @ an on-line dealer...You just gotta tap the muffler for the fitting.