There is also a cap design that came along after the tree huggers got a case of the vapors (pun intended) about venting a gas tank to the open atmosphere. It's called a "pressure/vacuum" cap. It allows a certain amount of pressure to build up in a tank and force the vapors to a charcoal canister, and it also has a vacuum-sensitive flap to allow makeup air to enter the tank as fuel is used. The problem with that type of cap on an older system is that it can cause enough pressure rise in an unvented tank to overcome the needle valve seal in the carburetor. Under high tank pressure caused by extremely hot weather or a clogged charcoal canister, the pressure/vacuum cap has a pressure override feature to allow venting to the atmosphere. Obviously, a pressure/vacuum cap should not be used on unvented tanks like stovebolts have.
Jerry
