N/A fuel caps are non-vented. They are no longer made to my knowledge which means you have to pick up a used one. The new ones on the market are for UK/EU and are vented (including locking ones). Ours (the non-vented) keep a partial vacuum in the fuel tank as gas tank fumes are sucked into the intake by way of the fuel separator system and charcoal canister(s). If you use a vented cap you will have a vacuum leak into the intake maifold (not sure how much). A friend that had got a new vented cap from UK said he could smell gas fumes and replaced it with a used non vented cap. (He lost his the same way you did). Considering how many cars went to N/A I'm surpeised they made them vented and not non-vented, especially as they often get lost.
Somewhere on the web I read you could plug a vented cap to make it non-vented but that may have been on the Spitfire. Not sure if/how you would do it on a TR7.
TR7 Spider - 1978 Spifire - 1976 Spitfire - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra
