tvander:

If you have a non-vented system with the vent tube to the charcoal canister up front, then maybe the vacuum that clears the canister is (or was?) pulling on your fuel tank. I don't know if it should or shouldn't, but I'd think there should be a bit of vacuum to pull fumes from the tank.

Or if your tank is not venting the way it was designed, then your fuel pump is sucking fuel out making the negative pressure condition. On Renaults (back around 1980 when they were marketed in NA by American Motors) there was a problem that if the venting did not work, the fuel being sucked out of the flat tank resulted in the atmospheric air pressure pushing the bottom of the tank in, collapsing it. The only indication was that the car would run out of gas because the pushed in bottom held the gage float up and the tank would only take a half fill. I don't think a Mini tank would collapse that way, but the punp would have to work hard.

A little highjacking this thread but.... My spi car has a non vented cap and in the past when I took the cap off it would have a suction on it. I put a new cap on it (made no difference). I'm not sure how the system works, or if there is even a problem I should look at. Just curious that it doesn't have a vacuum effect when I take off the gas cap like it once did.

Thanks for the tip...I'll check and hopefully it will be obvious!

Not sure about Inno Minis, but all others had a separate vent pipe/hose on top of the tank, which on later `70s cars was piped forward to a charcoal canister.
If yours has a vent pipe, fit a non-vented cap.

Hi all, Would anyone know if my 1973 Inno 1300 would take a vented or non-vented gas cap? Would one or the other that wasn't specified be harmful? Thanks!