I have the S2 1971 US setup in original configuration. Done about 3.5k miles without issues. After filling up (really topping off maybe a bit to much) and a few miles on the highway fuel starve set in. After stopping, heard the fuel pump ticking fast, hearing fuel gurgling I opened the (unvented) cap to a strong sucking noise with air going in the tank. So there was vacuum. Car ran great w/o the cap. I have the canister and the lines hooked up, never happened before. Whats going on? I‘d like to keep the original setup with the unvented cap or should I put on a vented one? Thanks for ideas.
Martin;
I would think one of the lines must be clogged. I would start at the charcoal canister at the front of the car.
There should be an open tube on the back of it. Also as you probably know the tube that runs under the car on the left side, other then the brake line, comes from the fuel vapor tank above the left rear wheel well. That needs to be open also.
The same thing happened to me, except car stalled several times before I was able to diagnose it (and that was by sheer luck). The SU pump develops only so much pressure between input and output. Once the input pressure falls around 4 PSI (8 inches Hg) into a vacuum, then output pressure is 4 PSI over that, or zero. This is a great safety feature…if the pump could make more pressure head, then the vacuum would increase enough to crush the tank. This does happen, but rarely…usually there’s just enough elasticity for the tank to recover, often with a big bang, when the cap is removed. Diagnosis is as Joel suggests.
Problem gone. Best I say no longer an issue. I ducttaped the filler opening and put air pressure in the half full tank. Air pressure stays put, pushing the ductape up. No audible escape through the charcoal filter up front. So when out and about today no more fiel starves. My resume is I had (deliberately) overfilled the tank and with that blocked the venting hoses from doing their job. So no overfill, no fuel starve. Thanks for replies.