Tank not venting/gas leaking out exhaust
 

I've got a 79 12A with the stock intake/carb/rats nest that's giving me two problems. First, the fuel tank isn't venting correctly and when removing the filler cap, I get a loud whooshing noise. It seems to happen after the car has sat overnight and also after the car is driven for more than 10 mins. After driving it for that amount of time, the idle lessens gradually until it finally stalls. After it stalls, I can pop the fuel filler cap off and put it back on and the problem goes away, idle returns to normal and remains ok for another 10 mins or so at which point the problem returns. I had a look at some of the components on the rats nest that pertain to the ventilation system, but those all seem to be working. I even swapped the venilation/check valve with another spare one I had (tested ok like the original valve) and that didn't change the symptom. Aside from the internals of the carburetor, is there anything else to check that would cause the tank to be doing this?

Second problem is something new that started well after the first problem above presented itself. When sitting overnight these past two nights, the carb has been leaking gas from the carburetor down into the engine and out the exhaust. That problem started yesterday after I returned home. I heard a boiling noise coming from under the hood, thought it was the cooling system, but it turned out to be fuel boiling over in the carb and flooding down the primaries. I had this problem happen once before on another engine and once it cooled off, the fuel stopped leaking and the problem went away. Well today, the carb had leaked even more fuel into the engine and the tank is once again pressurized.

Anyone have any ideas? I'm pretty sure that on the second problem, a full rebuild of the carb is in order. I'm thinking that the needle(s)/seat(s) in the bowls aren't seating fully and causing the gas to enter into the engine. As for the first problem, I'm not really sure what's causing that. If anyone has any ideas, please let me know. Thanks.

The SAs have the charcoal canister built into the air cleaner lid. Unhook the vac/fuel hose that goes to the top of the lid and see if both problems persist. If they go away, replace the canister lid. Good luck on finding one.

Rebuild the carb. Make sure the bowl vent works. Replace the charcoal canister if you have to pass emissions. If you dont have to pass emissions, just remove it. There are tutorials around here and I could walk you through it or someone else on the forum. You can keep your other emissions components and just remove the canister with no adverse affects. I would take an air compressor and blow air through the tanks vent line as well.