You certainly have a point. The only time that the "canister" can empty (evacuate) is if there is a negative pressure on the air-cleaner side, and if there is a positive pressure, it will force vapors back into the gas tank. Seems that start-up or idle would produce the most negative pressure. I'm running an air scoop. To solve the greater positive pressure issue, I installed a PVC valve "in-line" with the hose to the snorkel (Carburetor "hat"). This acts like an "anti-siphon" valve. I found a plastic one that requires very little pressure to close. (Used on my Nissan pick-up).
Sounds reasonable. Just curious, is the scoop sealed to the top of the air filter housing. Also, not sure if there might already be a check valve at the top of the tank. Has anyone ever seen one there? MD
When I bought this 1970. The previous owner gave me the original gas tank. He said that it had some pin-hole leaks on the bottom. He said that the gas tank that is in the car, was one that he got at an auto-wreckers that "fit". The original has no vapor emission hole - but, there is a (what appears to be) a hole blank. On the tank that he installed, it has valving. The previous owner said he had no idea how to "hook it up", so he found a big round doughnut shaped rubber gasket, cut out a piece of metal to go over the rubber gasket, drilled holes in it, and used the bolt slots to hold it in. Rebuilding the car, I took off his handy-work, and discovered 5 (or was it six?) rubber stoppers in holes on the tank. (I have no idea if that is stock!) Assuming that this was original, I made a light-weight aluminum washer (approx 0.020" thick), found a very light spring, and then screwed a shaft in that allows the aluminum washer to freely move up/down. The assembly acts like a one-way valve...when pressure builds up, it opens up. From an auto wrecker, I got the tubing & canister off of a '71. I re-built the canister. I bought a C4 transmission off a MC member in Tacoma, Washington. He showed me his gas tank. His (original tank) had a plastic cover over; where the rubber pipe connects to the steel tubing. He didn't know what the "guts" looked like. About my hood scoop: It's a 1970 Torino "Shaker" hood scoop that I bought on "e-bay". It had been butchered, so it would never fit with an original air cleaner. That was fine with me - just what I needed, and got the guy way-down in price. With some filing, J.B. Weld, new SS studs, I was ready to install. Using Spectre cold air intake parts, I strategically located a 3" intake & screw mounting holes, used 3" solid & flexible pipe, two 90 degree elbows, an in-line air-filter box, and a Weber snorkel intake hat on the Weber 32/36 - it worked out fine.
I have an open filter with the canister hose connected. I have no vapors smell inside a closed garage. I also have no idea if it works correctly or not because I have a problem seeing which way air moves.
I'm leaving it in. As we all know, the first year's had (and continue to have) gas tank problems because water vapor is able to condense, sink to the bottom of the tank, and rust through. At least with the early canister system (even though very primitive), it helps remove water vapor that is always present in atmospheric "air".
