Yesterday evening I was trying to understand all the tubes from the fuel tank to the carburetor and exhaust. I looked at photos of other ‘74s. It became obvious that over 48 years not every engine compartment looks like the photos in the Fiat shop manual.
People do take this stuff off...
The basic fuel tank plumbing is a fuel line from the tank to the fuel pump (doohickey bolted to the the engine block, right next to the oil filter, it is driven by a pushrod and a shaft inside the engine), then through the fuel filter to the carb. Then there's a second line from the carb straight back to the tank. This keeps fuel continuously circulating to prevent vapor lock. You need this to be working properly, and it's not a bad idea to replace the fuel lines if you don't know when this was last done - they can rot from the inside, shedding little specks of rubber that clog the internal passages of the carb and make you do that jets-out/carb-cleaner thing again.... and again... and again.
Everything else will only make sense if you're looking at the "Fuel evaporative emission system" page in the shop manual, it's page 55 in my copy.
All of the thinner tubes just open into the top of the tank to let air in and out of the tank - the tank venting system. The criss-cross tubes on the firewall right above the tank connect these to two liquid-vapor separators about the size of a spool of thread; these are designed to catch any condensation and drain it back into the tank while letting air and fuel vapors flow out to the three-way valve, a plastic blocky thing bolted above the driver-side wheel well.
This three-way valve lets air into the system when the pressure in the tank is lower than ambient. This is the essential "tank vent" function that we've mentioned above; if the pump keeps taking fuel out of the tank and no air comes in to replace, eventually a vacuum will develop inside the tank, the pump won't be able to pull fuel against the vacuum, and the tank may even collapse like a vacuum-wrap plastic bag. The three way valve also routes vapor from the tank to the charcoal canister whenever the pressure in the tank is greater than ambient; this stops fuel vapors from getting into the air. And the third function of this valve is as an ordinary safety valve - if for some reason the pressure in the tank is excessive and the flow through the charcoal canister isn't enough to relieve it, the valve will release the excess pressure to the atmosphere.
The charcoal canister is a cylinder bolted to the rear firewall near the expansion tank. There's a tube from the three-way valve so that the canister can capture fuel vapor from the tank, and another tube from the carb float bowl to catch those vapors. A hose from the exhaust manifold heat shield brings hot air to the canister; this cooks off the fuel captured by the charcoal and sends the vapor via a fourth hose to a vacuum port at the base of the carb so that it can be burned.
As long as air can get into the tank as fuel leaves none of this stuff needs to be working, and it's not present in the European models of the same vintage. However, if it is working there's no reason not to keep it that way - no one likes the smell of gas in their garage.
