The system you're asking about is present on '70 California cars and '71 50-state-plus-Canada cars. '71 was the first year all cars in North America (not just California cars) were equipped with evaporative emission control systems (ECS). This first-two-years system is more complex than the relatively simple '72-up system.
The '70-'71 setup uses a complicated fuel tank: there is a small inner tank at the top of the main tank and connected to the main tank chamber by a very small passageway, so that the main tank chamber fills up with the secondary chamber still mostly empty. The gas pump nozzle detects a full tank and clicks off, you hang up the nozzle and drive off. Over the next little while, the secondary tank fills up from the main chamber, dropping the fuel level in the main chamber so as to prevent fuel leaving the tank when things warm up and expand.
These '70-'71 ECS fuel tanks have
four vent fittings, one at each corner of the top of the tank. These are brought together in the trunk to a more-or-less vertical vent cylinder leaning against the left wheelhouse — that's what you're seeing. This vent cylinder has
five hose fittings: one for the hose from each corner of the tank, and one for a hose connecting to a line that runs up to the right rear corner of the engine bay.
it is not supposed to vent into the trunk!
Inside that vent cylinder, each of the four fittings from the corners of the tank continues vertically, each to a different height. The fitting to the engine bay continues inside the cylinder almost to the top. The idea with the 4-corner venting and the different-height pipes inside the cylinder is that no matter what angle the car is at, in what direction, there will always be at least one vent fitting above the liquid-fuel level, so there'll always be a vapour vent, thus preventing pressure buildup that would force fuel vapours out into the atmosphere.
The '70-'71 ECS is also unique and interesting in that it uses the
engine crankcase to store fuel vapours piped forward from that vent cylinder in the trunk and from the carb bowl. The interface point is a fitting on the fuel pump body on slant-6 cars, or extra fittings on the engine breather cap on V8s. When the engine is started, the collected vapours are drawn off out of the crankcase via the PCV valve. This system does a reasonably effective job of containing the vapours, since the crankcase is well sealed, but it can cause hot start/hot idle problems, since there is no control over when the vapours are extracted from the crankcase. Most of the unburned hydrocarbons wound up out in the atmosphere anyhow, having been first put through the engine (and emitted as black smoke). :roll:
For '72, things got very much better. At the back of the car, the inner tank was deleted; by clever shaping of the top dome area of the tank, the same function was achieved by means of a controlled air pocket at the top of the inside of the tank. The '72+ tanks also have only
one vent fitting. At the front of the car, the crankcase was no longer used to contain fuel vapors. Instead, a canister containing activated charcoal granules was mounted at the front corner of the engine bay. Lines lead to it from the carb bowl and the fuel tank vent. There is a purge valve mounted atop the canister, with a small diameter vacuum line teed into the distributor vacuum advance line, and a large diameter line from the underside of the purge valve, teed into the PCV valve hose. There's vacuum in the distributor vacuum line only above idle, so the purge valve only opened above idle. Voila, no more hot start/hot idle problems, since fuel vapors were only drawn off and burned above idle. This system works fine.
Starting in '73, additional simplification was phased in: the purge valve was deleted, and a special purge port was added to the carburetor. This port was placed so as to be above the throttle plates when they are in their idle position, so in effect the throttle plates themselves are the purge valve.
A charcoal can is easy to adapt to non-spec applications. Original types have three ports: TANK (tank vent), CARB or BOWL (carb bowl vent), and PURGE. If you find a replacement with only one vent port ("tank", these are common on fuel injected cars that don't have a bowl vent) you can simply tee the carb bowl vent line into the tank vent line near the canister, then connect this composite vent line to the one and only vent port on the canister. Size
does matter here; the late model units tend to be smaller because there's less vapour to handle (no hot carb pouring off vapours).
There needs to be a canister purge valve that only opens when the engine is above idle. Some canisters have these built in (looks like a round "flying saucer" atop the canister itself), but many do not, including Mopars after '72 and most late-model ones in which purging is controlled by the computer via a solenoid. Fortunately, standalone purge valves are readily available and inexpensive. NAPA Echlin # 2-28011:
Small fitting gets teed into the vacuum advance hose (which should have no vacuum at idle, full vacuum above idle)
Big fittings go inline with "PURGE" hose from canister, which gets teed into the PCV hose.
Whew! It's Miller time.
