TR4/4A Fuel tank venting

[tdskip]

If I wanted to install a fuel tank vent (humor me here) that goes to a carbon canister that seems simile enough, but how to then get air into the carbon canister? Would I need a one way valve of some sort?

 

[TR3driver]

The canister is normally open to the atmosphere at the bottom. What you need is a "purge line" that will pull fresh air through the canister while the engine is running, so the fumes will be released from the charcoal, sucked into the engine and burned.
Triumph usually did that by tapping into the constant depression area between the venturi and the throttle plate. No one way valve needed, since the canister is (theoretically) at atmospheric pressure so the flow is always towards the carb.

But assuming you don't want to tap into your carbs, I suspect that manifold vacuum and a very small orifice would do. If you want something more sophisticated (and effective), then maybe get a canister that has a purge valve built into the top and tee the control port into the (ported) vacuum advance line, so the valve only opens when the throttle is partway open. That way, the effective vacuum leak doesn't screw up the idle mixture and you can run more fresh air flow.

PS, it's very important that no liquid fuel get into the canister. I'm not sure offhand how a TR4 is arranged, but I know that on a TR3 all it takes is a hard left turn with more than 1/2 tank of fuel to slop liquid fuel out the original vent line. You may want to include a "vapor separator" in your setup.