For Sir RobynO - my ‘75 K25 still has the original two port canister, and with the original Quadrajet there was a dedicated port for the canister vent line.
When I switched to a newer carburetor I first just put a T in the PCV line to the carburetor and connected the canister vent there, but it seemed to run quite lean at cruise (I have a wideband AFR gauge). I now have the vent line disconnected and left it open in the engine compartment. Truck seems to run fine, but I know the vent should connected somewhere that pulls the vapor.
Where should I connect the vent line for a two port canister? Do I need an external purge valve to control it?
Thanks for any input.
Do you need an external control valve. It's not absolute requirement, but it really should have one and might solve your problem.
On a 5 or 6 port canister the control valve part is built in. If you run a 2 port without a control valve it will purge anytime the engine is running. An external control valve is going to have one line going to ported vacuum, which will prevent purge at idle and just off idle, until there is good vacuum at the ported port. It'll also cut the purge as you step on the gas sooner than it would on it's own. Most external control valves need to see around 10 inches of vacuum on the ported line before it'll allow purge. So if you are seeing that lean AFR at very light throttle, before ported would be fully open then adding an external control valve would do the trick.
Connecting the purge line to the PCV line is the correct way to do it.
What you really want is this >>>>
https://a.co/d/4wtwQQG <<<< connect it into the 3/8" purge line and connect the smaller "control" line to any ported source. Since fuel vapor does not travel though the control line it can be tee'd into any ported source and it does not matter if it shares that ported source with other devices. This is really the correct way to set it up.
Oh, also running around with the purge line disconnected as you currently are is fine for a while, but eventually the canister will load up with fuel vapors, it'll kill the charcoal and get smelly - timeline on that is hard to say.
Hopefully that helps, let me know if you have questions.