I am chasing the same problem.
The purge side of the carbon canister should enter the base of the carby somewhere.
I have checked the usual service manuals however no detail or reference is given.
Also I have asked several older Ford Trained mechanics.
Also checked an original untouched inlet manifold and carb setup.
Maybe there is a tee piece in the vent line I am missing.
No solution yet.
Interesting problem......