I see this thread and project is very similar to what I am working on.
The fuel tank vent hose on my car has been left open and tucked into the fender ever since I pulled the 4 cyl turbo 28 years ago.
It always smelled of gas and and if I plugged the vent hose it would develop an air lock and the engine would quit after a few miles.
To make matters worse, I unknowing bought a marine carb at the local swap meet and have been using it for the same time period.
The main difference with marine carb is there are no vacuum ports or spark ports, as boats rarely idle and the are rpms are fairly constant.
Left picture below.
The only port on it is above the choke plates at the very top - I've read it is to get plumbed to the fuel pump overflow on a boat to prevent spills.
I am now putting in a charcoal canister and purge system, but do not have a spark port to plumb the to the purge valve pilot.
The schematic below is the circuit I have come up with, I think in theory they work similar.
The difference this will purge all of the time even at idle, when plumbed to the spark port, the purge valve only opens to purge the canister at part throttle.
The temperature switch is to disable it when the engine is cold.
I will let you know how it works after installed, i'm a little concerned about the constant vacuum leak at idle, might have to play with orifice sizes.
Its amazing how we first spend all of our time and money making modifications, and later spend all of our time and money trying to put it back to original.
Eventually I will get rid of this carb and put in an EFI Carb unit.
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