[valiant67]

My 1968 Valiant has a 1973 slant 6 transplanted into it. When they did the swap, they kept the 1973 single barrel carb with has the vented bowl. They left the bowl vent on the carb open to the atmosphere which meant the float bowl dried out when the car sat and could likely be dangerous with gas vapors right on top of the engine.

I tried capping the bowl vent but when I did the carb leaked gas when the engine was shut off, which is never did before. I assume this was due to unvented pressure building up.

What is my best option? Route the vent away from the engine? Get a charcoal canister from a junkyard? Swap back to an older carb?

If I keep the car, I have a Super Six setup to install which will have the same issue.

 

[ImperialCrown]

The vent must be kept open to atmosphere. A charcoal canister would work.
1968 was pre-evaporative controls, just open to air (the fuel tank as well).
Keep the vent air filtered. I have seen a length of rubber hose from the vent fitting to an open-ended fuel filter mounted on the inner fender work well for venting the float bowl.

 

[68RT]

All older cars were vented to the atmosphere. Then they went to a vent that closed as you opened the throttle which gave much better control of the fuel mixture as the car changed altitude. Problem was, fuel vapors were allowed into the atmosphere and that was a pollutant that could be controlled. Carburators always were still vented to the atmosphere but became filtered through a charcoal canister to stop the pollutants. The float bowl drying out probably is not due to the vent but is from either an internal/external leak or from excess heat causing "after boil". Check to see if the heat riser is stuck or if you do not have a thermal block between the carb and the intake manifold.

 

[Bob Lincoln]

It would take months for the float bowl to evaporate out completely through the vent. And it's not a fire hazard to have the old style vent, just a pollution problem as 68RT said.

 

[TWX]

I like the charcoal canister idea. It's a known solution after all...

 

[valiant67]

The problem is this is a new style (1973) vented carb, so there is an open tube to the atmosphere on the carb. Now there may or may not be some type of valve (and if so, it may or may not be working properly) but all I know is when left open to atmosphere, the carb was always dry when I start the car which is about once per month.

I think my solution will have to be to run hose from that vent on the car to some type of filter or a charcoal canister. If I'm lucky maybe I can even find an A body canister in the junkyard.

 

[Bob Lincoln]

There will also need to be a hose to pull the vapors back into the intake from the canister. Someone should be able to point to a vacuum hose diagram.

 

[dana44]

Evaporation happens faster due to the heated engine and then sitting for the length of time. If the charcoal canister is added, it should also be continued to vent all the way back to the tank like original, which you would have to change the input of the sending unit fuel line and vent to the charcoal canister. This means the carb will need to be changed, can't just plug a rubber hose onto the vent and then run it to the carb top, just doesn't work that way. If you intend to install a Super Six setup later, wait until then and ensure you have the proper venting on the new carb you add to add the canister and vent line to the tank, but even this may not prevent the carb from being empty after a month of sitting. I have never seen or heard of an open vent carb producing enough fumes to have it cause a fire at any time in my life.