I have a 1972 big block and recently changed out my carburetor. (Of note, when I bought the car years ago it did not have the original carburetor on it.)

While installing the new carburetor, I was reminded that I do not fully understand my vacuum system. I have two questions:

Does anyone know what the second, smaller vacuum hose coming off the carburetor and connected to the vacuum canister does?

Second, should it be connected to full or ported vacuum? From the build sheet it "appears" to be full vacuum but on my new, old carburetor, it is ported vacuum.

Thanks.

PS - sorry for uploading a full size image. I didn't see how to downsize it.

 

As the diagram shows, the larger line Ts into the valve cover whereas the smaller line is dedicated to the carb. Think of the larger line as the “supply” line and the smaller one as the “sense” line.

I can’t speak to the specifics of the carb as you have not included that information.

 

The smaller vacuum line is to 'trigger' the vapor canister to purge contents into the larger line (which sends them into the intake charge at the base of the carb). That smaller line should be connected to a 'timed' or 'ported' vacuum fitting ont he carb. This is so the extra fuel/air from the vapor canister will not screw up the A/F mixture at idle or low throttle conditions. Once the throttle is open far enough that vacuum level in the ported/timed circuit matches the manifold vacuum circuit, the canister will start the purge process.

 

Thanks, this helps a lot but I need some clarification to make sure I fully understand what you mean. When you say "purge" the contents of the canister, what exactly do you mean? I thought the canister "content" was vacuum. Are you saying that once the smaller line reaches manifold vacuum it triggers the larger line to open and allow any fuel vapors it may have sucked in via the manifold vacuum line on the back of the carburetor to be purged. If so...

The vacuum line off of the back of the carb "sucks" air out of the canister creating a vacuum level. It retains that vacuum via the check valve.

The smaller line on the front of the carb activates once the port reaches a higher manifold vacuum and "sucks" air out of the canister (?) triggering the larger line to purge the canister as the manifold vacuum is higher than the canister vacuum. This tells me the larger line opens at manifold vacuum and "sucks" air/fuel/etc. from the canister into the manifold to be burned along with any other fumes released via the PCV valve.

At idle my engine generates 12-13" vacuum. At higher RPM it generates 20". So as I understand it, on startup, the canister vacuum (if it has bled off) is reset at 12-13". Then once the engine is operating at higher RPM generating 20" vacuum, the "trigger" line allows the larger line to open and because the manifold vacuum is higher than the canister vacuum, it creates a flow from the canister to the manifold thereby "purging" the canister. Do I have that about right?

Last question: If all of the flow is out of the canister, how does it suck in any air/fuel vapors as long as the system is tight with no leaks?

Bottomline, I never knew the canister would purge itself but it now makes perfect sense. Thanks!

 

The canister content is fuel vapors from the fuel tank via a steel line along the frame, connected to the canister at one of the top port nipples. In the diagram it appears to be the forward most port - my original canister has embossed wording indicating where the line goes/comes from and I don't remember which port is marked for that line. There are only two lines connecting the canister to the engine: the large line that is T-ed into the PCV valve and the small line going to the small carb port. The inlet air to purge the canister, when the small hose vacuum signal operates the spring and gasket in the canister cap, comes from the bottom of the canister where there is a fiberglass looking inlet filter.

 

7T1Vette/67:72, thanks for being patient with me. Clearly somewhere along the way I started thinking vacuum canister vice vapor canister. With your last reply everything snapped back in place. I now understand the engineering behind the set up and have my new carburetor hooked up correctly. Thanks again.

 

Yes, the vacuum reservoir is located in the same general area, but above the location of the fuel vapor cansiter. Glad some of that info helped you.....