Vacuum tuning carb

[User811780]

Simple question but where should I connect a vacuum gauge in order to tune the carb?  It seems to be running very rich. About 8mpg.

[Gary Lewis]

Most carbs have two vacuum connections - manifold vacuum, meaning always on, and ported, meaning on above idle.  But the Carter 1bbl usually has one connection and it is actually neither manifold or ported.

So I'd connect on the vacuum tree that is usually on the firewall.  It should have a vacuum connection from the engine and then vacuum lines to the HVAC system and to the transmission.  That should give you full manifold vacuum.

[ratdude747]

I'll have to double check, but I think the vacuum on the firewall is downstream of the vacuum reservoir and check valve... not ideal for vacuum gauges if you want an accurate view of what's actually happening in the manifold.

I usually pop the hose on the air cleaner's thermal valve when I need to connect a vacuum gauge on my 300. Connect up with a barbed fitting if the gauge doesn't have something that fits the hose. Doubly useful since often such adjustments require the air cleaner to be off anyway. But did 1981 have such a hot/cold dual intake setup? If not, just pick a free port on the manifold tree (there should be a few spare ones capped off)... and while you're there check all the caps as they like to rot and cause massive vacuum leaks (probably not in your case if you're running rich!)

[Gary Lewis]

In the pic below, from our page at Documentation/Vacuum Systems/Vacuum System Requirements, you can see the vacuum manifold on the firewall in the upper right and the tee/check valve in the upper center.  So if you pull a cap off an unused port on the manifold you'll be ahead of the check valve/tee.

[User811780]

Thanks, picture is very helpful!!!