Intake Manifold Question
 

Question on this stock intake manifold for my 1970 400. This appears to be a vacuum connection but I don't see what would have been connected here. (right next to the "firing order" text)
Seems like it would make a vacuum leak if it were uncovered.
Thanks for the help.

Manifold vacuum connection for the vacuum advance. I removed and plugged mine and used the vacuum connection on the carburator.

It's actually the vacuum source for the Thermac unit(s) on the air cleaner.

My vacuum advance appears to be connected to this unit cylindrical unit (in front of the spring) on the driver's side of the carb.


@ Rocky. That makes sense. However I don't think those have worked in years. Flaps in snorkels seem to be stuck open. (not sure how to test)

If that is the case, should I just plug this off, or just hook up the thermac unit?

Thanks for the suggestions.

Just cap it off

Thank you

Just a note on "capping" off vacuum ports. Seems like the rubber they are making the caps out of gets soft and fails pretty quickly these days. I see this ALL the time with carbs sent here, most of the vacuum caps are rotten.

Even a tiny vacuum leak can cause running issues, and bring in unfiltered air into the engine that can ruin the rings. I've been shoving a small amount of RTV into vacuum fittings that I'm not planning on using, then putting a plastic cap over them in lieu of a rubber one.

Most hardware stores have the plastic caps, they are color coded, and hold up to underhood temperatures just fine. I got mine at Lowe's in their hardware section.

Sort of related to this topic, I bought a 2012 Harley Davidson Ultra Limited and it developed a light miss and lean stumble around 5000 miles. I bought a tuner for it, but never could tune out the issues in that range. I then decided to purchase a Scream-in Eagle air filter assembly for it, and when I removed the factory air cleaner, found a vacuum fitting at the top of the throttle body with no cap on it. Nice, sucking all sorts of junk into my engine for a few days, glad I discovered it as soon as I did!

I mixed up some Marine Tex and filled the vacuum port, and put a plastic cap over it, and the engine now idles nicely and no lean off idle stumble........Cliff