Mc 2100 Vacuum Port Identity Double Check

‘72, 304, MC 2100

I just want to make sure I have these two ports identified correctly as I hook up my 3-port CTO.

Pretty sure port on passenger side of carb body under choke mechanism area is my ported vacuum source.

Is the port on the carb spacer base next to the larger PCV port my manifold vacuum source? I don’t see any other options for manifold vac because my air cleaner connects to the one by the heater hose outlet at the back corner of the intake manifold as I understand it.

 

There is a 72 here that hasn't been on the street in a long time. I went out and looked at its hoses and it is probably of no use thanks to a prior bubba. This one has the vacuum line from the spacer facing the firewall routed to the middle port of a sender coming off the thermostat housing. The passenger side carb port has a homemade plug on it, and the only vacuum hose on the air cleaner runs from the snout of the cleaner to a little gizmo on the firewall side of the base of the cleaner. Like I said, probably of no use, but thought I would throw it up here so that I didn't run outside, in the cold, in my flip flops, with no jacket, and no flashlight, for nothing.

 

Any port in the manifold is manifold vacuum.

Base of the carburetor is likely ported vacuum.

Re manifold vacuum, it does not matter where on the manifold it comes from; you can run long hoses and tee them and whatever to get the manifold vacuum sources that you need. Manifold vacuum is used both as a signal and as a power source.

The other type of vacuum is distributor or "ported" vacuum. This can only come from the carburetor. It's the same as manifold vacuum except it shuts off when the throttle is completely closed against the idle stop. At idle, the throttle plate physically blocks the vacuum port in the carburetor bore. Thus the vacuum goes to zero when the port is blocked. If you have the carb off, you can look in the bore and see this port. On many carburetors, you can follow the path the vacuum follows to the port. Usually ported vacuum comes from ports on the base of the carburetor, though it's possible to have an internal passage to somewhere else on the carburetor body.

This is easy to test with the engine running. At idle, if the vacuum goes to zero, it's ported vacuum. Otherwise, it's manifold vacuum. You can take vacuum from anywhere and run it to any device (mostly), as long as you have the right kind of vacuum connected to the device.

The small ports are all for small-volume vacuum operated devices, like smog and distributor advance. You should have a big port for the PCV connection, and a big port for power brakes. The power brake port may be plugged, and any ports you don't use should be plugged. If '72 has a TAC (thermostatic air cleaner), it runs from manifold vacuum.

The odd '72 vapor recovery does not use the air cleaner with the vacuum connection at its snout. Regarding the '72 TSM, it does, however use the TAC with a vacuum motor controlled air flap and heat sensor in the air cleaner. Check the TSM for a drawing and description of vapor recovery and TAC for that year. If you need to get this stuff right (for emissions or because you want everything to be original) you must consult the TSM for your year. The type, number and function of devices changes annually in this era.

 

Thanks gents. TSM hasn’t been much help on the actual routing and I know it was flavor of the month back then with emissions rules.

I’m cobbling things together, so I think based on what you shared and what I have to work with I’ll just use the manifold port originally for the air cleaner for my man vac and the ported vacuum port on the carb for my ported vac, and plug the one on the base of the carb facing the firewall by the PCV port and see how that works. Since I’m in FL, I’ll see how things go without the TAC set up altogether then.

If I run these two choices straight to the CTO switch’s #1 and #2 ports, and then a line straight from the “D” port on the CTO switch to the distributor, do you think that will make medicine? It would skip the vacuum solenoid switch, but my third gear trans switch to link up with that mechanism is broken anyway. Tim, I’ll have to get a cheap vacuum tester.

 

The routing is irrelevant. Connect the devices that need manifold vacuum to a manifold vacuum port. Connect the devices that need ported vacuum to ported vacuum.

Just feel it with your finger or lip.

 

This is correct.

 

Thanks again guys.

I looked around a little more online and stumbled across this diagram. This is essentially what I had concluded I would try based on the feedback. I’m just going to use the man vac port I have available to do this. I don’t have power brakes. I wanted to post it in case others were green like me and were still confused by the spaghetti and available TSM illustrations and are trying to go simple non-smog and non-TAC, etc. All the other illustrations I found introduced EGR complexities which I don’t have on my ‘72. I also don’t have any of the air pump functionality/parts. Getting closer to first start up on this first complete tear down/rebuild I’ve ever done. Learned a ton.