440 Fluctuating Idle Speed

Clean the idle circuits as described above, one side of the carb is trying to have clogged idle circuit. Rebuild it if this doesnt work.

Thank you all again for your comments. I think the carb is fine because I believe it is an electrical issue. At least two spark plugs are not firing. But I will probably give the carb a good clean anyway. It can't hurt anything.
I had my ignition coil tested and the primary winding was in good shape but the secondary was week so I purchased a new coil and installed it but the idle is still fluctuating. It will idle normal for a few seconds, then drop rpm for a few seconds. Normal, then drop. It's consistent. So I now have a new coil, new plugs, new wires, new distributor, and a new cap & rotor.
The car ran perfect before I took it to my buddy's shop to get some body work done. I drove it home which was a good 25 minute drive and it handled it fine until I pulled into the driveway and it stalled out on me. Hasn't ran the same since. I had been tinkering with the timing and the air/fuel mixture off and on for a couple of months but the fluctuating idle started happening just recently.
Thanks again for the help! I feel like I'm narrowing it down. It's only a matter of time before it hits the road again

Can I persuade you to shoot some starting fluid in the idle mixture screw ports

Make sure the throttle cable and the carb throttle blades/linkage/fast idle linkage ain't sticking & if good there then I'd open the carb

Sounds like a vacuum leak

Here is a picture of the port I was mentioning above. Is this where I hook up the vacuum gauge? The gauge only has one line and there's three ports. And is this thing supposed to be plugged? Thanks

Hi. Seems like you just found a huge vacuumn leak. All those ports is supposed to be plugged. Connect your vacuumn gauge to one of the ports. Remember to plug that port again when you are done. There should be no unplugged ports on your carburetaor or intake. Also check all hoses for any vacuumn leak. You can run it without pcv but your system might be set up for the controlled vacuumn leak the pcv provides. I am not 100% sure about this so hopefully someone else will shime in. Lars

Thank you Lars. I will plug two of them up and hook up my vacuum gauge. That should make adjusting the mixture screws a lot easier. I'll be sure to plug it back up when I'm done. I will see about getting a pcv valve and see if that helps the performance any.

Good. But I looked at the picture again and have to ask if the ports tree in the picture is bolted into the water pump housing. A little hard to tell from the picture. There is no vacuum source in the wp housing so that would have been strange, if not someone used a blocked vacuum tree to blend off a port in the wp housing?? Do you have a picture showing more of the surrendings?

that is a temp based vac valve in the water pump. there are no vac lines to it so it ain't affecting what is going on plus with the RPM rising and falling I'd think a vac leak would pretty much be a constant except possibly one inside the carb & I ain't even sure that a (vac) leak inside the carb would vary that much. Keep us posted

RapidRobert is right, that is a ported vacuum switch. It opens up internally as it gets warm but vacuum is supplied from a hose to the switch. It has no internal vacuum of its own.
You really need to sort out if this is carb or electrical related. Since you have replaced the rest of the electrical system, try taking your Ignition module in to AutoZone and having it tested. Make them test it several times until it gets hot to see if it starts to weaken.
Try spraying some starting fluid in the usual places to check for leaks - around the carb, vacuum hoses, and intake ports. Also, I've had vacuum leaks in the power brake booster do weird things at idle. Try temporarily plugging that hose/port.

It all depends on who you get Commando. I worked there as a manager for several years and if someone actually knows how to use the equipment, it can be a helpful place to start. I seriously doubt that is the problem on this motor but it can't hurt to check it out.

Vacuum leaks external to the carb can cause the engine rpm to wander at idle.

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Do you mean if the carb has a vacuum leak it can cause the rpms to wander? I think I might have one coming from the carb somewhere. There is a pretty loud air noise coming from the engine.