Hey guys, I have a '73 model 440 that suddenly wants to drop from around 1200 rpm to about 500 rpm while idling and then return back to 1200 and then drop again. If I put it in drive the rpm will drop all the way down to about 300. I tried putting new plugs and wires in. Now it just does it more rapidly. I used the Champion Copper Plus RJ12YC plugs that did seem a little shorter than the Autolites that I took out, but they were the recommended plugs from Auto Zone. I've never had this problem before. I put a light on the engine and got it adjusted where it sounds pretty smooth and then the rpm will just plummet. I thought maybe the fuel filter, but it is fairly new. I haven't tried cleaning the carb. Any thoughts would be appreciated. I'm still a rookie when it comes to engines just fair warning
Thanks!

What carb, where is the timing at?? I would up the initial timing then lower the idle screw for starters. Could be another problem like jumped timing chain? Need more info

Take out the idle mixture screws (count the # of turns to easily return them to their orig position) and shoot a 3 second blast of starting fluid into each hole (need the thin red straw to get it in there good) then replace the screws. If that ain't it then I would go thru the carb

Thanks for your replies. While the engine was running I pulled a spark plug wire and nothing happened. I pulled another one and nothing happened. Then I pulled one more and the RPMs dropped. So I now know that at least two of my plugs/wires aren't sparking. I tried putting a different plug in but it made no difference. So it's either the wires (which are brand new) or maybe the distributor cap (which is only a few months old). So confused..

I'd spray the carb base with carb cleaner to see if it smoothes out...would show a leak

Hook your timing light clamp to both of the offending plug wires (1 at a time) see if the light flashes. Next time I'd suggest grounding plug wires rather than pullling them off as electronics do not like open circuits & grounding/opening the circuit will disable it effectively the same (to check the RPM drop)

Are you running vacuum advance on the distributor? If so, disconnect it and plug the carb side connection. Reset the idle speed and see if the fluctuating idle goes away.

Never mind.I read the rest of the posts and changed my mind.
find the cause of the dead cylinders. Intermittant wire? You might try swapping wires/ or plugs to see if the problem follows the wire. Look inside the cap. Check for burned wire.

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?