Any thoughts as to what I'm hearing here? Check the exhaust sounds in my video attached below. Up under the hood when the engine is idling it sounds smooth and even and you don't hear anything odd but if you're back at the rear of the car you can hear this "donkety donk" sound coming from the exhaust. Does it sound like it's pipe hitting the frame or something? Or is that misfiring? I don't know what to make of it at all.

 

Sounds like both to me. I clearly hear what sounds like 2 mini exhaust backfires as soon as the video begins. Followed by exhaust pipe shudder. Seemingly caused by the misfire.

I had recently sorted out a similar issue on my 69. It was the air fuel mixture needing to be further adjusted in my case. At idle under hood sounds fine even when fully warm but mini pops were occurring out of exhaust. However till the air fuel mixture is ideal, you can also experience mini backfires from the carb while the engine is under load when driving. Can be triggered when you first hit the gas after waiting at a light. Or when you make successive give it gas, no gas, give it gas, movements.

The 2nd video posted, labeled before adjustments, here https://classicoldsmobile.com/forums...ds-etc-187428/.

You can hear the exhaust pops but mine did not audibly shudder the exhaust pipes.

 

To much ignition advance at idle will cause popping. How many degrees does the advance canister add to the base timing? Should be no more than 10°.
Verify the ignition system is not faulty...cap rotor points condenser wires plugs.
Read the plugs.

 

Also, are you using manifold or ported vacuum?

 

Mine definitely sounds a lot like what was going on there with yours. Maybe I'll fool around with the mixture a bit. I detect no issues at all with mine though off idle and during normal or hard driving......it runs great.

 

I have my initial timing at 9 degrees with the vacuum canister disconnected and plugged. The advance on my car runs off ported spark so it shouldn't add anything at idle anyways. I did verify that the ported spark connection worked properly.

Stock spec on my car is 7.5 deg BTDC

 

My car has ported spark, which I have verified that it works properly. No vacuum to the vac advance canister until you add throttle. I won't be switching it to manifold vacuum.

 

As stated, other probable causes include:
-Exhaust leaks. Exhaust leaks let oxygen in, which can ignite unburnt fuel in the pipes & muffler. That may indicate a rich mixture. The exhaust tips appear to be a neutral color. Stick finger in there...remove finger, examine finger color.
-Incorrect A/F mixture (and or possible carb probs)
-Read the plugs. What are they telling you?
-Bad plug or two, or wire, or cap, or rotor
-Dewll angel at 30°
-Vacuum leaks

Hook up a vacuum gauge to a direct intake port. With the engine at warm idle, what are you seeing? Does the gauge flutter more than a few inches of vacuum? Can you get the vac signal higher by adjusting the A/F mix & timing?
What happens when you hook up the distributor vacuum advance to a direct vacuum source?

With the car off, bang on the exhaust system with a fist. Is it hitting anything? Can you hear debris rattling in the muffler?
You might need to take her for a kill run to blow out the cob webs.

 

Excellent advice, thank you!

I do have some pretty significant exhaust leaks at the exhaust manifold to engine block connection unfortunately. It's not too noticeable at idle, although I can hear the ticking and it's quite noticeable under acceleration and pretty annoying. It's on my list of things to fix. I did pull a few plugs recently (I think I pulled 4 or 5) and they looked pretty normal. I could definitely pull the rest to be sure.

I have adjusted the A/F mixture using the vacuum gauge. With my vacuum gauge hooked to manifold vacuum, I'm seeing a little over 18" of vacuum and it flutters slightly less than 1" pretty randomly. I haven't tried hooking the distributor to manifold vacuum as my car is designed to operate off ported spark but I can try it to see what happens.

Hitting the exhaust never occurred to me, believe it or not! I will give that a try. I do take the car out on the highway and floor it quite often to help clear it out.

 

You can try manifold vacuum as an experiment, but my guess is more advance at idle is not what you need.

 

I do remember that I had my initial set at 12 deg recently. I was advancing it as the engine sounded like it was liking it so I thought I'd give it a try and just watch for pinging. I think that the "bonking" sound went away, but I might not be remembering that correctly. I'm pretty sure though. Pretty sure I backed it off to 9 deg a while back, because I was too paranoid, and the noises came back (7.5 deg is the stock spec on this car).

 

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Can be surprisingly random. Have gone for a hearty spin. Ran great on highway. Exit, come to a light wait, light goes green tap the gas and carb pop stalls off... restarts and won't do it again till whenever...

But I think I have her in a sweet spot currently. Gave her a good test the other day, no pops, no stalls, no stinky fumes, quick starts. After some intentionally janky driving, feels good...

Thumbs up on all of that. Sounds like the uneven idle shakes the pipe just enough to contact a too close for excessive play fitment...

 

Check how close the exhaust is to the passenger side emergency brake cable about where they both cross over the rear crossmember. Mine actually make contact there.

 

That's also a great idea. I really need to get under there and have a good look around. Looking at an undershot pic of someone else's '66 (not my pic), it appears there is definitely some potential contact points.

 

Yep, see what I mean. When I first discovered that on mine I was pretty surprised the factory set it up that way. So especially if you've changed the exhaust a bit (I took my muffler off and put glasspacks where the resonators would have been) then I noticed that my exhaust is actually pressed against that although mine doesnt actually bang. The muffler is supported by two heavy rubber hang down straps and if they're not perfectly situated ( Possibly your muffler had been replaced long ago) I could see that hitting for sure.

 

Is that what this is? I took this picture when I had the tank out of mine (I'm not at home so I can't take a look for myself at the moment).

 

Yes definitely and those 2 nuts and bolts below it is the end of the exhaust clamp holding it on. You can see if that clamp isnt precisely in the right spot how that could be hitting the cable. Also on the drivers side near the end of the door, the exhaust comes really close to the frame there and mine was banging there slightly until I fixed that.

 

This is actually very good info and I'm wondering if this could be the source of a banging noise I've been getting over harsh bumps in the right rear. I've been under the car with a pry bar reefing on various suspension components looking for play and haven't found anything yet. With the noise I've been getting it seems that it would be pretty easy to find but everything seems tight. I starting to wonder now if it's the exhaust rather than the suspension? I even swapped the shock absorbers to the other side to see if the banging followed. Definitely worth a look when I get a chance.