Long post, but contains as detail as I can provide.

Ok, so I had the 390 running a few nights back, and after it ran for a a few minutes holding the RPM up around 1200 or so (guessing, no tach) to help the lazy alt build up the battery, I shut it down. I then saw that the driver side exhaust manifold and pipe was glowing a nice shade of orange.
The manifold was glowing in the area of #6, and to a lessor degree #7, down towards the junction (between 6 &7), and the pipe itself for about 6" or so.
Passenger side was normal.
Engine does have a miss to it, but the plugs were supposedly changed just before it got parked, plug wires look good, and firing order is correct. The miss is not constant, it comes and goes. Kind of rules out valvetrain problems.
An external vacuum leak *should* cause all 4 of the cyls on that plane to be running lean, as the only place it could be leaking that much vacuum would be at the carb base, as there's no vacuum ports in that area of the runners.
Same thing would apply to one side of the carb having a fuel restriction causing it to be lean. All of the cyls should be running equally lean, and therefore both manifolds should be showing the same glow, just different areas.

I was thinking maybe the intake gasket is bad, letting it suck air around #6 & 7. This evening, I fired it up for about 5 minutes. I took off the oil filler cap, and there was minimal air sucking in, about what would be expected with the PCV valve still hooked up. Being an FE, an intake sucking air at the head *should* be sucking it from inside the engine, unless it's at the very front or rear. Obviously, 6 & 7 are in the middle.
Dual exhaust, and both pipes seem to be putting out pretty equal amounts
of hot air. The driver side actually puts out slightly more volume, since the exhaust manifold gasket is blown on #4. This would seem to rule out a restriction in the pipe.

Am I overlooking something that can cause the exhaust to get that hot, but only around 1 or 2 cyls? I would think a cyl not firing part-time due to either a bad plug or wire wouldn't cause the manifold to get cherry red.

BTW, tonight while running it, less than 5 minutes from a cold start (temp guage barely getting into the "normal" zone) at idle or just above, the pipe was turning a dull orange.

Since it was darkish (right about 9PM) I did look for any signs of 6/7 wires cross-firing. No signs of spark jumping, didn't get bit, and moving them around with it running did not have any effect on the miss.
The miss is more pronounced at idle speed, but can stick around well over 2K, or go away at any RPM.

I do have plans to replace the intake, since I just picked up a performer for dirt cheap. Plugs/wires were already planned with the intake/carb swap, as well as points/condensor. Cap and rotor look new, good brass/copper contacts, no aluminum.

I'm also pretty sure this issue has been around a while. The upper PS gear seal was leaking, and looked to have been melted. This part of the gearbox is very near the exhaust manifold outlet, and right above the pipe.


Am I on the right track, and there has to be either a leak between the intake and head, or something almost totally blocking the intake port(s)?
Dad did have the DS head off a few years ago.

 

I would definitely suspect an intake leak. Lean mix means delayed burn, so the mix keeps burning after the exhaust stroke.

What do the plugs on those cylinders look like? Any oil on them?

 

I've not yet pulled the plugs. I will be doing this pretty soon. It was a bit warm in the area of the plugs, as well as the entire state (100F @ 10 PM). Hard to get motivated to do much in those conditions......

I will add this though, I have not seen any signs of smoke from the exhaust. Well, not out of the exhaust. There was lots of smoke coming off the pipe after the PS fluid covered it..... That kinda surprised me as well, since the smoking started after less than 1 minute of run time. I think it was actually closer to 45 seconds. Just long enough to make a complete cycle of the steering, and shut it down to check the PS fluid level....

 

Couple strange things happened today. About 2:30 PM, some sort of huge dark gray "blanket" covered the sky, and then water started falling.......
This knocked the temp down from 103 to upper 80's.
When the water quit falling, the blanket stuck around, so I went to work on the truck.
Pulled the plugs from the driver side. 5 looked pretty normal, and fairly new. 6,7, & 8 were a bit wet, but looked,felt, smelled like gas not oil. Put them all back in, gap was pretty close, about .042
Pulled out #1, and it looked very similar to #5. Decided to pass on fighting the A/C box to get the other 3 out, for now.
Decided to fire it up, and test for vacuum leaks and such the old fashioned way, carb cleaner in a spray can. Nothing. Sprayed a little into the oil filler cap, nothing.

On a whim, I loosened the dist hold down, and started bumping the timing up. Every little bump improved how it ran. In the end, I think I bumped it by close to 20* before the vacuum advance got around to becoming too close to the bypass hose for my taste.
The intermittant miss is still there, and seem to be on the passenger bank. I'm betting it's one of those 3 plugs I passed on pulling out.
The exhaust leak from the blown exhaust manifold gasket at the top of #4 (top bolt missing) is MUCH quieter. Had to keep backing down the idle speed.
It doesn't sound to be trying to ping, and didn't try kicking back against the starter when hot. Fuel guage reads too low for me to feel safe trying to drive it, and the bank account is too low to add fuel for a couple more days......

In summary, yes, I did overlook the cause, since only one side was glowing. Had both been glowing, it would have been far more obvious.
Late ignition timing was the cause, very late timing..... Why only one side was getting that hot, is anyones guess.