Low Compression. Missing on cold start

[MIPS]

As the mornings are getting colder I'm starting to notice at least one cylinder has really lousy compression at odd times.
A few times I've cranked the engine during the summer and you could clearly hear the engine cranked faster on one cylinder but as soon as you started the engine it idled fine.
Since the beginning of the week however I've started the car in the morning and it does it usual thing in cold mornings:
-Engine cranks fine with no sounds of a low cylinder
-Starts with a tap of the  pedal to set the choke. Doesn't need a squirt of gas.
-Revs up to 1200 or so, drops down to about 600 with a bit of cold engine missing and then settles down to idle as the choke starts to open after 45 seconds or less and the carb heater warms up.

But now it's just stuck missing on at least one cylinder until you tap and rev the engine once or twice. You can drive away with it still being unhappy like that but OH MAN it doesn't like it. No black smoke or backfiring. You step on it and there's just no power and a lot of missing but once you are on the highway for a few clicks it's hot and happy. No missing, no power loss and the choke's full-open.
The valves, lifters and guides were serviced last summer so I'm doubtful one is sticking up in the cold but the block is due for a rebuild in the spring do to some bottom-end knocking. Does this sound like a ring that's on the way out and sticking? Oil consumption has been okay but I can't check the compression as the threads on my tool are not deep enough for the plug holes on the 258's head.....

[TheBirdman]

Theres not much I could speculate on without knowing compression numbers. You might want to get a compression tester that fits.

[AMC of Houston]

Don't get too focused on compression and ignore other possible causes; such as coil breaking down, choke pull-off not working, moisture in the dist cap, etc.   Good luck on the diagnosis.

[MIPS]

I checked this morning to verify the pulloff and the choke were indeed functioning fine and it was good. On the way home I dropped into Lordco and bought a deep plug thread adapter to fit the head and here's your mostly hot engine compression numbers:

1 - 142
2 - 150
3 - 150
4 - 145
5 - 152
6 - 160

Eh, not terrible but my issues were happening on a cold engine, so I'll try it again first thing tomorrow morning.

[MIPS]

And now the cold engine tests:

1 - 140
2 - 148
3 - 151
4 - 150
5 - 148
6 - 157

So the numbers seems pretty healthy over and hot/cold, except for cylinder #6 has exceptionally good compression all the time.

One thing I've noticed was that it has a lifter that's the loudest when you let the engine sit for a while so it's leaking down faster than the rest. During compression testing by the time I was at the 5th pot I could hear excessive pressure popping into one of the manifolds. Had to run the engine to pump up the lifters and test again.
My theory is that the #6 has a bad lifter and one of the valves are not opening fully.

[AMC of Houston]

Fairly common on these engines with high miles to have the rocker arm pivots worn down to where the valves aren't opening fully.   I tend to replace all of the pivots at 100K miles or so just for grins.   My 200K-mile waggie is almost due for them again!

[Illeagle1984]

I think if it was worn valvetrain parts, it would likely be more consistent and with accompanying clatter.  How's your oil pressure?  It could very well be a dirty lifter.  The tiny oil hole gets partially plugged, and while the engine is warm the hot, thin oil will make its way in there to pump up the lifter and lube the top end.  Sometimes it may sit and be fine.  Other times it may sit and run like a 5-cylinder for while.  It can seem random because the problem may only show itself when that particular valve is held open when you shut it off.  The cam & valvespring squeeze the oil from the lifter, and when you restart it takes time for oil pressure to pump that lifter back up.  There is a check valve inside the lifter to prevent this, but apparently it doesn't work when its dirty.  This could explain why revving the engine makes it go away (increased oil pressure overcomes the dirt).

Just my theory though, it can be tough to be 100% sure on something like that.  You can try repeated cold compression tests, or possibly shut it down once it's doing it and then test.  I had good luck fixing that exact problem on 80's Caravans back when they were still around.  I had one that all the lifters pumped down so hard it wouldn't even start!  The solution was an oil change and a bottle of Valve Medic.  You can try a motor flush too but sometimes those solvents can accelerate oil leaks on real old gaskets.

Also if you are able to get the lifters out, they're super simple to take apart, clean, and reinstall.  And you could inspect the entire valvetrain for wear/damage while you're there.  Easier if you can pinpoint which one it is.  If it's popping through the intake, it's probably an exhaust valve not opening.  Cylinder 1 looks sus...

[MIPS]

Last summer the head was also rebuilt and decked. The shop pointed out two worn pivots and one worn rocker. I didn't have new replacements on hand so I replaced them with considerably better pivots and rockers from an '84 head.
The rebuild included new lifters in the quotation so even though these ones are basically new we can change them out again when it goes into the shop.

[MIPS]

It did it again.

Yesterday I had the car rigged up with microphones for unrelated audio sampling. Started with a key bump, warmed up and went to idle perfectly normal. Drove around and did a 0 to 60 with no problems at all. I got the audio now to prove that it can run perfectly normal when it wants to.

This morning the temperature is the same. The weather is the same. Took a fairly long crank to start for no reason I can explain, left the driveway and immediately noticed it lacked power. Gunned it as soon as I had the chance and could feel it was sputtering or missing on heavy acceleration. Took it on the highway and put the transmission in a kickdown and WOW the kickdown happened, the throttle opened and the engine did not want to accelerate at ALL. Almost felt like an engine brake. Probably don't want to do that too many times on this transmission. 
I drove it more lightly the rest of the way to work and it didn't complain as much or stall.
A cold engine is a cold engine but I'm very suspicious if that is the cause when I can replicate exactly how I drove the car 24 hours earlier with wildly different results.
I'm beginning to suspect its an electrical issue rather than a mechanical problem, however there's still a lot to narrow down there between the pickup coil and the plugs.

[Canoe]

I had this behaviour with my '81 wagon, but I can't remember what fixed it. I tried several things over some months, and one day a simple electrical connection and all was good. Pretty sure it was something on the passenger side of the engine, near the starter solenoid & ignition/dizzy parts - although that doesn't narrow it down much... aaargh. It's hinting, but just not coming up out of my memory.

You might be getting multiple problems occurring, obscuring what's going on. Since you mentioned kickdown, one thing you could check is the throttle linkage adjustment. Although we call it a kickdown, a number of times someone has explained that on Eagles it's a throttle proportioning valve not a kickdown. Many have had issues that went away once it was adjusted as specified. (I don't know what the proportioning-valve/kickdown difference is, but following the procedure works.) Having a clean corrosion free loop at the top, and the correct bushing in good repair at the bottom linkage arm, is pretty critical to correct operation. 
http://amceaglesden.com/guide/Throttle_Cable/Rod_Adjustment_-_4_%26_6_Cylinder_Cars

[MIPS]

That does actually narrow it down a bit. That's where on most models the Duraspark module lives. Mine's always "worked" but it has cracked plugs that if you aren't careful the pins will slide out. Because new units are such a crapshoot and this one is the original AMC yellow grommet I've just left it alone hoping it doesn't fail. I guess this is justification finally to buy a replacement module and see if it continues to happen. While I'm in there I'll test the coil and pickup windings.

[Canoe]

If your Duraspark module is suspect, it's easy enough to drop it.
Instead of more of the same, replace it with the GM HEI four pin module. (Different suppliers have different quality parts - you'll have to do some research.) You power it direct from solenoid ~12VDC - NOT through the ballast resistor. Which means that all of the time (particularily during starting), the coil & module have the most voltage (current) available, for the most spark available. A TFI upgrade goes well with that. Somewhat better mileage.

I heated my Duraspark module box up and removed the insides. Then I mounted the GM HEI module inside the box, using the box has a heat sink. Instead of Heat Sink Compound, I cut some TIM (Thermal Interface Material) to match the module's footprint. Used the box's old pigtails for the updated connections.

[MIPS]

Yeah I gotta keep it mostly stock under the hood, so I'm stuck with sourcing another three connector yellow grommet module, OR if someone has a known dead module, mail me that and I can splice the good connectors in.

Also I did check my TV linkage and its still properly adjusted from when I last set it.

[TheBirdman]

Just say the word and Ill send you a pre-cec carter and duraspark, Ill even send you the little emissions sticker off this car to cover up the one on yours, then you can have it all nice and stock, and not be constantly plagued by mystery ignition issues. Ill even pay for the shipping, all you need to do is promise youll actually use it.

[Canoe]

... a pre-cec carter and duraspark ...

How would that play with the cec controlled spark? Does the cec adjust spark independent of or depending on mixture?
Carter is really really nice when in good repair and adjusted well.

I get my spark directly from dist pickup, to GM ignition module, to coil. Dist has direct port advance curve. So no cec spark advance adjustment (nor knock correction  , so far, not needed).

But I've looked at the GM 7 pin ig-module, considering if I can get the cec back into play. 7-pin lets you use the superior coil control (module limits coil current from the available battery/system VDC - great for startup, no ballest resistor), yet have the cec in play. It gets the reluctor input on pins N & P. Passes the timing pulse to the computer through pin "R" (reluctor) and gets the ECM adjusted (timing & dwell) pulse  back on pin "E" (ECM). Pin "B" (Bypass) high/low (5v/0v) lets you select module coil output as: ECM Adjusted timing & dwell, or Bypass for the original timing pulse using the module's dwell control. Correct use is it's on Bypass for startup (and initial warm up?)(I don't remember the official reason for this). I'm seeing a switch somewhere near the dash so spark can Bypass cec control if the cec adjusted spark is acting funny.) As I did with the 4-pin module, I'd mount this inside a duraspark box, using the box as a heat sink.
P.S. GM 8 pin is the same, but with a "G" pin for ground, for the harness going to the ECM.