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Most likely caused by the seat being cut off center from the valve guide, or the angles being off. A chunk of carbon will usually just break up and spit out-- the exhaust gas velocity is very high.

Small chance that the hydraulic lifter clearance was inadequate.

The low compression on #8 exacerbates the deposit buildup because of less complete combustion.

You should be able to hear me groan all the way from the far north.
Thanks Ross.

We used to go "blow the carbon out" in the old days. (Is EPA listening?) Even when proper, low compression and out of adjustment will increase deposits. (aka; coking reaction)
Instead of tuning carb by "ear", use exhaust gas analyzer/take to shop w/ one. That's about the only way to get the proper air/fuel ratio for today's gas. About all you can do is minimize it by keeping in tune. Another "we're spoiled" thing of today v back then.

If your getting much oil (blue smoke exhaust) in the cylinder, then it's a ring issue. So oil additives won't help, per se.
I've already stated what I think about the additives market so I won't indulge. And there's a bit of difference in materials then v now, so predicting performance w/o actual test data is speculative at best.

OK, I am reporting back. This is starting to sound like a Gus episode.

Just to summarize, I'm chasing a rough idle and discovered low compression on no.8. Leak down indicated exhaust valve.

Pulled the head and no. 8 exhaust valve. The valve was very slightly burned. My mechanic touched up the valve and the seat. Then we found that the lifter had a stuck check valve. We installed a new lifter and checked clearance. We thought problem was solved. Reinstalled head.

Started up this afternoon. Still had lifter noise and the miss. Pretty much same as before.
Ran the engine for a few minutes and still noisy with no smoothing out. ugh..

Checked the compression on No. 8 and it was almost 100.

Maybe some of the engine guys can point me in the right direction to go from here. I know where the nearest watering hole is....

Thanks guys,
Mark

Looks like you're tying two or three unrelated events together.
If you're getting a (dead) miss, look in the ignition system.
I've posted here before, you can take an induction timing light and check the flash on each plug wire to see which one or ones are misfiring. You don't just have to use it on #1. Power from a 12 V source if 12V light. A complete ignition "tune up" maybe in order if your going to drive it much.

The carbon can come from several things, and improving the compression is moving right. The air/fuel (fuel/carb) system may need some more work, if it persists, as noted before. If the tail pipe interior looks constantly black, for instance.

Lifter noise comes from a few sources; wear, adjustment, oiling, sticking, etc. If one was bad, noise remains, you might have others that are bad, or a common source problem like lube system issues; pluggage, pressure(oil pump), etc. You can check the oil pressure via a test gauge and see if it's correct.

Troubleshoot each system independently rather than try to find the magic bullet.
The overall picture, how well you expect it to run based on it's mileage?, will determine how much time and effort you'll have to spend. There's a point where doing a lot more internal work on a worn gunked up engine, a rebuild becomes an option.