Now engine runs for a longer time - 5min, advance ok, but there is knocking similar to valve with very big clearance.
In the beginning it was similar to not working cylinder(flop flop flop), which from time to time disappeared. Now it is like metallic knocking. Total work of the engine is less than 15 minutes, by portions of 2 - 3 min, with max 5 min this evening(I need to check with stroboscope). Tomorrow will change the oil, which will not help, it is sure. Now engine is filled with new oil, but unknown quality. When I changed the spark plugs, on 5A plug was tight to the end. When I put new spark plug, it was tight from the beginning, even squeaky sound. Best situ, probably is, broken sealant on exhaust manifold(there is some smoke coming from the engine). Worst is, some structural damage in one cylinder. What are Guru's suggestions, please?

 

This morning made video. Very interesting - now knocking disappears in a higher rpms, and appeared again in lower.
Link to video: Don't say: "It is valve seat, You must remove the head..."
Please, say: "It is dirty or little corroded valve stem, or other moving part seizing from time to time, which will be fine after some more minutes of work, with high quality oil! Meanwhile, it cannot ruin the head."

 

Hard for me to tell from video, but it sounds like an exhaust leak to me. There are 6 individual gaskets between the cylinder head and the exhaust manifold and are known to fail. My ‘88 had the rear gasket fail (6B); the noise sounded mechanical, not unlike a valve with excessive clearance and was very hard to pinpoint.
Exhaust leaks can be hard to locate but generally do no damage. I finally located mine using a piece of heater hose held to my ear on one end, and used to opposite end around the various potential leak points. You can also look for soot stains from top and underneath with a mirror or borescope camera.
The good news is that the manifolds can be removed and the gaskets replaced with the engine in the car. Pack your patience!

 

more & more - instead of oil, there was "cocktail Molotov" in the engine - more gas, than oil.

 

My sound card is AWOL, so no comment on the noise.

That cocktail is NORMAL in a V12 that is constantly started and shutdown within a few minutes.

AND

A shorted/ing Injector loom, causing Injectors to be held open too long, thus "flooding" the engine.

AND

Cold start Injectors, if fitted, dribbling, very common iissue.

BUT

Look on the bright side, the inside of that engine will be squeaky clean, BONUS.

The joys of a V12, priceless, I must get another one, haha.

 

I don't mean to be flippant (but I guess I am), the answer to your problem is quite obvious to me:
Keep the engine at higher rpms At All Times! That's plainly where it's happiest!

 

Sounds like it could be excessive clearance on a cam lobe, maybe a valve stuck open a little. It might clear if you drive it.
Can you do a leak down test?

 

I'm fighting with the visible defects still - today changed injector harness. Now I remember, there was backfires in the beginning, when engine started hard.

 

Yes, I never start for so short periods engines, but I'm afraid from hidden & dangerous defects. Oil itself was as water, and only 7 liters. And in the beginning it was clear oil, now, after around 15 minutes total work, I see it BLACK - it is the bonus You mean Oil pressure now raised to the half, but the engine is a bit rough, one cylinder stops/starts to work... my fight continues

 

Luckily, knocking disappeared, rough work was due to removed and forgotten 2 high voltage cables. There are removed temp deflectors, and, for example, vacuum hose of the gear case was damaged from the high temp. Also, it seems to me, this engine creates big heat in the eng. compartment. When I was at the tire shop, I see coming Jag XJ. Open the engine compartment! - I ordered, and the boy(25 years old may be) opened, V12 HE motor he said. There was no such big heat around his engine, engine was like new, car itself too! Garage car from Spain... Interesting, there was no O2 sensors, nor catalytic converters, vacuum lines - simple, not so complex as our XJS..

 

I started getting an intermittent knocking on a '76 V12, oh, 35 years ago, (after an overheat situation) it would start and stop, it was simply a valve seat that had come loose, and would reinstall itself for a while, then drop out again, and repeat until it finally decided tot to reseat.

Doug

 

Unfortunately it returns, when engine is warm.