Tom: the overheating of the engine causes white valves, but what I was trying to point out was burning of the valve alone. I think this is different from overheating of the whole engine, but I have not your expertise on diesel engines, I guess, so I do not know if I am understanding you well.
This "valve burning", is what I believe normally causes carbon deposits as points (clearly visible in the picture I posted). Actually you have to "de-carbonize" the seats on most diesel engine jobs. On Venom picture those marks could be dents, as you say, or carbon deposits (the photo of Venom is reaaaally out of focus).
As you point out, those are exhaust valves, and there are several problems that can cause overheat of exit valves, besides bad timing of cam shaft:
- excess fuel
- bent valve guides or seats that impede the valve from seating properly and allows hot gases to exit during ignition.
- worn valve stem oil seal, that allows oil to flow down the valve stem and carbonize at the valve seat
- hydraulic cam followers that are not operating well and don't allow the valves to seat properly
I bet for number one, as number two to four are more "catastrophic" and normally cause heavy carbon deposits and hot points or cause them only on the seats. The photo I posted show that the crown of the valve has broken, probably by metal fatigue on the stem that finally brought the valve to touch the cylinder head. The heavy damage to the cylinder head is shown here, an example of the "bad things" you mention:
Tolerances between cylinder heads and valves are another problem, because they are much lower than in gasoline engines.
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