Burn marks near SOME exhaust valves inside heads

As you can see in the picture, there are burn marks around SOME of my exhaust valve tops and not others (2 out of the 3 cylinders in the picture):


What could be the reason for these burn marks? Could it be pinging or premature combustion before the valves were sealed closed? Or perhaps that the valves weren't sealing tight. Since then, I have undergone a complete and detailed head and valve rebuild, cleaning all the valves, crud inside ports, lapped the valves and replaced the stem seals. I have also performed water tests to make sure the valves don't leak after the service I did.

 

just looks like a section where oil sludge accumulated. Nothing to worry about.

 

The same brown staining is visible in your picture on the riser that supports the valve cover. That is not a high heat area, so my assumption would be that the staining is not heat related.

 

my suspicion is that it may have TURNED TO BE a high heat area BY DEFECT if the valves used to leak (and hopefully won't any more after i serviced and sealed them tight) so the residue flames would shoot up the valve train and into the camshaft area. also, i imagine that can happen in the case of pinging or premature combustion if the spark hits before the exhaust valves are closed.

 

Agreed. I see no burn marks.

 

the dark sections on two out of the three spark plug risers

 

No. This doesn't happen even when the valves are leaking. The exhaust valve seal between the cylinder and the exhaust manifold, the intake valves seal between the cylinder and the intake manifold.

The only path between the valve seat and the cam area is via the valve stem seal .... which just isn't happening. Both the valve would have to be open and the valve stem practically nonexistent in order for anything remotely like you suggested.

 

Thanks, @Jimmyh, that makes a lot of sense. Now that you mentioned valve stem seals, I have a question about that which has not been answered.