Tearing down my 400 today and found something interesting. One exhaust valve on each head seems burnt. The corresponding plug has a different look as well. Engine ran well before tear down. Just cleaning, re gasketing and camming. So with all that said what could be causing this?
That cylinder is running leaner than the others. I wouldn't be concerned with that particular color on the valve, but the plug needs a closer look to be sure it's not too lean. I would be focused on why this cylinder is so different from the others. Do the cylinders with the tan exhaust valves have the exhaust crossover, by chance?
In the one photo I looked at three of the cylinders are getting a too rich air/fuel micture, hence the black sooty deposit on the spark plug insulator and the head's combustion chambers and valves - except for the combustion shamber with the very white spark plug. For that cylinder it is also getting a too rich a/f mixture evidenced by the sooty carbon on the head surface and intake valve surface. But, the exhaust valve being a much different color, and the white plug procelain tell me at some point their is either a very lean condition, or the plug is a much higher heat range than the other plugs. I have seen cases where folks put in a hotter plug for cylinders they find are getting their plugs oil fouled.
As interesting as I am with the two plugs that are indicating a lean mixture, I would want to find out why the a/f ratio is so rich. Some possibilities are the electric choke is not setting the choke wide open soon enough, and the choke blade being even just a little closed will pull more fuel through the venturis. Another is a ruptured power valve in the carburetor, If yo are running a 2100 or 2150 2 barrel carb, or a Holley carb, it is very kilely you have a leaking or ruptured power valve. Another cause is having a fuel bowl float set too high. And, finally, it could be due to foreign material stuck betweek the fuel inlet needle and seat.
As good as the engine was running prior to the tear down, I can assure you once you get the a/f ratio out of its excessive rich mixture situation the better your engine will run.