192,000 miles.......WOW, that is awesome. I would be afraid of suggesting anything with that kind of miles. There are some oil additives that are supposed to clean your valves, but I am not sure if they work or not. I have even heard some of the old timers say to drain the oil, fill it with kerosene, let it run for 20 minutes, if that long, and drain it all. They say that cleans all the gunk out of a motor, and doesn't hurt anything. I have never tried that, and am not sure I ever would. With that kind of miles on your motor, it might fall apart if you got it all cleaned up. Back to your original question, yes, it probably is carbon from the valves. If you try any of the above methods, please consult a professional mechanic before you try it. The oil additive may be the best place to start, and then change your oil and filter soon there after.
With that many miles, it may be best to rebuild it.
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