High carbon on valves

[Al 29Tudor]

Good Morning,
My "A" was running great showing light brown / gray color on the spark plugs.
It began loosing power and by shorting out the plugs determined number one cylinder had no power. Pulled the head off and found lots of carbon accumulated on the under side of the valves and a little down the guides. Also the plugs are heavily sooted included the pistons and the head. Shorting out each plug shows a blue / orange spark.
The points were too closed and i found the point block clamp screw was stripped. Reset the points and it started better and ran better.
I re-cored the radiator last year and now it runs real cool
What is the main/sudden cause of all the carbon.
Thanks for your thoughts.
Al