I have been trying to determine the cause of my spark plugs getting black soot on them after only a 20 mile run. The black soot does not feel oily but has no gas smell. The engine idles well....here are some things I have done.

I have leaned the carburetor to only one turn of the mixture needle outward.
I have overhauled the carburetor.
i have made sure the hole on the under side of the carburetor is not blocked by the base gasket.
I have double checked the exhaust heat riser to make sure it operates properly.
The engine appears to be properly timed and runs well without hesitation.
I have performed a compresion test and all cylinders are within 10% of each other.The cylinders vary from 90# to 105 #.
I have adjusted the valve lash.
The engine is a 1955 235 with a model B Rochester carb

I am not sure if this is fuel related or an oil problem....not sure what to check next.
The only thing I was thinking of doing now was to lower the carb float slightly to see if that might help.
If it does not help then i have to assume it is an engine problem or am I missing something else "fuel wise " that I could check ???
The truck does smoke a little and when I first start it there is a slight metalic pinging or popping for maybe 2 or3 seconds and then idles great with no issues..
Would timing cause an issue like plug fouling ?





1953 chevy 1/2 ton with 3 speed 318 on floor and 1955 235 engine