Sooty Plugs

A proper plug check is done after the engine is at operating temp.... Then the engine is taken to an "operating speed" of something over 2,500rpm, under load, for about 30/60 secs. ...

Then you kill the engine while keeping the throttle open. (less chance of vacuum pulling fuel into the cylinders) 

There are several variations on this, but this has always worked for me....

You are probably correct about the carb(s) being rich, in that most engine builders jet them rich as cheap insurance to heat problems.....   That said, your reading will mean virtually zip if you just pull the plugs after pulling in from a grocery run the night before....

If the car starts and runs well, you might try smaller idle jets to help with the milage as your engine spends most of its time under 2500/3000 rpm where the idle circuit furnishes most of the fuel....   Just go at this one step at a time.....

Personally, I wouldn't start buying parts just yet. Do you have a snail? Know how to synch the air, and adjust the linkage? Know how to properly adjust the idle mixture?

Do a valve adjustment first, check your timing, THEN properly adjust the carbs. Then do Leon's plug check. then get back to us with what you find.

My bet is the linkage is leaning on one carb more, and the carbs/linkage need a good adjustment.

This might help you some if you aren't familiar:

https://www.speedsterowners.com...-adjustment-and-sync