The great Smokey Yunick was probably the pioneer of spark plug reading when it comes to racing. Today’s younger generation are all about an AFR gauge and a lap top. Truthfully, a well trained eye can get more from observing the tip of the spark plug than the AFR alone. I have been trying to perfect my skill, not only as a race car tuner (of carbs and distributors) but also as a spark plug aficionado. During the initial tear down of the motor from the 71, this is what all 8 plugs looked like:
Just from what I have learned, I can tell the mixture is just about as spot on as I can get. If anything maybe a few .0001 lean. 🙂 The black around the top of the face and the darkened center electrode means its a bit rich at idle and the deposits on the ground strap are most likely oil. (as this engine has a LOT of hard miles on it. It takes some time and patience to make all of the best adjustments to get a carb to perform like it should.
This is not intended to be a tutorial as there are hundreds of opinions out there. One of the better tutorials can be found here:
The man and the master of his art.