We took out all my Bosch platinum plugs to change the heads and cam.

Most of them have that nice tan/brown colourization to them. But the colour changes as we move to the back two on each side. They get progressively darker and sooty looking. Is this a sign that the carb is running too rich or is it an indication that secondaries are too rich?

I am running a B hanger and AY rods. The plugs are less than a year old and are in excellent shape, other than the seemingly strange colour on the back ones.

 

that's not unusual for the rear cylinders on a SBC with carburetor. Don't know the cure. If your secondaries were too rich, you'd have the problem on all spark plugs. You could try a hotter plug in those cylinders. Otherwise, if it's not too bad, don't worry about it.

 

Hmm, OK.

Thanks for the info Mike!

It's not a problem. The engine always ran smoothly, no misfires. We've just taken the baseplate off the QJet and JB Welded the spots that will leakdown on you over a few hours. I wonder if the leakdown fuel tends to flow to the back cylinders for some reason and makes the plugs appear darker?

They're platinum plugs (sparked by a 50000 volt coil) and run a very hot spark, anyway, so it has never shown any detrimental effects.