Pull some plugs and let's look at the shell and the strap for the heat lines.
Don't bother with trying to read the insulator, unless you see it is bone-white. Low RPM driving and idle time won't let you accurately read them anyway. If you see the insulator is white, then we've certainly got a fuel problem.
Are you using a plug with a cad-plated shell? If so, this gets easier. If you examine the threads on the plug, you'll see the threads closest to the business end will be a slightly different color than the threads nearest the hex. How many threads are colored?
If the shell is a black shell (Autolite, etc.), then pull a few plugs one afternoon. If you have some older plugs, thread them in or spray some penetrant on some paper towel to wind into the open holes. Leave the plugs on the bench overnight (a really humid day is a good one). The next day, you will see the humidity has allowed some very light rust to appear on the threads. Once again, how many threads are colored with the rust?
Look at the strap on the plug, preferably with a magnifying glass in the sunlight. You will see the strap is not uniform in color, from the plug shell to the end. Where is the line with the color difference, in relation to the bend in the plug?
Something else to think about - how much compression ratio are you dealing with? And what are the specs on the camshaft? I see people wanting to run a bunch of compression with a wee cam profile and it drives cylinder pressure to the moon. Have you ever done a cranking compression test? What kind of numbers did you see?
Have you looked at the mechanical advance in the distributor, to be sure the weights are moving freely? The Government Motors HEI distributor is notorious for binding up weights at both ends, so make sure that isn't happening to you.
