Get BIG q tips (industrial size) and clean the threads in the heads too. And then get what you can inside the combustion chamber around the hole. Use a lot of them with brake cleaner on them! Any left in the chamber around the plug hole can revaporize under heavy load, and redeposit on the insulator of the next set of plugs u put in there!
Experience is what you get when you didn't get the results you expected!
We did a new 8-71 boat blower motor that would only run for 45 sec. to a minute and a half, backfire and stop.
We checked a few things, and it kept doing it! All new complete MSD system.
Replaced: coil wire, plugs, wires, coil, rotor, then hot wired it to the battery and MUCH more. About half the times, it would crush a float or two. Being that it was a drag boat, one of us would hit the starter, and the other one of us worked the throttle from outside the boat. We swapped positions after I lost part of my moustasche, and eyebrows from flames out of the backfire plate! Spent several hours dicking with it, wanted to replace the dist. but didnt have another small cap one that cleared the blower.
Bottom line: the terminals in the inside of the cap were 3/8" too short, the air in the cap ionized and the spark crossfired, killing the motor. After a few minutes of checking things out, the air in the cap would de ionize and the motor would again run for a short time. Being anal, and out of ideas, I measured the distance of the rotor to the distributor body, and the then, depth of the cap terminals. I bent the tip of the rotor up about 1/4" and it ran fine. Replaced the cap the next day, and shipped the boat.
MSD said they had never heard of that and didn't replace it!
To this day, I drill two vent holes in dist cap near the bottom, just because....
Trust me, I am too old to be learning this kind of crap!