All these folks have proposed excellent possibilities and solutions for the problem. The valve seals will definitely help but is not the root cause. The rings are sealing, ran great before the work, runs great for a few minutes, until the choke starts coming off then does not supply enough fuel for the vacuum leak in the intake valley, this leak also sucks oil vapor and fouls the plugs as you have shown. If you pull the intake and re-gasket it with appropriate sealing compound the problem will mysteriously go away, been there done that. Test, when the engine starts running poorly vape the oil fill orifice with ammonia vapor or starting fluid after plugging PCV, vacuum caps on the carb and the PVC valve, you will hear a change in the engine RPM to verify before pulling the manifold. Works more dramatically if you pressurize the intake valley slightly by sealing the Ether can or ammonia to the oil fill hole. One increases the idle the other decreases the idle. Shop air pressure is too much and will likely blow an engine seal out.
Those are aftermarket seals already. Replace with the same and you should be good. I have a pile of those from left over gasket kits. I'd gladly give you a set if you were local.
Big cam for a stock 71. I run that cam and it likes rich carb settings. Your engine tuner might have tuned the car to highest vacuum and that might be too rich. I was turning plugs black like that too but was running half decent still. What kind of carb are you running?
My eyes are not very good but the plug looked wet black instead of dry black. Dry black being too much fuel, wet black indicating oil contamination. At least thats the way it used to be when we had some lead and some octane in the pump gas. Uphill both ways.
