273 Commando w/front 4 Spark Plugs Black/Sooty

dibbons said
This is 1965 factory stock 273 Carter AFB, 273 factory intake, 273 factory exhaust manifolds on a stock rebuilt 318 short block with 2.5 inch TTI dual exhaust. Only about 2,000 street miles on re-build which, up until now, had a rock-solid smooth idle but now shaking noticeably at idle. Warm compression check shows six cylinders @ 160 PSI, one @ 150 PSI, and one @ 165 PSI. Wondering how plugs from cylinders 1, 2, 3, 4, could be so dry sooty black while the other cylinders (rear four) have normal whitish/tannish looking plugs? (choke is always tied open permanently by the way)

That's a single plane intake manifold. If you have the PCV valve in the port in the intake manifold, instead of at the carburetor, that could cause the rear four to run excessively lean which could make the front four look overly rich. The port in the intake is for power brakes and the carburetor port is for the PCV valve. So that's something to look at.