Need advice to fix rough running engine.

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I need some advice on where to look now for the root cause of my engine running like a slug up hill in a sand storm. My engine is a 39' 221 V8 that's stock except for finned heads and 2X2 carbs with a "super" type intake. Fuel is supplied by a Holley electric pump and regulator set at 2.5 psi. Ignition is a stock helmet type distributor with "tube type" coil and adapter. Symptoms: It starts ok and idles ok but with a slight hitch in the rhythm. There are spotty black deposits on garage floor below exhaust pipe. On acceleration, it cuts out, occasional back fire thru exhaust, and slugs down the road as if running on 6-7 cylinders, choke or no choke. It is one gutless SOB.
My unsuccessful remedies to this engine illness include the purchase and installation of: Rebuilt carbs (97s) by Uncle Max, rebuilt distributor by Bubba (recently rechecked by Bubba), two new coils and a new coil wire, three new condensers (NAPA RR174), new set of spark plugs, replaced inline fuel filter and flushed gas line.
Tests and procedures include: check for vacuum leaks (found none), checked spark plug wires and coil wire for spark on cranking (adequate blue color), checked accelerator pumps in carbs for gas flow (ok), compression check (lowest=80 all others 90-110#) with all plugs out and carbs open. Removal of plugs revealed two with lots of carbon deposits and all others clean.
Questions: Will a bad valve always reveal itself in a compression check? What should I check, and or, try next? Thank you.
Phil