Plug Fouling

[Vermonter]

During a 4th of July boat parade in a 25 kt howler in VT with party flags flying, black smoke started coming from my exhaust and the engine started to hunt, then gradually lost power and RPM dropped to an uneasy idle. With barely enough power to maintain steerage, I limped back to the mooring and found all four plugs totally black. Choke cable is working and gas bowl looks clear. Put 4 new plugs in and it fires up to full power, but after a minute, plugs are already fouling again. Carb throat has slight pooling of (rusty?) oil/fuel mixture (coming from manifold return line?). It has new coil, condenser, plugs, dist cap. New plugs show strong spark. Timing is TDC. Carb was new last spring. Compression is 140 in all cyls (usually 120 so I don't know if the fouling has increased compression somehow). Fuel is non-alchohol regular with a fresh 6 gal (with 4 oz MMO) into ~10 old gals.

Running super rich? Oil in fuel? Bluebirds in the exhaust?

Other ideas?

Thanks, Bob