Classic/Antique Car Repair: 64 thunderbird, holley carburetor, carburetor problems

Question
My son and I put on a Holly 4 barrel on this 390 and it ran great for quite a while.It has started to idle rough,and really rough while in gear but seems to run great, with more exaust fumes now. Changed points,plugs, condenser,rotor cap, dist cap and fuel filter,Still rough. Any Ideas, help please.

Answer
It sound like you have a carburetor that is dumping more fuel into the intake manifold system than it is supposed to. While the common folklore out there is that a Holley Carburetor will cure all ills I have found the opposite to be true. Well my personal biases aside, there could be a problem with one of the needle valves or floats or the power valve diaphragm could have a pin hole in it allowing the engine to run rough. If it is carburetor problems your spark plugs will tell the story. Reading a set of spark plugs is as good a read as any mystery novel. Pull all the plugs out in order and check the color of the firing end that is in the combustion chamber. If the plugs are sooty black then suspect the carburetor. If they are a tan color then the engine is running right. If they are an oily black then the engine is oil fouled and if they are white and some times heat blistered then the engine is running too lean as with a massive vacuum leak. Also they should all be the same color, if not there is a bad cylinder, plug, plug wire, or intake manifold leak for that cylinder. Let me know the results.
Brad