My failed smog test shows high hydrocarbons. Everything else appears fine. What are the possible causes if we assume all is well with the carburetor? It’s a Chevy Nova, '88, newly rebuilt motor, high mileage on the chassis. Runs nice, quick, strong, smooth running at speed and at idle. No hiccups or hesitations. Thanks for the help!
Likely it is running rich. How about checking the plugs to see what they look like. Look for deposits on the plugs, are they the same, what color etc.
www.autozone.com has some helpful information on your car’s feedback carburetor…how to check it, and how to set it. This is, also, information on the whole emissions controls systems. Take it (the website) for a drive! http://www.autozone.com/az/cds/en_us/0900823d/80/19/d5/4a/0900823d8019d54a/repairInfoPages.htm
I don’t think anyone would ever accuse the 88 Nova of running “strong”, I take that back, You could probably outrun one of the older 6 volt Rascals with it. 
Seriously though, try leaning out the mixture, make sure the cat is not partially clogged (doubtful if the car is otherwise running okay) ,also check the EGR valve to make sure it’s not stuck.
Ignition misfire (even slight), carburetor problem, retarded ignition timing, or possibly an oil consumption problem depending on the circumstances behind the newly rebuilt motor.
Knowing which engine and carburetor along with any camshaft changes might help if this is a modified car.
Why do you assume all is well with the carburetor? These carburetors are pretty complex and you could well need a rebuild if one has not been done recently. Also look at the oxygen sensor. If this has quite producing voltage, the computer will command full rich at idle and cruising speed. Since the computer is not OBDII capable it might not set a code if the oxygen sensor is not swinging through zero. Look at your read out to see if they list oxygen and carbon dioxide content of the exhaust. Any excess oxygen indicates misfire, poor combustion, exhaust valve leakage, cold catalytic converter, or dead catalytic converter.