Our 1991 Mercury Capri Turbo 5 speed manual failed emissions.

If the car was sitting for a while yes I would agree do a minor tune up at least also try higher octane gas at least while you have it tested. Good news is you only have two more years and you won't have to do emissions inspections anymore.

When Hydrocarbon (HC) emissions are high, that is a result of an engine misfire, excess fuel entering the cylinders, or a catalytic convertor not working as well as it should. Replacing the plugs is a good place to start, perhaps the 02 sensor as well if it is not to expensive and relatively easy to replace. Make sure you make a good freeway run before getting the vehicle tested, you want to be sure the catalytic convertor is fully warmed up and functioning at its best. If you still don't pass you can plug the results of the test into a Lamda calculator - http://www.smogsite.com/calculators.html#lambda. This calculation will help determine if the problem is to much fuel or something else (misfire or tired cat).