93 aerostar 4.0 e4wd timing question

My 93 aerostar 4.0 e4wd 140,000 miles failed HC emission test by over 300 ppm. I had new plugs, wires, dist. cap, dist. button and fuel filter installed. Still failed HC test by over 500 ppm. Timing was set 3 degrees high and it passed. What problem(s) would cause timing to have to be set high?