190E 2.3 Failed Emissions Tests

I have just purchased a 190E 2.3 (1987)MB. Except for its high mileage (some 239,800 original miles, I think), this car is in excellent condition with all working electronic features (even the clock!) However, it failed the emissions test three times. The emissions test diagnostic results indicated that HC is the problem (some 1.98 gram/mile, and the limit is 0.8 gram/mile) I have replaced spark plugs, air and oil filters,and the works. The latest replacement was an oxygen sensor, but to no avail. I am going nuts here trying to figure out what can be done, without spending my kid's college fund, on this problem. High HC seems to be the main problem with the test. Do you have any suggestions? Thanks a bunch!

C.B.