1990 audi low rpm misfire
 

I have a 1990 audi sport coupe. I have fixed all the vacuum leaks. Replaced the pcv valve, distributor cap, rotor, ignition tested good, timing belt,alternator, and water pump. I checked the gap on the plugs. The first time I pulled the plugs only one showed good clean burn. Now they are all burning better I believe. The car still has an ignition or fuel issue somewhere. I checked all the fuses today for the ecm, etc. All good there. I cleaned the ISV valve and it is working. I am going to check the cat. convertor and install a new fuel filter. The car has 120,000 miles on it. What is a good way to check the fuel mix, the injectors?? Help some one please.