A faulty injector will cause this, it can spray too much fuel into the engine cause it to run rich. You will also want to check your timing and compression.
Fouling of the entire set might result from an incorrect heat range spark plug or an over-rich air/fuel mixture caused by a clogged air cleaner filter element, a sticking heat riser valve, or a faulty choke. Fuel injectors that malfunction can also lead to this condition.
Other causes include weak ignition system voltage or an inoperative pre-heating system (carburetor intake air) or poor cylinder compression.
That is what i was trying to say about the injector. The injectors however could have gone wrong in two different places. The injector or injectors or the computer.
You can test the wiring and the computer with a noid light, or a test light.Hook up one of those too the injector connector, and crank the engine, the light should blink.You could do one of two things with the injectors. The first and best would be pressurize the rail, then fire them by using a battery positive source, and a ground to the other terminal. (these are the terminals on the injector) Then check to see if the F/P pressure drops.You could just also check the resistance of the injector, or see if you can get to open close off the car by powering it.Sounds like there are injectors stuck open which could be due to a bad ECU/PCM.
Well it could be stock open manually and that would test if it's stuck like that. If you disconnect it and raw fuel is still coming out to it then yes it's open. However you should also check the current to it to make sure that the injector is coming on and off rather then just have power at all times. Also if you do the disconnect check if you get fuel on the spark plug again.