Sorry for the delay!
I did some more digging for you and have some things for you to test to determine the cause of the engine running too rich.
Obviously, be sure to check the engine coolant temp sensor before getting into all sorts of electrical diagnoses. The engine running rich could be something as simple as that!
If that checks out, you could have a bad or lazy oxygen sensor. The heated oxygen sensor is very important to control not only the cars emissions but it also helps the vehicles performance! If it is not working properly it can result in a number of problems like rough running stalling hesitating bucking and jerking. But more common than that if it is lazy, the cars gas mileage can drop considerably, and sometimes not even set a check engine light on the dash!
The fuel control Heated Oxygen Sensor (HO2S 1) is mounted in the exhaust manifold where it can monitor the oxygen content of the exhaust gas stream. The oxygen present in the exhaust gas reacts with the sensor to produce a voltage output. This voltage should constantly fluctuate from approximately 100 mV (high oxygen content - lean mixture) to 900 mV (low oxygen content - rich mixture). The heated oxygen sensor voltage can be monitored with a scan tool. By monitoring the voltage output of the oxygen sensor, the PCM calculates what fuel mixture command to give to the injectors.
1. Verify with a spark tester that the tester can be fired with it adjusted to 25 kv.
2. Try adding fuel, does the additional fuel help this accelerate.
3. With the timing light, verify the base timing and then what happens with the timing on acceleration when the problem happens.
4. Verify fuel pressure and monitor the spray pattern of both injectors.
Exhaust Gas Recirculation (EGR) Valve
Secondary Ignition System
1. Disconnect the injectors and run engine on carb spray or propane to see if it will run good.
2. If not check the Exhaust Gas Recirculation (EGR) valve for seating, secondary ignition and engine mechanical condition with a compression test.
3. If runs good on alternate fuel, check fuel quality and flow bench test the injectors.