The thermo-sensor/thermostat on the fuel injection unit senses engine coolant temperatue to adjust the fuel mixture from rich(engine cold) to normal(engine warm). If the cooling system thermostat is stuck open and the engine always runs cold then, naturally, the thermo-sensor will tend to maintain a too rich condition.
Of course, the thermo-sensor on the injection itself or the 'piston' that it operates could be defective/stuck. There are other sensors, mechanisims and valves on these mechanical injections that can fail. Before you attempt repairs, find a detailed shop manual on this injection system to see what you might be getting into. Experimenting with gas and fuel systems can be hazardous. You might want to check out that Houston MB shop that Steve suggested.