These systems are very hard to deal with without proper test equipment.
Your mixture is worse than just rich if your plugs are all sooted up. With the D-jet system this probably means fuel pressure or manifold pressure sensor although open circuits on the temp sensors (air - in air filter and coolant - at front of intake manifold) can also send it that far rich.
You should verify manifold vacuum. I would do it by teeing to the manifold pressure sensor vacuum hose (7.5mm id rubber hose). The vacuum should be as close to 17in as possible. It will take good timing and proper running to get this much vacuum. To test the manifold pressure sensor try pulling a vacuum on the MAP sensor (manifold absolute pressure) higher than the engines vacuum and see if it leans out (to do this plug the cars hose and suck directly on the sensor). A gas analyzer sure helps at this point or a scope to evaluate the mixture by measuring injector pulse width. It you have either of these available I can give better instructions.
The MAP sensor is adjustable with an Allen screw under the black plastic cap (opposite end from the vacuum fitting. You probably can get idle mixture correct no matter what the problem with this adjustment. The problem is that you can't sit there and adjust for every situation so these are just tests.
To evaluate the temp sensors first make sure they are connected and both wires intact. Measure ohm value if you like, but the real test is to backprobe the sensor input at the controller. The signal comes out at 5v disconnected. This is brought lower by passing through the resistance of the variable temp sensor resistance. By testing at this point the voltage a true measure of what the computer sees is at hand.
Bosch Master, ASE Master, L1
33 years MB technician