Lots of potential causes. First off, you need to confirm that the wetness and color is due to fuel fouling and not oil fouling. Normal excessive rich fouling results in a sooty black appearance unless the fuel system is dumping raw gas into the manifold. Oil fouling will, depending on the severity, be shiny and borderline sticky. Assuming you have fuel fouling:

Make sure your air filter is not plugged with dirt or fouled with oil fumes from the crank case ventilation system.

Check your fuel pressure. The later versions of the D jet run at 30 psi. If your pressure is higher than that it will run rich.

Check the connections to the engine coolant temp sensor (the one at the front of the block) and the air temperature sensor. Test the resistance of the sensors to make sure that they are within specification (as per the Volvo service manual).

Make sure that your cold start valve is not leaking fuel when it is shut off. Check for leakage on individual injectors. Because of the batch fire nature of the Djet system, a really vigorous leaker can affect the fuel mix on more than one cylinder.

Always good to confirm the correct operation and setting of the throttle switch; but, I don't think incorrect setting would cause gross rich operation.

It gets more complicated if the preceding do not address the problem.