The most common cause of rich running is a dead or stuck warm-up thermostat. It is very easy to check not necessarily that easy to fix.
At the rear of the pump is a metal air filter. Remove the filter. With the engine idling cold place ones finger over the hole. It should kill the motor. Now try it again with the engine hot. While suction will be felt there should be no change in rpm..
In the tube (with water hoses attached) above the filter is the water warmed t-stat. It pushes a plunger into the pump which does two things. One it controls air added for fast idle cold and two it enriches the govering mechanism. The mixture is harder to measure but the airflow is easy. Same action does both. If there is significant drop in engine speed hot then the choke correction mixture is also off.
The whole arrangement disassembles quite easy and the piston inside is often frozen (usually in the cold setting). Just pay attention to all shims. They are in with the t-stat and beneath the assembly. Don't loose any untill we get to the bottom of this.
Bosch Master, ASE Master, L1
33 years MB technician