My Lumina is stalling.

I have a Chevrolet Lumina APV 1990 model and a mileage of 75,000. The car was working fine before. All of a sudden it just started stalling when I put it in drve, reverse and even when I on the AC. It got to a stage that when I start it it will just die off. I called my mechanic, he checked it and told me that the catalyst under is blocked. I changed it but the slow running is not regular. At times it will just go very high. We change the complete ignition module, the slow running got improved a little bit but it is still not okay and it brings small black smoke and it still stalls whenever I put it in drive, reverse. When we remove the socket on the right nozzle of the dual nozzle carburatur. The slow running will get stabilized. I did the servicing about two weeks ago. We have changed the MAP Sensor. Please what could be the problem because the mechanic is telling me that he needs to open the cylinder in order do some cleaning.