Simple advices/solutions.

for the following specific problems.

1. The engine has an ordinary carburator. Check the air filter. If you already have changed it, may be you changed to a filter that is too small for this engine.
   It happens that people change the spark plugs many times before the real problem is addressed. It is so simple, that it many times is forgotten.
   
   
2. The engine has a fuel injection system. Check all the connections for the electrical wiring that is governing the injection. Oxidation is a problem on most parts of metal. And on connecting ends, in electrical wiring, oxidation is annoying, especially if you are struggling without finding out about the source for your problems.
   
   One example: Mercedes Benz 190 E (1987) has, under the air filter, the air flow meter (where the air goes in) and a “centre” for the injection. Mounted on its back side is an electrically governed injection regulator. It has 2 pins for electrical contact. When they are corroded, they transfer wrong voltage, if any. Get rid of the corrosion here and, if necessary, also on all other involved contacts from different sensors and components. This worked for me. The black smoke and the jerking vanished. When I looked at some internet sites, trying to find an answer to my problems with this car, none gave me the correct answer.
   
   Another example: Some cars have the electrical governing of the injection, mounted on/by the injectors. Check the electrical contacts there.
   
   Important: If the engine, after it has reached working temperature, gets problems idling or running on lower revs, and jerks when you try to accelerate, the injectors probably need to be cleaned. Many of the additives in gas are unfortunately deposited in the injectors. This can easily removed by a bottle of "Injector Cleaner", poured into the gastank. One 250 ml bottle, made by STP, mixed with a almost full tank did the trick for me. Before the tank was run empty, the engine behaved like it is supposed to.