Hi
You may need the ECU checked and check the cold start injector on the back of the plenum chamber.
Have you had the mixture checked ? (Screw on the meter, the black box by the air filter)
Check all the Butterfly valve in the flapper system.
Check all the injector seals and pipes for leaks.
Check the pipes at the Fuel filter (in the drivers side rear wheel arch)
Check all the pipes to and from the meter/air filter for holes/splits
If this does not work check the fuel pressure regulator at the rear of the engine and the one way solonoid under the plenum chamber.
If none of these work have the cylinder pressures checked for warn rings or bores.
Black smoke can also be a sign of warn bores as can grey or blue
Hope some of this may be some help
Charlotte
If I think of anything else