Rough Idle & Black Smoke

Latley every now and then when i start it up the idle is pretty rough and the engine sounds/feels like its choking. It continues to do this even holding it at 3000rpm. When its idling theres a decent amount of black smoke. My exhaust tips constantly need to be shined up from the carbon. This problem also started around the time when i went back to 2 1/4" catback with a chambered muffler(no packing inside the muffler) My megan muffler was strait threw but had packing which im assuming soaked up some of the black smoke making it less visable. Plugs need changing? Im getting near 4500kms since my last oil change but ive never had this before. What should i be looking at doing as ive never had this car really tuned up.

take ur plugs out and check them

Im also leaning towards my fuel injectors…This can cause the car to run rich and cause a rough idle. Engines got 210 000kms and i see 250-300kms a tank…roughly.

ive allready changed my injectors on my sohc if u need/want any help just gimme a call, i dont charge, i did it like 4 times cause i fucked up