Nissan Repair: engine miss firing, nissan pulsar n13, air fuel mixture

Question
Hello sir,
My car is nissan pulsar n13 built 1990, 4 cylinder, injector, manual transmission and I also got the service manual I already read it the trouble shooting section but it did not help ,the problem is: Miss firing at idel speed and also when driving on the road at top  gear 4 stil miss, I,ve changed all 4 new spark plug it went OK for one day but miss again the next day, i check the all new 4 spark again and all black with cabon deposit , I suspect the air fuel mixture motor,but is not adjustable it's control by the ECM, do you have any suggestion I'm wondering around in circle.  

Answer
Bronson,

I would think you should check the fuel pressure.  Also, if yours is a throttle body injected model check the injectors to make sure they are not leaking since it sounds like yours is over rich.  Lastly, check the distributor for oil on the optical shutter wheel.