has any one had this problem before

ok well time for an update. i changed out the spark plug and the smoked seemed to be a bit better and can manage the smoke was blackish grayish color and less fuely smelling. but on the other hand wens the car is running for while and the you slow down to stop the rpms will keep going down and if u rev up to keep them going is the only way to keep the car going and if you let it idle the car will die out and is a pian in the ass to get started again i have to hold the gas down. and before all this the car like spits wen u frist start out and if you dont keep the rmps up and the car driving the rpms drop out and the car will cut out. is my timing just that off on the car??????