Mitsubishi Repair: 1992 Eagle Summit Wagon, air fuel ratio, eagle summit

Question
I have a 1992 Eagle Summit Wagon, 2.4L and it recently started to idle very rough. I have checked for spark and there are no problems there. The rough idleing seems to be somewhat inconsistant. I can start it and let it idle for a while with no problem, then once i drive it for a bit and come to a stop, its starts idleing rough again. I have also noticed carbon deposits at the exhaust pipe. Any suggestions?
Thanks, Dave

Answer
Dave,
Poor idle can be do to a number of factors. A dirty air filter, dirty fuel filter, weak fuel pump, clogged PCV valve, clogged EGR valve, or a slipped timing belt can all contribute to a rough idle.  Access buildup of black soot on the tail pipe can indicate that your engine is running abnormally rich (air/fuel ratio has too much fuel).  If you've noticed a slow decline in your gas mileage, this is another good indication you are running rich.  Running is commonly caused with a worn out oxygen sensor and/or worn out catalytic converter.  Have all the items I've mentioned checked out by a mechanic.  
Good luck!