1997 Toyota 4Runner Consumption Problem

My car has a consumption problem,that is consumes more fuel now,and sends out black carbon at the exhaust pipe. This was not so before as you would hardly see any carbon at the exhaust pipe end. I have replaced the spark plugs bought direct from the USA and air filter to no avail. I am in Cameroon and does not have the facility to run diagnosis even though the engine check light at the controls has not come on to indicate there is a problem.Please assist.