Hopefully someone can help me with my Ranger problem.

I have a 1994 4.0L 6c Auto trans ranger that just got a new Mass Air Flow Sensor, Flue Pump, Battery, and rear brake line. I received a call from the mechanic saying that the truck has a rough idle with popping coming from the tailpipe. He said that it was probably something to do with the exhaust valve/gasket. He did not fix that problem citing that he would need to take the engine apart and said it was too much money and not worth it because it was probably just built up carbon. Is there anyway i can get rid of this build up carbon and return the truck to normal. Oh it just started happening like a week ago. Thanks.