My truck when idling has this "popping" sound coming out of the exhaust. Is this a mal-adjusted carb or a carb that is needing rebuilt? This is a 1979 F-250 4x4 with the 400 and 4 speed manual. I don't know exactly what carb it is. It is a 2-bbl motorcraft. Which I assume is the stock carb but not sure. It does run real strong though. I think it is suffering a bit in the mileage area. Even though economy and these trucks don't often reside in the same sentence. Any help would be greatly appreciated...thanks.