Having issues with a 3406b in an 85 Freightliner. Once the truck gets above 60-62mph it starts spitting and sputtering and smoking. Doesn't seem to be an engine rpm issue as it only happens in 15th on a 15 speed roadranger. Any ideas as where to start looking? Air filters were replaced 5 months ago (less than 1500 miles).
3406b question
