Engine Performance problem
1986 Nissan Truck 4 cyl Two Wheel Drive Manual 227000 miles
My 1986 720 series truck runs rough at idle and surges/stumbles at mid-range RPMs. The engine vacuum is good with no vacuum leaks. The carburetor was just replaced with a rebuilt unit from a good rebuild company. The truck ran the same way before the carburator was replaced. The EGR valve opens at the proper engine speeds. I tried to isolate anything that uses engine vacuum by disconnecting or pinching off hoses, but that made no difference. The ignition system is functioning properly and the engine timing is correct. The truck starts up fine and then starts to run roughly pretty bad until the choke is done functioning and then idles at a good speed but is still rough, sounds like a small " miss". The choke does open completely. The biggest symptom is at mid-range when it surges/stumbles. The truck has so many vacuum operated components that it is difficult to if something is properly functioning. I managed to " misplace" the Haynes and Chilton repair manuals I have for this truck, even though those publications only provide limited information. I'm suspecting the EGR system, oxygen sensor, or one of the many vacuum operated valves that litter the intake area. I have owned the truck for 23 years and is has been well maintained.