You probably have a vacuum leak that is making a few cylinders run lean. Does the engine have a PCV valve? Try removing and plugging the PCV, and putting a little Propane into the crankcase through the oil filler or the PCV grommet in a valve cover. (don't overdo it!) If the engine speeds up or smooths out, especially if the vacuum goes up, look for a leak at the bottom of an intake passage where it meets a cylinder head that's drawing air from the crankcase. That was a favorite trick of the NASCAR guys who had to run restrictor plates- - - -create a deliberate vacuum leak into the crankcase, and rely on the big reservoir of oil in the dry sump oil system to let the engine suck up a little oil on pace and caution laps without losing oil pressure. Those cars smoked heavily during a caution, and cleared up as soon as they got back up to racing speed. I watched that happen at Talladega on several occasions.
Jerry
