Fred, you say the plugs are sooty black so it's running rich during normal driving, right? If you change only the rods like Chuck says,try to select ones that have similar smaller diameters to keep the high speed enrichment the same.The backfire from the carb when opening the secondaries is a temporary lean condition before the secondary jets can flow enough fuel.At the same time, the primary metering rods have raised into the richer power mode, so leaning the "cruise mixture" won't have any effect on this.
