I'm having a fit trying to tune my 350 sb with a road demon carb. Too rich at idle, then stumbles off idle. Too lean, too rich back and forth aagh! So I inspect the plugs and the front four cylinders look good, but the rear cylinders(5,6,7,8) are black dry sooty. What gives? Can half the engine run rich? Everything I read says the mixture screws should be equalized. The carb has four corner idle screws. What about the butterflies? Should they be opened the same amounts, or should the primaries be set to a proper setting and then close the secondaries to adjust the idle speed? ...maybe i'ts not the carb.