...If it has Backfired Through the CARB...Just One Time, You have a Blown 'Power Valve' RUBBER Diaphragm on the Primary Metering Block. Too Much Fuel, Flooding Out! If it is a 'Double-Pumper', there is also a Power Valve on the Secondary Metering Block! Check/Test these by Removing and Sucking on them...No Kidding, they should 'Hold' the Vacuum.
MJ